Potentiometry

   Definition of Potentiometry

Potentiometry is the field of electroanalytical chemistry in which potential is measured under the conditions of no current flow. The measured potential may Subject:then be used to determine the analytical quantity of interest, generally the concentration of some component of the analyte solution. The potential that develops in the electrochemical cell is the result of the free energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition has been satisfied.

                                    

This concept is typically introduced in quantitative analysis courses in relation to electrochemical cells that contain an anode and a cathode. For these electrochemical cells, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell.

                

If the reaction is conducted under standard state conditions, this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must utilize the Nernst equation to determine the cell potential.

              

Physical phenomena which do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion concentration gradients across a semi-permeable membrane. This can also be a potentiometric phenomena, and is the basis of measurements that use ion-selective electrode.

Nernst Equation :

Reactions at electrodes are redox reactions

Aox + ne <--> Ared

Nernst eqn: how electrode potential is related to thermodynamic activities of reduced and oxidised species.

E = Eo -	RT	ln(	ared	)  ...............(1)
	nF		aox

E = electrode potential
Eo = electrode potential in the standard state (constant value for particular electrode at a particular temperature)
ared & aox are thermodynamic activities of Ared and Aox respectively.
R = gas constant (8.314 Joule.K-1.mole-1)
T = temperature (K)
n = no of electrons in the half cell reaction
F = Faraday constant (9.65 x 104 Coulomb.mole-1)
ln, as usual, denotes natural logarithm (base e)
Terms in the Nernst equation have units VOLTS.
(RT/nF reduces to Joules/Coulomb, which = volts)

Usual case is solid electrode in equilibrium with an ion in solution. As activity for the solid species = 1, the Nernst equation simplifies to..

E = Eo -	RT	ln(	ai)  .................(2)
	nF

The -sign becomes + if the ion in solution is the oxidised form. But sign of voltages in electrochemistry is a matter of convention.
If T = 298.2K (ie 25oC) and use 2.303Log10 instead of ln, then as 2.303RT/F evaluates to 0.059 volts, Nernst equation becomes...

E = Eo + 0.059log ai  ........................ (3)

Use Nernst equation to evaluate the activity of an analyte ion in solution if you can measure electrode potential by potentiometry

 

Theory of Potentiometry

theoretical basis of potentiometric analysis; the dependence of electrode potential on analyte concentration. Definition of a Nernstian electrode.

uses and advantages of potentiometric analysis

practical reference electrodes: ideal characteristics and some common reference electrodes

factors affecting measured cell potentials of galvanic cells. Reasons to minimize current flow in potential measurements. Description of iR drop.

origin of junction potentials. Salt bridges.

electrode polarization in galvanic cells: effect and explanation. Definition of overpotential.

Metal Indicator Electrodes

metal electrodes of the first kind. The most useful metal electrodes.

metal electrodes of the second kind and how they work

inert electrodes (metal and nonmetal)

Membrane (Ion Selective) Electrodes

problems with metal indicator electrodes

general description of ISEs and their response to analyte concentration.

categories of ISEs

glass ISEs; the pH electrode

crystalline ISEs. The fluoride ISE, and ISEs based on membranes of silver salts. "First-" and "second-order" Ag ISEs.

liquid ISEs. Description of calcium ISE.

Methodology and Characteristics of Potentiometric Methods

the need for ionic strength buffers

sources of error in potentiometric measurements; factors ultimately limiting the precision advantages/disadvantages of potentiometric methods

HISTORY :

1889	Lecturing in analytical chemistry introduced by Dr. Nicola Dobrev.
1891	The first textbook on analytical chemistry published by Dr. N. Dobrev.
1904	The Chair of Inorganic and Analytical Chemistry established; Head, Prof.  Dr. N. Dobrev.
1924	The Chair of Analytical Chemistry established; Head, Professor Dr. Z. Karaoglanov     (scienific contributions in gravimetric analysis; mechanisms of precipitation reactions; electroanalytical methods; electrochemical kinetics; qualitative analysis)
1943–1971	Head, Professor Dr. Nicola Pentcheff, Corresponding Member of Bulgarian Academy of Sciences (scienific contributions in analytical chemistry and geochemistry of noble gases and Bulgarian mineral waters; gravimetric analysis).
1971–1972	Head, Assoc. Prof. Lyulyana Kotcheva (ion exchange separations and chromatography; qualitative semi-microanalysis).
1972–1982	Head, Assoc. Prof. Dr. Radka Christova (potentiometry; ion selective electrodes; determination of anions; ion exchange separations; analyses of ores, metals and alloys).
1982–1991	Head, Professor Dr. Stoyan Alexandrov (atomic emission spectroscopy; neutron activation analysis; radiochemical tracers; preconcentration by sorption, copreciptation, extraction; determination of noble metals and mercury; analysis of high purity substances).
1991–1999	Head, Professor Dr. Panayot R. Bontchev, D.Sci., Corresponding Member of Bulgarian Academy of Sciences (Catalymetric analysis; analysis of drugs and pharmaceuticals; coordination and biocoordination chemistry; electron paramagnetic resonance; organic and inorganic structural analysis).
2000–	Head, Professor. Dr. Dimiter L. Tsalev, D. Sci. (atomic absorption spectrometry; hydride generation; cold vapour AAS, dithiocarbamate extraction; chemical modification in electrothermal AAS; hyphenated hydride generation–ETAAS; on-line sample pretreatment in flow injection systems; biological and environmental monitoring; speciation in trace element analysis).

During the period of 1889–2005, numerous staff members have contributed to the development of analytical education and science (in alphabetical order): Dimiter BALAREV; Stefan BELTCHEV, Stefan CHRISTOV; Hristo DAIEV; Elena GANEVA, Dimiter GERILOVSKI; Radka IRIBOINOVA; Dimiter IVANOV; Maria IVANOVA; Nikolai JORDANOV; Spas KOLEV; Petko MANDJUKOV; Vesselina MICHAYLOVA; Michail MICHOFF; Georgi NIKOLOV; Georgi KANAZIRSKI; Georgi KANDILAROV; Charalampi KARASTOYANOV; Elena KOLEVA; Nikolina KULEVA; M. KURCHATOV; Donka NONOVA; Malina NOVKIRISHKA; Boris SAGORTSCHEV; Lyubka SHISHKOVA; Ivan TRIFONOV; Jowtscho TSANEV; Dragan TSCHAWDAROW; Peter TISCHKOV; Emilia VASSILEVA, Bisera Evtimova.

