PHYSICOCHEMICAL MEASUREMENTS STANDARDIZING

THE MEASUREMENTS

IN NONAQUEOUS

MEDIA

M. Y u .

Gorina

OF pH

and D. K. Kollerov

UDC

389.63:541.132.3

Measurements of pH of nonaqueous media are used in chemistry, medicine, and biology for analytical and research purposes, and they also have an important practical application for controlling technoIogical processes in the c h e m i c a l vacuum-robe, pharmaceutical, food, and other industries. The conditions of pH measurements of nonaqueous media are characterized at present by the Use of electrometric method with the application of pH meters comprising measuring (glass) and comparison electrodes, but lacking such important means as buffer solutions and a pH scale. Therefore, measurements by means of aqueous buffer solutions are made without taking into consideration the phase difference of potentials, and thus providing a low measurement precision. The latter is damaging to our national economy. For instance, the existing error of 20.5 pH units in these measurements reduces the production of synthomycin levomycetin by 5~ Below we examine on the basis of the existing data in literature the possibility of standardizing the measuremerits of pH of nonaqueous media. For this purpose there exist two procedures related to two possible methods for establishing a pH scale for nonaqueous media: a scale for each single (given) solvent and a common scale [1, 2]. Scate for a Single pHs Solvent. The standardization of pH measurements of nonaqueous media with the establishment of a single-solvent pHs scale can be accomplished by means of a test scheme similar to the "Test scheme for means of measuring pH" (RS 1923-69) in aqueous solutions. The establishment of the pI4_ssc$1e is based, as in aqueous media [3], on determining the pHvalues of a number of buffer solutions in the given solvent. In this scale the activity and activity factor are referred to an infinitely diluted nonaqueous solution as a standard condition and are denoted by a* and )/* respectively. Therefore, we find that pHs = - logat~+ = pa N. At present two methods are used for determining the pH value of buffer solutions in nonaqueous media by means of the pHs scale [2-5]. In the method developed by the NBS the circuit e m f of a buffer solution in an a l c o h o l - h y d r o g e n mixture with an addition of different quantities of a chloride (or in general halide) of an alkali metal is measured without tramfer by means of a hydrogen-silver chloride element, and extrapolated onto a zero concentration of chloride. In this method all the required quantities are determined experimentally, with the exception of IogyC 1-, which is calculated on the basis of any given condition, for instance, of ~C1- = YHCI" This is the disadvantage of the NBS method, since the conditionality in evaluating l o g i c 1- contributes a certain uncertainty in obtaining paI~. The method of Romanian investigators [5] is f0rmally free of this defect of the NBS method. According to this method l o g i c l- is determined experimentally in the circuit without transfer by means of a standardized potassium or sodium amalgamated electrode in the presence of a buffer solution containing a K+ (or Na+) ion, for instance, of potassium biph~halate or borax and salt. The disadvantages of this method consist of a greater volume of measurements as compared with the NBS method, operations with mercury, and the impossibility of measuring the e m f with an amalgamated electrode at the solution's ionic strength -< 0.05 m, thus making it impossible to extrapolate in calculating the standard potential of the amalgamated electrode.

Translated from Lzmeritel'naya Tekhnika, No. 1, pp. 82-84, January, 1974. 9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or trc~smitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

135

In both methods the known quantity for determining the value of pat~ in a l c o h o l - a q u e o u s solutions consists of the standard potential of a silver chloride electrode in these solutions. Data on the value of E~ in m e t h a n o l aqueous and e t h a n o l - a q u e o u s media are provided in [3, 4, 6]. Discrepancies between the data of various authors amount to 0.3 mV. The most important quantities for standardizing the pHs scale consist of pat~ values of buffer solutions in the given solvent [2, 3, 7, 8]. It c a n be considered that almost the entire pHs scale of methanol (logKp = 16.7) is covered by buffer solutions. Considerably fewer buffer solutions are known for ethanol. Buffer substances consist mainly of mixtures of organic acids with salts. According to [4, 9] the error in the values of pat~ of buffer solutions in a l c o h o l - a q u e o u s m e d i a . o b t a i n e d in circ u r s without transfer amounts to about ~ 0.005 pa H units and in absolute alcohol to ,0.01 pa H units. For a l c o h o l aqueous media with a high content of alcohol a correction is applied for the degree of chloride (halide) dissociation. The c a l c u l a t i o n of dissociation constants for 70-100~ alcohol is provided in [4, 8]. The method for measuring pa H in circuits without transfer is the most precise, but also most labor consuming and, therefore, it can be used only for establishing and reproducing a nominal scale of the hydrogen ion activity in nonaqueous m e d i a . The buffer solutions whose pa H values are obtained by means of this method can be used as reference buffer solutions. For p r a c t i c a l e v a l u a t i o m of pits measurements in circuits with transfer are used:

