T H E D E T E R M I S A T I O S OF T H E HYDROGES IO?: CONCESTRATION I N GOLD SOLS BY T. R. BOLAM AND J. CROWE
Introduction Despite the large amount of research which has been carried out on gold sols, considerable uncertainty exists regarding the use of the hydrogen electrode for the determination of the hydrogen ion concentration in these systems, Herstad,’ who appears to have been the first to apply the method, states that by careful working reliable results could be obtained. Actually his pH values are uncertain by a t least i 0 . 5 pH. The next reference to the subject is to be found in a paper by Kautzky and Pauli,* who reported that it was impossible t o obtain a steady potential with red gold sols and that the latter were coagulated by the hydrogen. These conclusions were based on an investigation by Adolf and Pauli,s an account of which appeared in the following year. Dialysed sols, prepared by Zsigmondy’s method, were the object of study and the hydrogen electrode was found to be extremely sensitive to either dissolved or colloidal gold. To quote from the paper itself, “Diese Veranderung der Elektrode tritt schon bei so geringen Goldspuren ein, dass sie unter Umstanden jedem feinen analytischen Goldnachweis gleichkommen oder sogar iiberlegen sein kann”. KOdifficulty was encountered in measuring the hydrogen ion concentration in ordinary solutions of potassium or barium chloride, or in the solution (“Flockungsfiltrat”) obtained by coagulating a sol with a sufficiently large amount of these salts and removing the gold by filtration. In the latter case the hydrogen ion concentration was found to be about I X IO-5 pi. The experience of Tartar and Lorah$ was the same as that of Adolf and Pauli. These workers were investigating the influence of pH (over the range 2-9) on the protective action of gelatine on Zsigmondy sols and found that the hydrogen electrode gave erratic results unless the gold was carefully eliminated. The gelatine itself did not cause variations greater than 0.2 millivolt. WO. Ostwald5 also reports that he encountered the same difficulties as Pauli and hdolf. Beaver and Muller in a paper6 describing a very careful study of the action of ultraviolet light on various gold sols make the following statement. “Electrometric titrations of the chloroauric acid with 0.1 S potassium carbonate were therefore made at room temperature ( 2 2 - 2 5’). X curve was then plotted of C.C.of potassium carbonate against pH of the solution for the given gold content, and from this curve solutions of definite pH were made, heated to 6 j oand reduced.” There is no mention of any difficulty with the hydrogen electrode due to the presence of gold in the solution. I n one series of experiments the pH varied from 3-10 approximately. The latest work involving the use of the electrode in gold sols is that of Keinders and B e n d i e ~ In ~ this
HYDROGEN ION CONCENTRATION I N GOLD SOLS
603
case the sols were prepared by Bredig’s method, and gelatine and casein were added to them. Here again there is no suggestion that the E M F . measurements were troublesome. The following is an account of an investigation undertaken to ascertain whether it is possible to define conditions which permit of the accurate determination of the hydrogen ion concentration in gold sols by means of the hydrogen electrode.
Apparatus Some difficulty was experienced in finding a suitable form of hydrogen cell. The apparatus shown in Fig. I was finally devised and found to be satis-
FIG.I
factory. A is the hydrogen electrode, B is a saturated potassium chloride calomel half-cell, and C a bridge of saturated KCl. By opening the tap D the pressure is released when the rubber stoppers E and E’ are inserted in the Il-tube. Taps F and G are opened only when a reading is actually being made, D being then closed. h spiral of platinised platinum or gold wire was found to give better results than a sheet of platinised platinum as the electrode itself. The hydrogen is introduced at the lower end of the spiral by means of a narrow turned-up glass tube H. Aistream of fine bubbles is thus formrd which fills the inside of the spiral and passes out between the coils. In this way rapid equilibrium is established between the electrode and the so1 or solution to be investigated. I is a glass cap, and J a rubber ring holding the electrode in position. The advantages of this form of apparatus are:
604
T. R . BOLAM AND J. CROWE