History 
By Professor Bengt Nygård

. In the report from the Research Committee of Science 1945 concerning the future requirements for research it was suggested to establish a central analytical laboratory at the University of Uppsala. secure a high standard for this laboratory right from the beginning the head should be in a position of associate professor.

According to the proposals the new chair was established on July 1, 1946 and Folke Nydahl was the first to enter the duty in 1947. Nydahl, also lecturer in chemistry at the Agricultural College of Sweden, was earlier director of the Official Laboratory of Agricultural Chemistry in Kristianstad. Nydahl's previous research activities in analytical chemistry design of the institute. However, Widman obviously tried to provide for different interests and the building probably corresponded quite well to the requirements of a modern chemical institute at the turn of the century. It was inaugurated on the 2:th of September 1904. Cleve was on leave from the beginning of the term and retired in February 1905. At this point the designations of the chairs were changed and Widman was made Professor of Chemistry (ordinarius) with teaching responsibilities in organic chemistry. It was decreed that the holder of the other professorship should teach general and inorganic chemistry.

Widman's research work had slown down during the building period but it was now revived to its full extent and directed towards oxido compounds. Sven Bodforss and Henrik Jörlander were also working in this field; G. Karl Almström was investigating pyrrole derivatives and Arthur Bygden organic silicon compounds.

The investigations of Assistant Professor ('docent') Håkan Sandqvist on phenanthrenesulphonic acids may perhaps be regarded as a late extension of the naphthalene work of Cleve and Widman. This field, however, turned out to be a very difficult and tedious one.

Widman retired in 1917 but continued his activities as an experimental scientist for another few years.

Folke Nydahl .

Nydahl's research activities are mainly connected with wet chemical analytical techniques and characterised by a profound feeling for solving analytical problems. Together with a number of co-workers Nydahl has improved and developed analytical methods for minerals and rocks with the purpose to secure the standardisation for the more rapid but sometimes less accurate instrumental procedures. Among these investigations may be mentioned chelometric determination of aluminium, calcium and magnesium in the presence of each other (M. Mortsell), the hydrogen peroxide method for spectrophotometric determination of titanium (R. Bryntse), determination of minor amounts of magnesium after separation from calcium by coprecipitation with nickelhydroxide (E. Johansson), extraction of calcium and strontium by coprecipitation (Allan Bengtsson)..

To analytical investigations of more general interest belong separation of sulphate and hydrogen sulphate ions from interfering substances by adsorption on aluminium oxide, prior to sulphate determination (with L. Gustafsson 1953), 4-amino-4'-chlorodiphenyl as an analytical reagent for sulphate (Arthur Bengtsson 1957), the influence of sulphate on the determination of phosphoric acid as molybdophosphate (G.-B. Nordstrom), determination of the solubility products for triphenyltinhydroxide, -fluoride and -chloride as well as the acid constant of triphenyltinaquoion (G. Brodin 1958).

L.-H. Andersson (diss. 1962) carried out a comprehensive investigation of two methods for the determination of silicon as silicic acid and L. Danielsson (diss. 1967) performed a systematic study of the sorption of about thirty metal ions on ion exchangers from solutions containing large amounts of iron. Advanced instrumental techniques were used for the quantitative determination.

Initiated by the requirements of accurate analytical methods for elements in water (biological research projects in the Baltic) Nydahl and L. Gustafsson have successfully improved some existing methods. One example is the determination of total phosphorus in natural waters (1973). Recently (1976) an important investigation by Nydahl has clarified the reaction parameters at the reduction nitrate - nitrite giving a more accurate determination of nitrate in natural waters.

Later on, electroanalytical techniques were more consequently introduced in the analytical department. A comprehensive study of some new graphite paste electrodes by J. Lindqvist (diss. 1970) proved the possibility of using this type of electrode for quantitative voltammetric analysis in oxidative processes.

After 24 years as associate and, later, full professor in analytical chemistry, Nydahl retired in 1971. As his successor, Bengt Nygård was appointed.

 
Beng Nygård

The basic research in analytical chemistry has so far by different circumstances been given a more independent profile. The earlier research activity of Bengt Nygård was directed towards physical techniques in analytical chemistry. In collaboration with members of the department of organic chemistry, mainly Arne Fredga, Goran Bergson and Lennart Schotte, electroanalytical techniques, especially polarography, had been proven to be a useful tool for studies of redox properties of organic sulphur and selenium compounds. Experiences obtained could confirm the potential possibilities of the voltammetric techniques in organic chemistry for direct analytical purposes as well as for structural correlation of electrochemically measurable parameters. Consequently, a focusing on electroanalytical techniques started in the department during the 1960's was further intensified during the 1970's. As a lucky coincidence a sort of revival of the classical voltammetric techniques came a few years ago. With the help of modern electronics new principles as pulse voltammetry could be realised in commercially available instrumentation of excellent performance. New types of electrodes with graphite as a matrix, e.g. the paste electrode mentioned earlier, could considerably expand the available potential range for voltammetric analysis in anodic direction. Reductive as well as oxidative properties of organic compounds could then be used for    electroanalyticalpurposes.

At the end of the 1960's an instrumental co-operation had started together with the department of electronics. Upon a physicist, Rolf Danielsson who had been established in the department of analytical chemistry, the task was imposed to contribute to the development of instrumentation and techniques of measurement in electroanalytical chemistry. This scientific joint work proved to be very fruitful for improving the instrumental research facilities in the department of analytical chemistry. Danielsson's dissertation in electronics (1973) dealt with the integrated subject "Some examples of electronic instrumentation in analytical chemistry". As a research assistant Danielsson is now responsible for the construction of a new advanced measuring system designed mainly for electroanalytical techniques.


