A. K. GRZYBOWSKI
550
the high concentration range. This is in qualitative agreement with the theory since this corresponds to high surface coverage, and consequently there is much interference between polymer segments. The experimental isotherms appear to be quite flat, though in some cases the amount of polymer adsorbed continues to increase as the solution concentration goes up. Further adsorption experiments a t much higher solution concentrations would be required to determine whether the asymptotic limit of adsorption is approached slowly. 2. The theory predicts that adsorption from a poor solvent is favored over adsorption from a good solvent which is in agreement with the fact that adsorption of polyvinyl acetate from carbon tetrachloride solution is much greater than adsorption of the polymer from solut,ions in better solvents. 3. The theory predicts enhanced adsorption
Vol. 62
with increasing molecular weight, but that chain interference near the surface would tend to decrease the molecular weight dependence perhaps in an extreme case causing the over-all adsorption to decrease with increasing molecular weight. The results of the experiments indicate that polymer adsorption increases with increasing molecular weight, but that this effect is most pronounced in a poor solvent where chain interference near the surface is greatest. This latter is in contradiction with the SFE theory, but probably could be brought into agreement if the known modifications of the random coil statistics of the polymer chain in a good solvent were specifically introduced into the theoretical calculation. 4. The theory predicts that adsorption may either iiicrease or decrease upon elevating $he temperature. The experiments show an increase in adsorption at higher temperatures.
THE STANDARD POTENTIAL OF THE CALOMEL ELECTRODE AND ITS APPLICATION I N ACCURATE PHYSICOCHEMICAL MEASUREMENTS I. THE STANDARD POTENTIAL BY A. K. GRZYBOWSKI Department of Biochemistry, University College, London, England Received August $8,I967
Details are given of the preparation of a reproducible calomel electrode suitable for use in cells without liquid junction. The standard potential of the electrode has been determined a t 5" intervals from 0 to 60". The cubic equation relating the EO to absolute temperature is Eo = 0.266469 (3.46466 X 10-4)(t - 30) - (2.86482 X 10-6)(t - 30)2 (8.5384 X 10-9) (t - 30)a. The thermodynamic quantities associated with t'he cell reaction and the activity coefficients of some HCI solutions (0.001, 0.002, 0.005, 0.01 and 0.02 mole of HC1 per 1000 g. of water) have been calculated at 25" and compared with the values obtained in other investigations.
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I. Introduction
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with two modifications, namely, an additional set of saturators for the calomel electrode compartment and a tap be-
The very accurate value of the standard po- tween the two compartments. The whole cell could thus tential (*lo p . ) obtained by Hills and Ives1V2at be kept thoroughly deoxygenated by bubbling .with hydro25" for an improved calomel electrode suggested the gen (British Oxygen Co.) freed from oxygen by "deoxo" catalyst purifiers. This is particularly important possibility that it might prove superior to the platinum as the calomel electrode is known to be very sensitive to Ag ;AgC1 electrode in accurate physicochemical dissolved ~ x y g e n . ~The ! ~ tap was used to connect the two measurements involving cells without liquid electrodes only when measurements were made in order to junction. This investigation was undertaken to minimize any reduct,ion of mercurous ions a t the hydrogen although no considerable alteration of the e.m.f. study the behavior of the calomel electrode and to electrode, was in fact observed (HCI solutions) when the two compartmeasure its standard potential over a wide tempera- ments were allowed to remain in contact for a period of time. ture range in the hope of applying it in accurate The cells and electrode vessels were rendered hydrophobic determinations of acid dissociation and metal by treatment with a 1% CC14 solution of silicone fluid Silicones Ltd.) MS 200/1000 C.S. and baking a t complex stability constants, together with the (Midland 180" for several hours, the excess silicone being subsequently associated thermodynamic quantities, of com- removed by washing with pure CCla. This was done in the pounds important in biological systems. hope of reducing ion exchange between the solution and the The method of preparation of the electrode it- glass and to prevent the formation of a film of solution between the mercury and calomel of the electrode and electrode self is based on that of Hills and Ives;2 the cells vessel. a and electrode vessels, however, have been rede(i) The Calomel Electrode.-A.R. grade mercury was signed to yield reasonably rapid and accurate purified by distillation under a reduced pressure of oxygen results with not unduly large amounts of test and washing with dil. .HNO3-Hg2(N03)2 mixture. Just before use final purification was carried out by distillation solution. in vacuo in an all-glass still; oxygen was excluded by displacing the air in the still before evacuation with nitrogen or 11. Experimental Procedures The cells were essentially similar to those described by Ashby, Crook and Dattaa for use with Ag;AgCl electrodes ( 1 ) G . J. Hills and D. J. G . Ives, Nature, 166,530 (1950). (2) G . J Hills and D. J. G. Ivsa. J . Chem. SOC.,305 (1951). ( 3 ) J. H.Ashby, E. M. Crook and 6 . P. Datta, Biochem. J . (Lond o n ) , 66, 190, I98 (1954).
hydrogen. The calomel was prepared by the addition, with continuous stirring, of an excesa of bromine free 2 N HCl to a solution of purified Hg2(N03)z(about 20 g. in approximately (4) R. H. Oerke, J . Am. Chsm. Soc., 44, 1684 (1922). (5) M . Randall and L. E. Young, ibid., 60, e89 (1928).