Pt, Hz

Standard buffer soln. or X soln. in nonaqueous solvent

KC1 Nonsaturamd aqueous solution

HgzC12, Hg

(1)

The value o f pH x measured with instruments can then be determined from the equation ,

9

Ex --

pH x -----p H b +

Eb RT

2.3-

F

'

(2)

-

where pI~x is the pH s value of the tested solution in a nonaqueous solvent, pH b is the pH s value of the standard buffer solution in a nonaqueous (the same) solvent, Ex and Eb are the values o f the circuit emfs,(1) in measuring with the tested and standard buffer solutions respectively. According to the data in [2, 4] the pH b values approach pa H of buffer solutions in alcohol aqueous media under favorable measurement conditions and the pH b values approach -log a H of aqueous solutions. Favorable conditions here are the same as for aqueous solutions, namely, the ionic force does not e x c e e d 0.2 m, the m e d i u m is not too acid or alkaUne, and the buffer substance is a reference one. The method of measuring pH* in circuits with transfer and the application of buffer o l u t i o n s in the same solvent c a n be used for transferring pa H units from circuits without transfer to pH H units in circuits with transfer. Buffer solutions in which pH~ = pa H c a n serve as reference means of measurement. The measurement error of pH b then is ~ 0.05 pH* units. A number of investigators [7, 10-12] have shown that the measurement of pH* of nonaqueous solutions is also possible by using aqueous buffer solutions. It was found that the p o t e n t i a l of liquid compounds (diffusion and phase) at the boundary of nonaqueous and aqueous solutions is independent of the nature and concentration of the dissolved substance, and depends, apparently, only on the composition of the nonaqueous solvent. The value of pH of a nonaqueous solution measured in a circuit with transfer when the instrument is adjusted by means of aqueous buffer solutions is r e l a t e d to the value of pH*, obtained by means of nonaqueous buffer solution adjustment, by the expression p H - - p H * --= 6 = E~ -- lgu ~ . ~ corgi 9

(3)

HT

where Ed is the diffusion p o t e n t i a l in pH units; 7~+ is the c o m m o n zero ion activity factor of hydrogen, or the effect of the m e d i u m . The value of log 7~+ characterizes the value of the phase potential in pH units, since 5 is a constant quantity for the given composition of the solvent, it is possible to obtain the value of pH* by measuring pH and adding to it the c o r r e c t i o n 6 .