As a specialist on complex chemistry and general solution chemistry Åke Olin entered the department in 1972 as an assistant professor. Also under his responsibility is the development of electroanalytical techniques based on precision potentiometry.


Comprising this introduction means that the research activities are concentrated in three areas within basic and applied electroanalytical techniques namely voltammetry, complex chemistry with potentiometry and instrumental-measuring development. In the following these research projects will be presented in more detail.

Finally it can be mentioned that a limited research program is proceeding in collaboration with industry. One example is physical-chemical investigations of the mechanisms at gel permeation chromatography with special reference to simple inorganic salts. The present capacity of the department for education on the Ph.D. level is about ten students

POTENTIOMETRIC DETERMINATION OF CAPTOPRIL INPHARMACEUTICAL FORMULATIONS

Abstract

A simple, precise, rapid and low-cost potentiometric method for captopril determination in pureform and in pharmaceutical preparations is proposed. Captopril present in tablets containing knownquantity of drug was potentiometrically titrated in aqueous solution with NaOH using a glass pH electrode,coupled to an autotitrator. No interferences were observed in the presence of common componentsof the tablets as lactose,microcrystalline cellulose, croscarmellose sodium, starch and magnesiumstearate.analysisresultsobtained by applying the proposed method compared very favorablywiththose obtained by the United States Pharmacopoeia Standard procedure.Recovery of captopril fromvarious tablet dosage formulations range from 98.0 to 102.0%.

Keywords: potentiometric determination; captopril; pharmaceutical formulations.

Introduction

Captopril,  chemically  know  as  1-[(2S)-3-mercapto-2-methylpropionyl]-L-proline,  (CPT) isused  therapeutically  as  an  antihypertensive  agent11, 15], which is also prescribed for the treatmentcongestive hart failure [6].Several types of analytical procedureshave been proposed for the analysis ofcaptoprilinpharmaceuticalsformulations. These proceduresinclude capillary electrophoresis [13], high-performanceliquid chromatography (HPLC) [3, 4, 5, 9],polarography [10], voltammetry [23], coulometry

20], amperometry [18], conductometry [19], fluorimetry

[12] , colorimetry [1, 2, 7, 14, 22, 24] andflow injection methods.[12, 21, 28]. Some of thesesprocedures are not simple for routine analysisand required expensive or sophisticated instruments.Potentiometric methods with ion-selectivemembraneselectrodes (ISE’s) can provide valuableand straightforward means of assaying captopril inpharmaceutical formulations because of the possibilityto determine directly the active ions in thesolution. ISEs’ low-cost, easy of use and maintenance, and the simplicity and speed of assay procedure,and the reliability of the analytical informationmake them very attractive for the assay of pharmaceutical

products.Potentiometric methods based in the reactivityof thiol  group have been used for captoprildetermination in pharmaceutical formulations [8,17, 26].To the best of our knowledge, despite theadvantages of the glass membrane pH electrode,there are no previous reports for the potentiometricdetermination of captopril based in the reactivityof this carboxyl group using the glass pH electrode.For this reason, the purpose of this workwas to develop a potentiometric method for directdetermination of captopril in tablet dosage formulations. The method is based in a potentiometrictitration of captopril (carboxyl group) in aqueoussolutions  with sodium hydroxide solution using acombined glass electrode, coupled to anautotitrator.  The proposed method is simple, rapid,precise, accurate and inexpensive.The results agreed fairly well with thoseobtained by the United States Pharmacopoeia(USP) standard procedure [27] (iodimetric titration).The influence of interferents normally foundalong with captopril in tablet dosage formulationsin also studied.

Experimental

Apparatus

Potentiometric measurements were carriedout using a Metrohm autotitrator, model 716

(Metrohm Ltd., Herisau, Switzerland). The indicatorelectrode was a Metrohm combined pH electrodemodel 60234.100. A thermostated titrationcell (25.0±0.1)OC was employed. Volume measurements(±0.001mL) were performed with a Metrohmautomatic burettes model 665.

 

 

Reagents

All chemicals were analytical grade andsolutions were prepared with deionized water (conductivity= 18.2MÙcm). Captopril (standard substance)was purchased from sigma (St Louis, MOUSA). It was analysed as prescribed in the USP [27]and contains 99.7% of 1-[(2S)-3-mercapto-2-methylpropionyl]-L-proline, calculated with referenceto the dried substance. It was used as a standard.The captopril tablet(25mg)manufacturedrespectively by Medley (Campinas, Brazil),Azupharma Gmbh (Gerlingen, Germany) andApotex (Toronto, Canadian) were used for theanalysis. Sodium hydroxide and sodium nitrate werepurchased from Merck (Darmstadt, Germany).

Solutions

A freshly prepared 1.000 x 10-1 mol L-1aqueous of solutions of captopril (substance)was used as the stock solution.The sodium hydroxide 2.00 x 10-2 mol L-1was prepared, standardized and stored accordingto recommendation of the literature [25].The ionic strength (I) of the final solutionsused for the potentiometric determination was keptconstant at 0.500 mol L-1 by addition of sodiumnitrate.

Recommended procedures

For pure form

A standard solution (100 mL) of captopril1,000 x 10-2 mol L-1 (I adjusted to 0.500 mol L-1 withNaNO3) was prepared by suitable dilution of thestock solution with water.Analiquot of 15.000 mL of this solutionwas transferred to a thermostated glass cell(25.0±0.1)ºC and potentiometrically titrated with astandard solution of NaOH 2.00 x 10-2 mol L-1 (I =0.500 mol L-1 in NaNO3).