STANDARD POTENTIAL OF THE CALOMEL ELECTRODES
May, 1958
500 ml. of 0.1 N HNOa). After 12 hours the supernatant was replaced by fresh 2 N HCl and the stirring continued for a further 12 hours. The precipitate was washed f i s t with HCl'and then with conductivity water till no chloride could be detected in the washing. The resulting dark grey intimate mixture of calomel and mercury was subse uently thoroughly dried in uucuo over PzO5. No basic &loride formed even after several months storage (viz., Hills and Ives2) presumably because of the complete exclusion of water from the calomel. A small amount of the dried calomel was allowed to form a thick skin over a little pure mercury, part of which was then transferred onto the surface of the mercury in the electrode vessel where it spread out in a thin uniform grey film. The electrode vessels were (Fig. 1) attached to tubes mounted in a ground glass cone, each unit comprising two electrodes. Before filling with solution, the cell with the calomel electrodes in position was swept out with hydrogen which was then used to force in the presaturated solution. The hydrogen electrodes were inserted, both electrode compartments bubbled with hydrogen for about 15 minutes and the cell, tightly stoppered, was left standing overnight before e.m.f. measurements were made. (ii) The Hydrogen Electrode.-The design and method of preparation of the electrodes was the same as that described earlier by Ashby, Crook and Datta3 but with some modification in the formation of platinum black. To ensure even deposition, each platinum plate of the electrode was used alternatively as cathode or anode, the current (0.3 amp./cm.Z) being reversed a t 10 see. intervals for 3-5 minutes. After transfer into the cells the electrodes were bubbled with hydrogen a t the rate of about 100 bubbles per minute. Polythene tubing was used for most of the connections. (iii) Solutions.-Several batches of constant boiling HCle were mixed, freed from HBr by treatment with chlorine, redistilled and finally analyzed by two different methods. (a) Three analyses by differential potentiometric titration7 and (b) four analyses by the gravimetric estimation of precipitated AgCl, the amount of AgCl remaining in the mother liquor and washings being estimated nephelometrically .* The over-all agreement among the results was about one part in 2,500. The HCl was used to prepare the test solutions by weight dilution with conductivity water. (iv) Measurements.-The cells were kept in a paraffinbath provided with three heaters and a refrigerating system; the temperature being maintained constant within ~I~0.01 during measurements by an electronic controller connected to a nickel resistance thermometer. The temperature could be set within f0.1' of the required value using a calibrated mercury in glass thermometer, the exact value being determined with a platinum resistance thermometer constructed in this Laboratory to the design of Barber.9 The resistance a t the triple point of water was checked periodically in a triple point ceWo with the ratio Rjoo/Ro assumed to remain constant. All e.m.f.'s were measured in absolute volts with a Tinsley vernier potentiometer, type 4500 A, in the range 1.8 to 0.00001 volts. The platinum resistance thermometer, the standard resistances and the standard cells have been calibrated by the National Physical Laboratory, London.
111. Theory The cell used in the determination of the standard potential of the calomel electrode was of the type P t ; Hz, HCl(m), Hg2C12; Hg RT In mH+mCI- YE+YcIE = EO - -
F
(1)
(6) A. I. Vogel, "Quantitative Inorganic Analysis," Longmans Green and Co., London, 1951,p. 228. (7) D. A. MacInnes and M. Dole, J . Am. Chem. Soc., 51, 1119 (1929).
(8) T.W. Richards and R. C. Wells, ibid., 27, 459 (1905). (9) C. R. Barber, J . Sei. Instr., 27, 47 (1950). (10) C. R. Barber, R. Handley and E. F. G. Herington, E d . J . A p p l . P h y s . , 6, 41 (1954).
551
A. -
B. Fig. 1.-Pt;
H2-Hg; Hg2C12 cell: A, extended diagram; B, plan.
The extrapolation function used herell can be obtained from (1) by combination with the DebyeHiickel equation12 for the activity coefficient for a uni-univalent ion, 2nd subsequent rearrangement, Le. log
yi =
-
AP/a
1
+ Bu*I'h
I = ionic strength = m(mola1ity of HC1) RT E 2 In 10 - (log m - Am1/?) F RT = EO + Bu*m'/z EO - E - 2 In 10 - log m) F
+
(
(3)
by plotting the L.H.S. of (3) (y') against m'/2. (EO - E - 2 In 10 RT/F log m) (x'), a linear plot is obtained with the intercept a t x' = 0, equal to Eo and the slope equal to Ba*. Since Eo occurs in 2' the correct value can only be obtained by successive approximation. Alternatively equation (1) can be rearranged to (4) E
+ 2 In 10 RT - log 7n - 2 In 10 F EO + m'/z __
= B~*EO
-~
u (*E
+ 2 In i o RT -F- log m )
(4)
the L.H.S. can be fitted by least squares in terms of the variables l/m'/B and ( E 2 In 10 RT/F log m) and the three constants Ba*Eo, Eo and Ba", yielding both the Eoand a* directly.
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IV. Results and Calculations The concent,ration range of HC1 solutions used was 0.005 to 0.02 mole per 1000 g. of water. Only a fourfold concentration range was employed as the MacInnes and Brown equation does not yield linear plots above va.lues of I much greater than 0.02. Further, since the principal object of this work was the determination of the standard potential, the lack of data at higher concentrations (11) D.A. MacInnes and A . S. Brown, J . Am. Chem. Soc., 57,1356 (1055). (12) P. Debye and E. Hackel, Physik Z.,24, 185 (1923).
A. I