136

The measurement of pH of nonaqueous media when the pH memrs are adjusmd with aqueous (standard) buffer solutions is very promising, since the measurements are then made in the simplest possible manner without requiring any new organizational or mchnical measures to be adapted in order to ensure uniform measurements. The problem consists of investigating 6 and finding the possibility of its standardization. Since 6 comprises two quantities: the diffusion potential and the effect log 7~t+ of the medium, it was of interest to evaluate the relation to 6. Analysis of the data on 6 and log 7~+ obtained independently from each other and of the approximate values o f E d calculated from the above values, shows that small values of 6 are obtained as the difference of large values of Ed and log y~t+. Therefore, the errors of the latter two quantities can have a substantial effect on the error of the former and accordingly on its standardization. Corrections of 6 for methanol-aqueous and e t h a n o l - a q u e o u s media obtained experimentally are provided in [2, 7, 10]. These data indicate that the values of 5 are large for compositions containing less than 90% of alcohol. The data of different authors have discrepancies for m e t h a n o l - a q u e o m media and methanol no t exceeding 0.01 pH units, for e t h a n o l - a q u e o u s media up to 0.12 pH units, and for ethanol 0.5 pH units. The presence of a considerable potential in liquid connections leads to the necessity of its investigation. The effect of various types of Iiquid connections (capillaries, porous glass filters, glass microsections, and asbestos threads) on the reproduction of 6 was investigated in ['11]. It was found that, within the limits of the experimental error in measuring with a glass electrode, any differences in the types of connection for methanol-aqueous and ethanol-aqueous media can be neglected, with the exception of connections by means of asbestos threads. The values of pH for a number of buffer solutions in mixtures of water with alcohol (methanol, ethanol, propanol, and isopropanol) have been determined by adjusting the pH meters by means of aqueous buffer solutions [7, 13, 14]. However, in using a glass electrode which is normally utilized for current measurements of pH in nonaqueous solutions, the potential of asymmetry can change in transferring the electrode from an aqueous media to a nonaqueous one, or vice versa [2, 15]. Therefore, the means for adjusting and checking pH meters can consist of two types of buffer solutions: those in the given solvent and aqueous ones. The former can be recommended for more precise current measurements, and the latter for less precise measurements. The utilization of aqueous buffer solutions in cases when high measurement precision is not required is economically advantageous, since it does not require industrial production and certification by metrological institutions of new buffer substances, and it serves to unify the working means for measuring pH of both aqueous and nonaqueous solutions. It was found in a number of investigations that silver chloride and c a l o m e l electrodes can be used as comparison electrodes in measuring pH of such nonaqueous media as alcohol, as well as of alcohol-aqueous mixtures, acet o n e - a q u e o u s mixtures, etc. [1, 2]. Data also exist on the application of glass electrodes in these and other media. Thus, for the solution of the problem of standardizing the measurements of pH in nonaqueous and mixed media in terms of the pHs scale, it is necessary to develop buffer solutions for these media and determine their pa H values for circuits without transfer, to work out a method for transferring pa H units from circuits without transfer to pH* units in circuits with transfer, to evaluate the corrections for 6 in transferring pH * units o f buffer solutions in nonaqueous and mixed solvents to pH units of aqueous buffer solutions, and to investigate the stability of the potentials of comparison and measuring electrodes. C o m m o n pA scale of the acidity of nonaqueous media is considerably more convenient than the single-solvent pHs scale, since it can be used for comparing the acidity of solutions in various solvents, whereas the pHs scale permits the comparison of acidities only in a given solvent. The establishment of a c o m m o n scale would make it possible t o link all the pHs scales and standardize pH measurements in all nonaqueous media by means of c o m m o n aqueous buffer solutions. In this connection several practical methods [16-18] were suggested for measuring acidity in various solvents; however, they have substantial deficiencies which are examined in [1, 2, 8, 19]. N. A. Izmailov made a substantial contribution to the development of methods for establishing a c o m m o n acidity scale for nonaqueous media. He suggested that the c o m m o n measure of acidity should consist of the absolute activity of a solvatized proton a H + (M) referred to a diluted aqueous solution as a c o m m o n standard condition. The effect of the medium then becomes subject to measurement: pA = - - lg an+ (M) = - - lg an+ -- lg y~

~- pHp - - lg y~+.

137

TABLE 1 Metha no I

Authors Pleskov Griinwald Gammet Izmailov Popovich Ligny and Alfenaar

5| etha no 1

9~o ethanol

Ethanol

0.35 0.35 3.2 2.09

0.25

2.2

4.7

O.40

1.6

4.2

1.45 ~0.27

It was suggested in [2] that for determining log Y~t+ the mean activity factors of HC1 ions should first be used and then the sum and difference of the chemical energies of ion solvation should be calculated and the values thus obtained extrapolated. However, this method provides a certain ambiguity in the results Cup to * 0.5 log 7~t+ units) [19]. The method suggested in [29] for determining log 7~t+ is based on dividing the energy of ion solvation equally between cations and anions. The method for determining log Y~I+ developed in [20] is based on dividing the chemical potential of ions into an "electrical" and a `'neutral" part and equating the latter to the chemical potential of a noncharged particle of the same size as the ion. The latter two methods use ions of sufficiently large sizes, thus raising considerably the precision in determining log 7~+, although these methods are not strictly thermodynamic. Table 1 provides the values of log),~+ obtained by various authors for methyl and ethyl alcohols. The arithmetic mean of these values for methanol amounts to 1.49 which is the nearest approach to the value of log7~+ obtained by Ligny and Alfenaar (1.45 *0.27). These authors suggested a `'universal`" pH scale for methanol and methanol- aqueous mixtures [21]. This scale can be implemented by using PHst of a nonaqueons buffer solution and log 7~+ pH(PA)st = PHst-log 7~+ as well as by using PHst of an aqueous buffer solution and the potent i m difference between the liquid connections from the aqueous solution of KCI (salt bridge) to the aqueous buffer solution on the one hand and to the nonaqueous buffer solution on the other, ~d(H20) :--Ed(S): pH (pA)st = pHst q-