Tablets analysis

Fifteen tablets were weighed tocalculate the average tablet weight. Theywere finely powdered and homogenized. Aportion of the powder equivalent to about217.3 mg ofcaptopril was accurately weighedand dissolving with 40 mL of water bysonicating for 20 min in an ultrasonic bathThe resulting mixture was filtered and its ionicstrength was adjusted to 0.500 mol L-1 withNaNO3. Finally, this solution was diluted withwater in a 100 mL flask and analysed underthe same procedure described for captopril

in pure form.Results and discussionEcl. Quím., São Paulo, 28(1): 33-44, 2003

 

Potentiometric titration

The development of the titration curvesionizationconstants. Theoretically, there will be one inflectionfor each labile hydrogen. However, so thatan inflection is associated to an adequate variationin pH, it is necessary in the first place for the relationshipof the respective ionization constant tothe next one to be greater than 104 (K1/K2 > 104 orpk2 –pk1 > 4). In the second place, it is necessaryfor the corresponding ionization constant not tobe that of a very weak acid.Captopril is dibasic acid having dissociationconstants pk1 = 3.7 (carboxyl group) and pk2 =9.8 (thiol group). Observing the values of pk1 anpk2ofcaptopril, it can be foreseen that the titrationcurve presents a clear inflection for the first pointof equivalence since k1 = 2 x 10-4 and the relationshipof k1/k2=106 (pk2 – pk1 = 6). However,captopril is a very weak acid in relation to its secondhydrogen (k2 10-10) that its titration curvedoes not present a perceivable inflection for thesecond point of equivalence.Figure 1 (a) shows a typical potentiometrictitration curve with only one inflection point. Inthe proposed method changes at the titration endpoint were enough pronounced to give potentiometrictitration curves of satisfactory shape for anaccurate and reproducible end point detection. Thetime required for the analyses of captopril (after the

samples preparation) in tablet dosage formulationsby potentiometric method was 8 min. per sample.

            Figure 1. (a) typical potentiometric titration

curve of captopril (sample A) with NaOH solution; (b)first derivative plot supplied by the autotitrator.Figure 1(b) shows the first derivative ofthe potentiometric titration curve generated bythe internal algorithm of the autotitrator. Theevaluation of the potentiometric titration curve by the autotitrator is fully automatic yielding anaccurate end point. The determination limit ofthe proposed method determined as describedby Leite [16] was 180 g mL-1.

1.Effect of interferents

To assess the usefulness of the proposedmethod, the effect of the common components

(additives, adjuvants and excipients),which often accompany captopril in tablet formulations(lactose, microcrystalline cellulose,croscarmellose sodium, starch and magnesiumstearate) were investigated in a concentrationrange of least 20 times higher than that ofcaptopril. No interferences were observed ithe presence of the substances tested.

Analytical applications

The proposed method was successfullyapplied for captoprl determinatioin tablet formulations. The results presented in Table1, agreed fairly well with those obtained by theUSP standard procedure [27] (iodimetric titration).

For further confirmation, the standardaddition method was applied to test the reliability

and recovery of the proposed method.The recovery studies were carried out after adding

formulations.The results presented in Table 2 showthat the percentage recoveries were found tobe close to 100%; the SDs were within 0.48 –0.85. These results point out the accuracy andprecision of the method and the absence of significantmatrix effects on potentiometric measurementsat least for the samples analysed.

 

 Table 1. Determination of captopril in pure form and inpharmaceuticals using the proposed method.aFor the assay of captopril in pure form, an amount equivalentto 217.3mg of the standard substance (Sigma) wastaken.bLabel to content for tablets: mg unit–1.cAverage of five determinations ± SD, performed within 2– 3 days.dRelative standard deviation (RSD)

 Table 2. Recovery data for captopril spiked to pharmaceuticals.

aAverage ± SD of three determinations.

 

 

 

Conclusion

Compared with many of the already existingprocedures for the determination of captopril,which required special instrumentation, reagents,precautions and experience, the propose potentiometricmethod employing the glass pH electrodeexhibited the advantages of simple operation, reasonableselectivity, fast response, lowcost,andsufficient accuracy for the determination of captoprilin pharmaceutical formulations.

Potentiometric Determination of N-(2Mercaptopropionyl)-glycine

Using an Electrode with AgI-based Membrane

INTRODUCTION

N-(2–mercaptopropionyl)-glycine (MPG), also namedtiopronin, is a synthetic aminothiol antioxidant. It isused in treatment of cystinuria,1,2 rheumatoid arthritis,liver and skin disorders,3 and as an antidote to heavymetal poisoning. Recent studies have shown that MPGcan function as a chelating, cardioprotecting and radioprotectingagent.4,5 Along with its desired effects, it macause some side effects such as muscle pain, yellow skinor eyes, sore throat or fever, changes in taste and smeetc. Moreover, this drug produces a dose-related nephrosyndrome.6 Therefore, sensitive determination MPG in biological samples and pharmaceutical prepais highly desiraA number of spectrometric,7–9 fluorimetric,10,11 chemiluminescence12–15 and chromatographic methods16–20have been developed for MPG determination. All thesetechniques are highly efficient but very expensive andtheir application is rather complicated. Electrochemicalmethods are popular for many applications because theprocedures are simple and fast, and the cost is low. Recently,

Siangproh and co-workers21 reported MPG determinationin commercial tablets using cyclic voltammetry.In this paper, a novel, simple, sensitive and affordablepotentiometric methods for determination of MPGin solution as well as in pharmaceutical preparations ispresented. The proposed procedure is based on MPG reactionwith silver (I) using an indicator electrode withAgI–based membrane. Response of the applied chemicalsensor to MPG (also designated RSH) is explained bythe formation of sparingly soluble RSAg in the reactionsolution and/or on the exposed surface of the sensor. Inaddition, a method for MPG determination in commerciallyavailable tablets by potentiometric titration in anaqueous solution (0.1 mol L–1 HClO4, pH = 1) with standardsolution of silver nitrate is described.