Only 3 or 4 C.C.of sol are necessary for a determination. The column of poorly conducting liquid between the electrode and the saturated KC1 is very short. A high resistance is thus avoided and the null point can be accurately determined. (3) The hydrogen electrode attains equilibrium rapidly. (4) The simple form of the apparatus permits of quick and thorough cleansing. ( 5 ) The cell can be immersed in a thermostat. The hydrogen was generated from zinc and hydrochloric acid and purified by passing through ( I ) conc. KOH, ( 2 ) alkaline KMn04, (3) neutral KMn04, (4) distilled water, ( 5 ) sol (or solution). The E M F . values were obtained by the usual compensation method, the instruments employed being such that individual determinations were accurate to 0.I millivolt. A11 experiments were carried out a t 20’ C, the cell being immersed in a glass-walled thermostat up to the level indicated in Fig. I . Two saturated calomel electrodes (S.E.) were used as working electrodes and checked by comparison with two N, I O calomel electrodes (D.E.). All four were prepared from carefully purified mercury, electrolytic calomel, and Kahlbaum KCl. The E.M.F. of the combination, Hg, Hg2C12, satd. KC1, N / r o KCl, Hg2C12, Hg was found to be 0.0886 volt, with both sets of electrodes, so that taking 0,3379 volt as the standard electrode potential of the D.E.8a t zo°C., that of the S.E. was equal to 0.2493 volt. Clarks gives 0.2492 volt. A test was made of the hydrogen electrode with three of Sorensen’s standard phosphate mixtures (pH = 5.91, 6.47, 6.81 respectively).r The electrode rapidly attained equilibrium and gave very steady and reproducible potentials, the derived pH values differing from Sorensen’s by not more than 0.01. (I) (2)
Zsigmondy and Nordenson Sols Repeated trials showed that it was impossible to obtain a constant E.M.F. with a sol* prepared by Zsigmondy’s method (reduction by formaldehyde of gold chloride made alkaline with potassium carbonate). There was a continuous increase in the E.R.I.F.,which amounted to about 3 0 millivolts in the first two hours. I n no case however could any sign of coagulation of the sol during an experiment, or even hours after, be detected. A pronounced drift in the same direction also took place with the liquid obtained by precipitating the gold with potassium chloride and removing the precipitate by filtration. I n contrast with the Zsigmondy sols, those prepared by Nordenson’s method gave very satisfactory results. The principle of the method is the reduction of gold chloride by hydrogen peroxide under the influence of ultraviolet radiation. I n the present instance a small quantity of potassium carbonate was added, which, while insufficient to give a neutral or alkaline sol, *AU the sols used in this investigation, whatever the method of preparation, were bright red in colour, showed no sign of opalescence, and proved to be very stable. Further details of preparation, etc., will appear in a subsequent communication.
HYDROGEN ION CONCENTRATION I N GOLD SOLS
605
was found t o increase the degree of dispersion. Table I contains the data for the sol prepared from 2.0 C.C. of 0.6% HAuC14, 4H20 (Merck) in 125.0 C.C. conductivity water 0.6 C.C.of 0.2 N K2C03 (Kahlbaum) 0.4 C.C. Perhydro1 (Merck), which is quite representative. h separately prepared sample of the sol was used for each experiment.
+
+
TABLE I Nordenson Sol Preparation
Time
E.M.F. (volts)
(mine.) I9
I.
0.4772
52
74 I22
I1
45
0,4777 0,4777
65
0.4779
90
0.4766
3.91
30
0.4779
3.93
I1
I1
IJ
71
25
3.
3.92 3.92 3.96 3.97
0.4774 0,4793 0,4799 0.4802
32
2.
PH
3.93 11
I1
52 171
I t was thought that if the Zsigmondy sols were buffered by the addition of small quantities of suitable electrolytes it might be possible to get steady E.M.F. values. Accordingly 20 C.C. Zsigmondy sol (2.0 C.C. 0.67, gold chlor3.0 C.C. 0.2 N K X 0 3 3.0 C . C . 0 . 4 7 formaldehyde ~ ide in 1 2 0 C.C.water (specially prepared) were mixed with 1.0C.C. M I I S Na2HP04(A.R.) 1.0 C.C. M / I KH2POa ~ (A.R.), and three portions of the mixture tried. The results are given in Table 11.
+
+
+
TABLE I1 Zsigmondy Sol Expt. I
7.10
Expt. I1 E.M.F. pH (volts) 0.6637 7.12
0.6642
7.11
0.6648
0.6652
7.12
0.6650
If
7,
,,
,,
Time (mins.)
E.M.F.
pH
IO
0.6620 0.6630 0.6633
20
30 60 84
(volts)
,, ,,
+ Phosphate Buffer
7.14 7.15
,, ,,
Expt. I11
E.M.F
pH
(volts)
7.13 7.15
Jf
,,
J,
,f
,,
,f
606
T. R. BOLAM AND J. CROWE
I t would appear that the change in E M F . observed with the original Zsigmondy sol was not due to any deleterious action of the gold upon the electrode but to the absence of equilibrium between the solution and the hydrogen, i.e. to change in the hydrogen ion concentration. There is every reason to believe that the relatively high concentration of potassium carbonate used in the preparation of these sols is the source of disturbance. I n dilute unbuffered alkaline solutions (the above Zsigmondy sol had pH = 9 approx.) the participation of any C 0 2 present in the acid-base equilibrium is of great importance.8 We may therefore attribute the E.M.F. drift in Zsigmondy sols to the removal of COa by the hydrogen stream.