Ed(H~O ) -- E d (S) 0.05916

It is noted in [19] that the authors of [21] have committed an error and, therefore, the values of the corrections Ed(HzO)-gd(S) are greatly overestimated. Moreover, the authors consider the value of Ed(HzO) as independent of the electrolyte composition, and this should be confirmed. A method is suggested in [22] for determining log 7[-I+ by assuming a "constant surface potential`" in aqueous and alcohol electrolyticsolutionswhich have on their surface heptyl alcohol. The method is based in practice on several other assumptions that the dipole moments of aliphattc alcohols are equal and that the heptyl alcohol does not affect the liquid connection's potential. Therefore, in view of the above and the evaluation by the authors of the error of this method, it appears to be insufficiently precise and the values obtained by its means differ substantially from the mean arithmetic value and the data of Ligny and Afenaar. It will be seen from the above considerations that a further refinement of the evaluation of log 7~i+ in various media should be made. In order to carry this out it is necessary to develop a number of experimental and computation methods for determining log 7~+ in nonaqueous media, and to refine them by comparing the results and agreeing them, it would appear, on the level of international consultations, as well as developing a method for determining pH of nonaqueous media in terms of a pA scale by means of aqueous buffer solutions. LITERATURE 1,

2.

138

CITED

N. A. Izmailov, Electrochemistry of Solutions [in Russian], Khimiya, Moscow (1966). R. G. Bates, Evaluation of pH Theory and Practice [Russiantranslation],Khimiya, Moscow (1971).

3.

4. 5. 6. 7. 8. 9. I0. II. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

V.V. A leksandrov, D. I<. Kollerov, and I. L. Skorik, Transactions of the Instituteof the Committee of Standards Measures and Measuring; Instruments,No. 68 (128), Standartgiz, Moscow-Leningrad (1963). M. Yu. Gorina and L. N. Seregina, ~lektrokhimiya, 8, No. 6 (1972). C.L. De Ligny, P. F. M. Luykx, M. Rehbach, and A. A. Wieneke, Rec. Tray. Chim., 799(1960). G. Popa, C. Luca, and O. Enea, Z. Physik. Chem., 230 (1965). M. Paabo, R. G. Bates, and R. A. Robinson, Anal. Chem., 37, No. 4 (1965). R.G. Bates, M. Paabo, and R. A. Robinson, J. Physik. Chem., 67, No. 9 (1968). W.J. Gelsema, C. L. De Ligny, and G. F. Visserman, Rec. Tray. Chim., 8_~4(1965). M. paabo, R. A. Robinson, and R. G. Bates, J. Am. Chem. See., 87, No. 3 (1965). C.L. De Ligny and M. Rehbach, Rec. Tray. Chim., 7__99(1960). W.J. Gelsema, C. L~ De Ligny, A. G. Remijnse, and H. A. Blijleven,Rec. Tray. chim., 85, No. 7 (1966). R.G. Bates and R. A. Robinson, Chemical Physics of Jonik Solutions, Wiley and Sons, New York (1966). H. Berge and P. Jeroschewski, Z. Anal. Chemie, 203, No. 2 (1964). O. Fillaux, R. Gaboriand, and R. Schaal, Comptes Rendus Acad. Sci. Set. C., 263, No. 17, Paris (1966). M. Paabo and R. G. Bates, Technical Note, National Bureau of Standards, 271 (1965). V.A. Pleskov, Usp. Khim., I__66(1947). M.S. Zakhar'evskii, Measurement of Oxidized Media [in Russian], Khimiya, Moscow (1967). B. Gutbezahl and E. Grflnwald, J. Am. Chem. Soc., 7-5 (1953). O. Popovich, Anal. Chem., 38, No. 4 (1966). M. Alfenaar and C. L. De Ligny, Rec. Tray. Chim., 8__66,No. I0 (1967). M. A lfenaar and C. L. De Ligny, Rec. Tray. Chim., 86, No. Ii (1967). Yu. F. Rybkin and T. N. Seredenko, ~ektrokhimiyaj, No. i (1972).

139