EXPERIMENTAL

Apparatus

All potentiometric studies were carried out with a millivoltmeterIskra Model MA 5740, coupled to a personal computer.A double-walled, thermostated reaction vessel, maintainedat 25 ± 0.1 °C was used. Cell potentials were measuredwith an indicator electrode with AgI–based membrane(Orion 9453) versus a double-junction reference electrode(Orion 90-02-00), with 10 % potassium nitrate solution asthe outer filling solution. The selective electrode was drystoredbetween measurements and overnight. Before each setof measurements, the ion-selective membrane was polishedwith a special polishing strip (Orion 94-82-01). During measurements,the solution was stirred using a Teflon-coatedmagnetic bar. Stirring speed and electrode distance were kept

constant throughout all measurements.

Reagents

All chemicals were of analytical-reagent grade and solutionswere prepared in MilliQ deionized water. MPG (0.01mol L–1) stock solution was prepared by dissolving an appropriateamount of MPG (Sigma-Aldrich) in 0.1 mol L–1perchloric acid(Merck,Suprapur) and stored in a dark bottleat 4 °C. Working solutions of lower concentration wereprepared daily by appropriate dilution of the stock standard

solution with 0.1 mol L–1 perchloric acid.Silver nitrate (0.1 mol L–1) stock solution was preparedby diluting a titrisol of silver nitrate solution (Kemika) to 1L with water, and was kept in the dark. The solution wasstandardized by potentiometric titration with sodium chloride(0.1 mol L–1) using an indicator electrode with AgI–basedmembrane (Orion 9453). Working solutions of silver nitratewere prepared daily from the standard solution.

The applied basic buffer solution (pH = 2) was preparedby mixing acetic, boric andphosphoric acids of finalconcentrations 4 ´ 10–2 mol L–1. Buffer solutions withhigher pH values were prepared by mixing the basic buffersolution with sodium hydroxide solution, c = 2.0 mol L–1.The appropriate pH value was checked using a Metrel

(HEC 0102) pH glass electrode.The MPG containing drug, Captimer, was obtained

from MIT Gesundheit GmbH, Germany. At least 10 tabletswere weighed to obtain themean weight per tablet. An accuratelyweighed tablet was left overnight in 50.00 mLHClO4, c = 0.1 mol L–1, partly dissolved, and then crushed.The obtained suspension was filtrated through filter paper(Blue ribbon, S&S, Germany). Solid residue on the filterpaper was washed with ca 40 mL of perchloric acid. Thefiltrate was collected in a calibrated flask, and diluted100 mL with perchloric acid.

Procedure

Direct Potentiometric Measurements. – The response of thecell with the indicator electrode with AgI–based membraneto the Ag+ ion was measured by serial dilution of the standard0.1 mol L–1 AgNO3 solution with 0.1 mol L–1 HClO4solution. The potential response of the indicator electrodeto MPG was recorded under standard addition. Before additionof MPG, 50.0 mL 0.1 mol L–1 perchloric acid and 10mL 0.01 mol L–1 silver nitrate were added, as a backgroundsolution, into the thermostated reaction vessel. In the otherexperiment, the background solution consisted of 50.0 mLbuffer solution (pH = 3)and 10 mL 0.01 mol L–1 silver nitrate.The potential–time response of the electrode was measured in a regular analytical setup. The background solutionwas stirred and monitored under successive additionsof known quantities of MPG. The potential values were takenafter a steady–state potential had been established. Potentiometric Titration. – Potentiometric titration studieswere carried out in the manner of conventionalpotentiometrictitrations. Unless otherwise indicated, the total ionicstrength and pH were kept constant by addition of 0.1mol L–1 HClO4 solution. During the potentiometric titrationstudies, the titrant was delivered in 0.05–0.10 mL steps, usinga Hirschmann micropipette. The end-point volume wascalculated mathematically from the second derivative data.From the collected data, the concentration of MPG insample solution and the solubility product of RSAg werecalculated.»Kinetic« Determination. – 50 mL of 0.1 mol L–1HClO4 was accurately pipetted into a reaction vessel and 1mL of AgNO3, c = 5 ´ 10–4 mol L–1, was added. Duringmeasurements, the solution was stirred with a Teflon–coatedmagnetic bar at a suitable steady rate to avoid splashingand bubbling. After the steady–state potential had beenreached, 1 mL of solution containing different amounts ofMPG was introduced into the cell. The change in potentialwith time (1 minute intervals) was recorded for each solution.After each experiment, the reaction vessel and theelectrodes were washed with dilute nitric acid and distilledwater. Between the sets of experiments, the sensing membraneof the electrode was polished. Before the next run, the

electrode was soaked in 1 ´ 10–3 mol L–1 Ag+ and 1 ´ 10–3mol L–1 I– ion solutions for 10 minutes and washed twicewith water.

RESULTS AND DISCUSSION

In this experiment, the indicator electrode with AgI–basedmembrane used in combination with a reference electroderesponds primarily to the activity of Ag+ ions in

the solution or on the phase boundary surface membrane/solution, according to the Nernst equation:

 

 

 

 

Potentiometric Titration

Reproducible results for MPG were obtained when thetitration was carried out in 0.1 mol L–1 HClO4 as backgroundsolution. A white precipitate containing MPG

and silver in a 1:1 ratio was formed during titration inacidic media (pH = 1–3).The titration of MPG with silver performed in a lessacidic medium (pH = 4 or 6; Figure 2b) was non-stoichiometric.Our results show that titration is possible forMPG amounts from0.4 to 1.0 mg (Figure 3; Table II)and from 1.0 to 3.0 mg (Table III). Amounts outside thisrange were also investigated, but with low accuracy ofthe results. Titration in mixtures of water and organic solvents,such as ethanol and tert-butanol, was also achievedbut without any particular advantage.

»Kinetic« Measurements

Addition of various amounts of MPG to (0.1 mol L–1HClO4) silver solution alters the concentration of Ag+ inthe solution and the potential of the cell according to Eqs.