Experiments with Y!itrate” Gold Sols If the above explanation is correct no difficulty should be experienced in obtaining a steady and reproducible potential in sols prepared by replacing potassium carbonate in the Zsigmondy procedure by tri-sodium citrate. Table I11 gives the results in the case of a “citrate” sol containing 0.31 millimols per litre of sodium citrate. Two quite separate preparations were examined.
TABLE I11 “Citrate” Sol Preparation I.
Sample (a)
Time
E.M.F.
(mins.)
(volts)
‘5
5.11 J 2
30
,,
42 I2
PH
0.5469
5.12
J )
I,
2.5
40 2.
9)
I2
5 . IO
23
5.11
33
,,
Variation of E.M.F. with Composition of Sol Three series of sols were prepared by reduction with formaldehyde in the absence of potassium carbonate, the first containing increasing amounts of sodium citrate, the second, of disodium hydrogen phosphate, and the third, of sodium hydroxide (from the metal). I n every case the reduction mixture contained 1.0C.C.0 . 6 7 ~gold chloride (diluted with sufficient water to give final volume of 1 2 8 c.c.) 0.4C.C.R I sodium citrate IO, (C6H507?;a2. j+ H 2 0
+
HYDROGEN ION CONCENTRATION IN GOLD SOLS
TABLEIV “Citrate” Sols Final concn. of Sodium Citrate (rnillimols/litre)
Concn. of Na (rn!lligrm.equivs./litre)
E.M.F. (volts)
PH
0.5461 0.5553
5.11
I .os
I .;I
0,5801
0.64
I .92
0.71
2.13
0.78
2,34 2 ,S5 3.18 4.26
0.5857 0.5923 0.5982 0 5989 0.6063 0.6165 0,6236 0.6274
5.69 5.79 5.90 5.95 6 .02 6.14 6.32 6.44 6,51
0,31 0.35 0.57
0.93
0.85 I
.06
I .42 I .80
5 .40
2.13
6.39
’
5.27
TABLE V “Phosphate” Sols Final concn. of Xa,HPO! (millimols/litre)
*Concn. of Na (mil1igrm.equivs./litre)
E.M.F. (volts)
0.08
I
0.16 0.31 0.78
I.25
0.5722
1.55
0.5927 o ,6295 0.6497 0.6623 o ,6722 0,6767 0.6806
.09
2.49 3.27
I .I 7
1.56 2.34
4.05
7.28 7.36
2.73 3.52
7.42
0.5578
PH
5.31 5.56 5.91 6.54 6.89 6.11 7.28
7.36 7.42
TABLE VI “Hydroxide” Sols Final concn. of NaOH (rnillimols/litre)
‘Concn. of Na (mil1igrm.equivs./litre)
o ,0780
1.008
0.156
I
0.31
I.24
0.47 0.62 I .56
I,55
2.49
3.12
4.05
x
,086
I .40
E.M.F. (volt) 0.5611
0.5787 0.5961 0.6194 0,6428 t(o.777 *I t(O.895*I
PH 5.31
5.67 5.97 6.37 6.78 (9’ 1 *) (I122
*)
M sodium citrate present in every case. f I n the case of these preparations there was a drift in the E.M.F. The initial values are, however included to indicate the order of magnitude of the pH. ‘0.31
10-3
608
T. R . BOLAM kVD J. CROWE
by Kahlbaum). To this was added the appropriate quantity of citrate, phosphate, or hydroxide and then, after heating, 3.0 C.C. o.4y0 formaldehyde. E.M.F. determinations were made on two separate portions of each sol and the mean values are given in Tables IV, V, and VI. With the exception of the last two preparations in Table VI, the E. M.F.’s were very constant and reproducible. Duplicates agreed generally to 0.I millivolt, and were never farther apart than 0 . j millivolt. If the curves showing the variation of the p H with concentration of sodium are drawn (Fig. 2 . ) it will be seen that they occupy positions to be expected from a consideration of the relative degrees of alkalinity of sodium
FIG.2
hydroxide, phosphate and citrate, This affords strong evidence that the potential of the hydrogen electrode in these sols depends in the normal manner upon the activity of the hydrogen ion. Mortone in the course of a very accurate electrometric titrationof 0.01M moles base citric acid with 0.01 31 sodium hydroxide found that when the ratio molesacid was equal to 2.769, the pH was 6,40j, the concentration of citrate in the mixture being 2.65 X 10-3 M , Extrapolating the data in Table I V to this concentration of citrate we find that the corresponding pH is about 6.60. I n calculating the base/acid ratio for comparison with Morton’s figure, account must be taken of the acid produced during the reduction of the gold chloride. Assuming that the reduction is complete and wholly due to the formaldehyde, then according to Vanino and Hartllo the liberation of z atoms of gold re-