(6) and (12) (Figure 4).For all MPG concentrations, the voltage differences,

DE, for 2 and 10 min time intervals were calculated (Figure5). The relationship between the potential change (for2 min) and the MPG concentration was found to be linearfor more than one-decade range of the amount ofMPG. It should be stressed that this linear or analyticalrange was found when the response of the electrode wasunstable and voltage differences were calculated with nonsteady-state potentials recorded 2 min after the additionof MPG. For these measurements, the analytical signal,DE, was taken in the kinetic region of the reaction.The investigated »kinetic« method provides a simple

and rapid technique for the determination of MPG in aqueoussolution. However, when applied to the MPG determinationin pharmaceutical preparations, it gave poor reproducibility.

ApplicationsThe proposed potentiometric titration method was applied

to the determination of MPG in pharmaceutical preparations.The results obtained and the labelled contents aresummarized in Table III. There were no significantdifferencesbetween labelled contents and those obtained bythe proposedmethod. Recovery studies were performedby adding a known amount of MPG to the sample beforethe recommended determination by potentiometric titration

in 0.1 mol L–1 HClO4. Recoveries ranged from 97–103 %.Table III also summarizes recovery results.The proposed method for analysis of MPG in pharmaceutical

preparations has the following limitations:MPG cannot be determined in non-aqueous solutions orif the reaction solution has pH > 3.

CONCLUSIONS

The potentiometric methods described in this work aresimple, economic and rapid techniques for the determinationof MPG. The potential response of the indicator

electrode with AgI based membrane to MPG is based onthe reversible chemical reactions involving the RSAgcompound on the exposed surface of the sensor. Whenthe direct potentiometric method was applied, a linearresponse was obtained in the concentration range from2.0 ´ 10–5 to 1.5 ´ 10–3 mol L–1. The kinetic potentiometricmethod makes it possible to determine MPG inthe concentration range from 5.0 ´ 10–3 to 7.5 ´ 10–4

mol L–1. The best accuracy and reproducibility wereachieved with the potentiometric titration method. Thismethod, involving 0.1 mol L–1 HClO4 as the backgroundsolution, was applied to determination of MPG in pharmaceuticalpreparations.

 

PREPARATION AND APPLICATION OF POTENTIOMETRIC SENSOR

Objective:

 Starting from past research experience and literature data, the proposed research program predicts a study of the preparation of a potentiometric sensor suitable for the flow-through injection analysis (FIA). Investigations are planned for the purpose of improving the laboratory design of the FIA-system with a potentiometric detector based on commercial ion-selective electrodes or by potentiometric sensor prepared in the laboratory. The obtained results and the experience gained in the development of the flow-through injection potentiometry (FIP) are intended to be applied for anal. det.
Goal:

The proposed project is planned basically for the needs of scientific training, education and progress of teachers in the scientific field of analytical chemistry. In view of this the maturity and competence in research will be checked by processing the investigation results for the purpose of their publication in international journals. The planned investigations will contribute also to the construction of the measuring equipment of interest to the teaching requirements in postgraduate research studies.

Summary:

In order to achieve the essential characteristics of the flow-through injection potentiometry: small sample volume, high-rate response, signal reproducibility, ..., a multi-purpose potentiometric flow-through detector with different sensors will be constructed. The conditions regarding the planned investigations will be satisfied by the incorporation of a tubular sensor with a reference electrode into the flow-through injection system and by using the measuring instruments for the recording andpenicillamine, coenzyme A, ...). The determination procedure will be optimised by changing the parameters of the measuring system: flow-rate, sample volume, sensor geometry, ...; the species and the stoichiometry of the reactions will be discussed on the boundary sensor/solution. The coefficient of selectivity, the dynamic range and the detection limit will be determined. Depending on the nature of the chemical reaction product on the sensitive surface of the sensor, the constant of the solubility product or the stability constant of the complex will be established. Initially, for the preparation of the tubular potentiometric sensor, the AgI and CuI-based electrode material will be used, previously tested by applying the classical potentiometric measurements. By using the CuI-based sensor the flow-through injection method for the determination of I- and Cu2+ will be developed by successive injection of the samples with iodide and Cu2+respectively, without renewing the sensitive surface of the sensor. The rate and the stoichiometry of the complexation reactions of metallic ions with different ligands will be studied by applying kinetic potentiometry and the flow-through injection potentiometry. The complexation processes will be discussed as to the interpretation of equilibriums in the water environment of interest to the formation of toxic (non-toxic) compounds of some metalspenicillamine, coenzyme A, ...). The determination procedure will be optimised by changing the parameters of the measuring system: flow-rate, sample volume, sensor geometry, ...; the species and the stoichiometry of the reactions will be discussed on the boundary sensor/solution. The coefficient of selectivity, the dynamic range and the detection limit will be determined. Depending on the nature of the chemical reaction product on the sensitive surface of the sensor, the constant of the solubility product or the stability constant of the complex will be established. Initially, for the preparation of the tubular potentiometric sensor, the AgI and CuI-based electrode material will be used, previously tested by applying the classical potentiometric measurements. By using the CuI-based sensor the flow-through injection method for the determination of I- and Cu2+ will be developed by successive injection of the samples with iodide and Cu2+respectively, without renewing the sensitive surface of the sensor. The rate and the stoichiometry of the complexation reactions of metallic ions with different ligands will be studied by applying kinetic potentiometry and the flow-through injection potentiometry. The complexation processes will be discussed as to the interpretation of equilibriums in the water environment of interest to the formation of toxic (non-toxic) compounds of some metals

 

 

 

 

 

 

Experimental errors

In relation to stability constant determination there are some considerations over and above the usual chemical ones. Chief amongst these is the control of experimental error. The accuracy and precision of calculated stability constants depend on the magnitude of systematic and random errors respectively. Good accuracy requires that systematic errors be reduced as far as possible. The use of analytical grade reagents will reduce errors due to purity of reagents such as acid or alkali and the salt used for ionic background. Errors in temperature control are systematic errors. Electrode calibration error is also a systematic error, of particular importance when comparing duplicate titration curves.

Good precision requires that random errors be reduced as far as possible. All instrumental measurements are subject to random error. The magnitude of this error is instrument specific and, in the case of spectrophotometric measurements is also dependent on the magnitude of the measured quantity. The objective of the stability constant refinement is to calculate values that correspond to experimental observations within experimental error. This means that estimates are needed of the random errors present in the experimental measurement

 

 

 

 

Determination of fluorides in urine - Method by ion specific electrode / Potentiometry

Index

1. SCOPE AND RANGE OF APPLICATION

2. PRINCIPLE OF THE METHOD

3. REAGENTS

3.1. Distilled and deionised water

3.2. Glacial acetic acid                 

3.3. Sodium chloride

3.4. Sodium citrate

3.5. Sodium hydroxide

3.6. Sodium hydroxide 5 M

3.7. Sodium fluoride

3.8. Tampon for fitting total ionic power

3.9. Fluoride standard solution

3.10. Ethylendiaminetetraacetic (EDTA) acid

4. APPARATUS AND MATERIAL

4.1. Polyethylene or propylene container

4.2. Precipitate flask of 1000 ml and borosilicate glassware

4.3. Magnetic stirrer

4.4. Specific electrode of fluoride ion

4.5. Reference electrode of simple connection

4.6. pH meter

5. COLLECTION OF SAMPLES

6. ANALYSIS PROCEDURE

6.1. Cleaning of material

6.2. Preparation of the sample

6.3. Preparation of standard and calibration curve

6.4. Potentiometric determination

7. CALCULATION

8. PRECISION

9. BIBLIOGRAPHY

APPENDIX A

APPENDIX B

1.   SCOPE AND RANGE OF APPLICATION

This method describes the procedure to follow and the equipment needed to determine fluorides in urine, by means of Ion Specific Electrode technique (EIE) in a range of concentration from 0,2 µg F^-/ml (*) to 200 µg F^-/ml of urine (10^-5 M to 10^-2 M).The method is applicable to the surveillance of occupational population potentially exposed to inorganic fluorides.

The ingestion of substances containing fluorides in the diet or in the consumed water, as well as the contribution due to dental treatments should be considered as potential interferences in the interpretation of the results. The presence of some metallic ions such as the Al⁺³, which may affect the reading, can be neutralized adding ethylendiaminetetracetic acid (EDTA) to the urine sample in the ratio of 2 mg of EDTA per each 100 ml of urine.

2.   PRINCIPLE OF THE METHOD

The urine sample is collected in a polyethylene container, containing 0,20 g of EDTA. The urine is treated with a tampon for adjustement of the total ionic strength, and the soluble fluorides present in the sample are determined by means of the technique of ion specific electrode.

3. REAGENTS

All reagents used should have at least the specification “for analysis”.

3.1. Distilled and deionised water

The water will be of quality 1 in accordance with the standard ISO 3696 (9.7.)

3.2. Glacial acetic acid

PRECAUTION: CORROSIVE SUBSTANCE.

3.3. Sodium chloride

3.4. Sodium citrate

3.5. Sodium hydroxide

PRECAUTION: CORROSIVE SUBSTANCE.

3.6. Sodium hydroxide 5 M

Dissolve 20 g of sodium hydroxide in bidestilled water up to complete 100 ml of solution.

3.7. Sodium fluoride

PRECAUTION: TOXIC SUBSTANCE

3.8. Tampon for fitting total ionic power

Pour 500 ml of distilled water into a 1litre container. Add 57 ml of glacial acetic acid, 58 g of sodium chloride and 4 g of sodium citrate (3.4.). Shake until obtaining the solution. Put the container in a water bath to cool. Add it slowly to the sodium hydroxide 5 M controlling the pH until it is between 5 and 5,5 units. Cool at ambient temperature and take it up to 1 litre with water according to paragraph 3.1.

Alternately commercial solutions, already prepared, can be used with the same purpose, bearing in mind the work pH.

3.9. Fluoride standard solution

3.9.1. Fluoride 0,1 M solution (1900 µg/ml). Dry the sodium fluoride (3.7.) in stove at 120° C over 4 hours and let dry in desiccators. Weigh 4,200 g of sodium fluoride, dissolve in water (3.1.) and complete up to 1 litre.

3.9.2. Fluoride 10-2 M solution (190 µg/ml). Dilute 10 ml from the solution 3.9.1. with water and complete up to 100 ml

3.9.3. Fluoride 10-3 M solution (19 µg/ml). Dilute 10 ml from the solution mentioned in paragraph 3.9.2. with water and complete up to 100 ml.

3.9.4. Fluoride 10-4 M solution (1,9 µg F^-/ml). Dilute 10 ml from the solution in paragraph 3.9.3. with water and complete up to 100 ml.

The solutions prepared as indicated will be kept in polyethylene bottles. Fluoride 0,1 M solution is stable during 2 months. The rest of the standard solutions will be prepared every time that the analysis is performed.

3.10. Ethylendiaminetetraacetic (EDTA) acid

4. APPARATUS AND MATERIAL

4.1. Polyethylene or propylene container of 100 ml capacity.

4.2. Precipitate flask of 1000 ml and borosilicate glassware 3.3. in accordance with the standard ISO 3585 (9.9.).

4.3. Magnetic stirrer with small-size stirring bars.

4.4. Specific electrode of fluoride ion

4.5. Reference electrode of simple connection

4.6. pH meter with scale in milivolts expanded (sensitivity of 0,5 mV).

5. COLLECTION OF SAMPLES

Urine samples are collected in polyethylene or polypropylene containers of at least 100 ml. If it is suspected the presence of substances that may produce interferences, such as metallic ions (Al⁺³), add 0,2 g of EDTA (3.10.) per each 100 ml of urine collected. The samples that are not going to be analysed immediately can be kept refrigerated at 4° C during at least one week (see table 2 Appendix A).

6. ANALYSIS PROCEDURE

6.1. Cleaning of material

6.1.1 All glassware used in the analysis, after being washed with detergent, must be soaked in nitric acid at 50 % (V/V) for a few minutes and afterwards rinsed thoroughly with water (3.1.).