HYDROGEN ION CONCENTRATION IN GOLD SOLS
609
quires I I molecules of Pu'aOH. I n the present case thismeans that 0.626 X 10-8 moles per litre of base are neutralised by hydrochloric and formic acid. We therefore have for the citrate equilibrium, moles base - ( 3 X 2.65)-0.626 moles acid 2.65 2.764: i.e. almost identical with Morton's value. The difference of 0.20 between the pH values is probably connected with the fact that the citrate itself acts as a reducing agent, but the information necessary to make allowance for this is lacking a t present. I t should be emphasised that in none of these experiments could there he detected the slightest tendency on the part of the gold to coagulate in the hydrogen cell. Moreover if an electrode was placed in a standard solution immediately after use in a gold sol it invariably acquired the correct potential within 2 0 minutes. If the gold exerted any ill effect upon the electrode it was certainly of a very transitory nature. Conclusions The results of this work establish beyond reasonable doubt that under circumstances the hydrogen electrode may be relied upon as a means of determining the hydrogen ion concentration in a gold sol. It has been shown that, a s would be predicted from the behaviour of ordinary unbuffered solutions, the sol must be either sufficiently acid or suitably buffered in order to secure steady and reproducible potentials. The unsatisfactory working of the electrode observed by Adolf and Pauli3 and Tart,ar and Lorah4 can hardly be due to the absence of the above conditions. The dialysed sols studied by Adolf and Pauli contained about I x IO-^ i Y hydrogen ion, and gelatine, which exerts a buffering action, was present in the sols of Tartar and Lorah. Moreover both pairs of workers report that, provided all the gold was removed, no difficulty arose in the use of the hydrogen electrode. As far as the available information goes, the essential difference between the sols employed by these investigators and those examined in the experiments here described, appears to be that the former contained complex organic substances. Adolf and Pauli dialysed their sols by means of parchment paper membranes and the work of Wintgen" and of Thiessen and Heumann" has shown that appreciable quantities of some protective colloid passes into the sol from such membranes. Wintgen and his co-workers found that this substance could be present in the sol to the extent of 5oc7, of the weight of the gold, and Thiessen that it was not eliminated by prolonged use and washing of the membranes. Gelatine of course was present in the sols of Tartar and Lorah. I t may be tentatively suggested that the hydrogen electrode gave erratic results owing to some combined action of the gold and the foreign material. The sols investigated by Reinders and Bendien' also contained gelatine but unfortunately these workers give no description of their E.M.F. measurements.
610
T. R. BOLAM AND J. CROWE
The thanks of one of the authors are due to the Carnegie Trust for a Teaching Fellowship, of the other to the Department of Scientific and Industrial Research for a grant, and of both to Sir James Walker for his interest, and to the Earl of Xoray Research Fund for assistance in obtaining materials.
References Herstad: Kolloidchem. Beihefte, 8, 409 (1916). 2 Kautaky and Pauli: Kolloidchem. Beihefte, 17 303 (1923). Adolf and Pauli: Kolloid-Z., 34, 30 (1924). Tartar and Lorah: J. Phys. Chem., 29, 794 (1925). 5 Ostwald: Kolloid-Z., 40, 204 (1926). BBeaverand Miiller: J. Am. Chem. Soc., 50, 308 (1928). 7Reinders and Bendien: Rec. Trav. chim., 47, 97’8 (1928). 8 Clark: “Determination of Hydrogen Ions”, pp. 285, 456, 114, 267-9 (1923) 9 Morton: Trans. Faraday So?., 24, 14 (1928). ‘0 Vanino and Hartl: Kolloid-Z., 1, 272 (1907’). ‘1 Wintgen: Kolloid-Z., 40, 301 (1926). 12 Thiessen and Heurnann: Z. anorg. Chem., 148, 387 (I92 j). Chemastry Department, University of Edznburgh. July 19, 1930.