6.1.2. Clean polypropylene and polyethylene bottles and material by soaking in nitric acid diluted at 10 % (V/V) for a few hours and afterwards rinse them with water (3.1.)

NOTE: All material both plastic and glass must be fluoride-free.

 

6.2. Preparation of the sample

6.2.1. Pour 10 ml of urine sample, once it has stabilized at room temperature, into a 100 ml precipitate flask and add 10 ml of tampon to adjust the total ionic strength (3.8.), shake it to homogenize the solution. The sample prepared in this way is ready for the potentiometric determination.

6.3. Preparation of standard and calibration curve

6.3.1. The standard solutions to perform the calibration are prepared by diluting 10 ml of each of the work solutions stated in paragraph 3.9. with 10 ml of the tampon solution in paragraph 3.8. in both polyethylene and polypropylene precipitate flasks, shaking them to homogenize. The standard solutions prepared in this way are ready to carry out the potentiometric determination.

6.3.2. Prepare at least three standards that cover the whole concentration range of the samples to be analysed. It is advisable to work within the lineal range of the measurement instrument. If there are interferences, the standards will be matched in the same way as the samples in order to eliminate them or minimize its effect.

6.3.3. Calibration curve. The readings, in milivolts, obtained in the potentiometric determination (6.3.) of the calibration standards (6.2.1.) are plotted versus its concentrations, expressed in micrograms of fluoride per millilitre of urine (µg/ml), in semi logarithmic paper. The fluoride concentrations will be represented in the logarithmic axis and the readings in milivolts in the lineal scale axis.

6.4. Potentiometric determination

6.4.1. Put the electrodes into the solution and fit the container position and the agitation rate.

6.4.2. When the meter equipment is placed in the milivolt scale and once the reading is stabilized (deviation less than 0,5 mv/min), write it down.

NOTE: For most samples, the reading is stabilized in one or two minutes. The stabilization takes longer when the concentrations are low.

6.4.3. When the operation is finished, wash the electrodes with water and dry softly before proceeding to perform the next determination.

NOTE: The samples and the standards must be at the same temperature to perform the determination.

7. CALCULATION

7.1. Determination of fluorides concentration in the calibration curve

The fluoride concentration in the sample, expressed in micrograms per millilitre of urine (µg/ml), is determined directly by interpolation of the reading obtained in the calibration curve.

NOTE: It is habitual to express the concentration in milligrams of fluoride per litre of urine (mg/l), for which it is not necessary any numeric transformation.

7.2. The results may be referred to the amount of creatinine present in the sample (9.8.) by means of the following expression:

	mg fluorides/l urine
mg fluorides/g creatinine =
	g creatinine/l urine

The determination of creatinine is described in appendix B.

8. PRECISION

8.1. The precision and accuracy of the method and the stability and the conservation of the samples have been determined in accordance with an established validation protocol (9.11.)

8.2. The precision estimated for the method, expressed in terms of variation coefficient, over the whole range tested, is less than 2 % (tables 1 and 2).

8.3. The bias of the method has been evaluated using reference materials (9.12.). The result of such evaluation is listed in table 1.

8.4. The range of application for this method goes from 0,2 µg F^-/ml to 200 µg F^-/ml of urine (10^-5 M to 10^-2 M).

9.APPENDIX B: DETERMINATION OF CREATININE IN URINE

B.1. PRINCIPLE OF THE METHOD

The method is based on the measure of the velocity with which creatinine reacts in an alkaline medium with the picric acid (Jaffe reaction), forming a coloured compound, which is determined spectophotometrically at 520 nm.

B.2. REAGENTS

All reagents must have at least the specification “for analysis” and the water used will be at least of grade 2 of purity, in accordance with ISO 3696 (9.7.).

B.2.1. Sodium hydroxide

PRECAUTION: CORROSIVE SUBSTANCES.

B.2.2. Picric acid

PRECAUTION: TOXIC AND EXPLOSIVE SUBSTANCES

B.2.3. Hydrochloric acid (min 30 %)

PRECAUTION: CORROSIVE SUBSTANCE

B.2.4. Creatinine

B.2.5. NaOH 0,4 N solution

Weigh 16 g of NaOH dissolving and completing to 1 litre with water

B.2.6. Picric acid solution

Dissolve 2,0000 g of picric acid in water, completing to 1 litre

B.2.7. Creatinine standard (1mg/ml)

Weigh 1,0000 g of creatinine. Transfer it to a 1 litre volumetric flask with the help of a small volume of water. Add 8,5 ml of hydrochloric acid and shake the solution completing the volume with water. Stable, at least for a month.

 

 

B.3. APPARATUS AND MATERIAL

B.3.1. Spectophotometer or colorimeter capable of reading at 520 nm

B.3.2. Chronometer

B.4. ANALYSIS PROCEDURE

B.4.1. Dilute the urine with water 1 → 100

B.4.2. Add 0,5 ml from the diluted urine to 1 ml sodium hydroxide solution. Mix and leave to stabilize over 5 minutes at room temperature.

B.4.3. Add 1 ml of picric acid solution. Mix and pour it immediately into the spectophotometer container and after exactly 1 minute measure the absorbance (A₁) at 520 nm. 5 minutes exactly after the first measurement, measure again the absorbance (A₂) at 520 nm.

B.4.4. Proceed similarly with 0,5 ml of the creatinine standard solution (B.2.7.).

The creatinine reaction with the picric acid is very sensitive to temperature so all samples and standards must be at the same temperature. When the temperature is higher than 30 ºC the first absorbance must be read after 30 seconds.

B.5. CALCULATION

The amount of creatinine is calculated according to the following equations:

	A2 - A1
c (mg creatinine/100 ml urine) =		x 100
	Ap2 - Ap1

where:

A₂ y A₁: are the sample absorbances after 5 minutes and 1 minute respectively from the reaction starting (B.4.3.).

Ap₂ y Ap₁: are the absorbances corresponding to the creatinine standard

Creatinine concentration in grams per litre of urine is obtained according to the following expression:

	c (mg creatinine/100 ml)
C (g creatinine/I) =
	100