-1COAIPARISON O F SILVER .CiD LEAD SOLS MADE BY T H E BREDIG METHOD* BY HELEN QUINCY WOOD.4RD
Considerable work has been reported on the influence of various ions in precipitating colloids. Comparatively little has been done on the influence of various ions in stabilizing colloids prepared by electrical methods, however. Hence it has seemed desirable to compare the properties of silver and lead sols prepared by the Bredig method‘ in different electrolyte solutions. Apparatus and Materials The apparatus and materials used in the preparation of the lead colloid were the same as those previously reported.* The same apparatus and electrical connections were also used in the preparation of the colloidal silver. The silver used was Baker & Co.’s fine silver wire, I+ mm. in diameter. The anode was a disk-shaped coil of this wire, the lead-in being in one piece with the disk. The cathode, from which most of the disintegration took place, was a straight piece of wire fed down with a screw feed. Method The method for the lead colloid has been described in the previous paper. The method for colloidal silver was substantially the same, the current used being 8.0 0.2 amps D.C., and the temperature range a t the level of the arc I O ’ - ~ ~ O C . The silver was determined by the thiocyanate method3, which gave a precision of + 0.01mg., the amounts determined being 0.1 mg. t o 1.0 mg. All the sols, both silver and lead, were centrifuged for 5 min. with a force of 1000 X gravity, and all concentrations reported are for centrifuged sols. Results When arcing was done between silver electrodes in suitable solutions, colloidal silver began t o form as soon as the arc was started. The concentration of the sol rose to a maximum, and then, upon further arcing, fell to zero. Prolonged arcing in the supernatant liquid remaining after this precipitation did not result in the formation of any further colloidal silver. With some electrolytes the curve obtained by plotting the concentration of the sol against the weight of silver disintegrated in the arc per I O O cc. of sol formed showed a sharp peak; with other electrolytes the peak was more rounded. *From the Huntington Fund for Cancer Research, Memorial Hospital, New York City. Bredig: Z. angew. Chem., 1898,951. 2 Woodard: J. Am. Chem. SOC.,50, 1835 (1928). 3 Scott: “Standard Methods of Chemical Analysis”, 4th Ed., 1, 456.
COYPARISON O F SILVER AND LEAD SOLS
I39
This is shown in the typical curves in the accompanying figures. These curves showed good reproducibility when the time required for arcing to complete precipitation was short. With the more concentrated sols, where 1-2 hours. arcing was required before the sol precipitated, i t became increasingly difficult to control the steadiness of the arc, and the reproducibility fell accordingly.
FIG.I Silver sols made in solutions of S a c 1 S KaC1 Curve A = sols in .oo14 i Curve B = sols in .mz5 N NaCl Concentrations are for sol silver only
I n the previous paper the author reported that arcing between lead electrodes resulted in the rapid rise of the concentration of the lead colloid to a maximum, and that further arcing did not result in significant change in concentration. This was considered as a fundamental difference in the process of sol formation between the lead colloid and colloidal gold', silver2, and platinum3. Since that time over I O O lead sols made in o.ooozz N KOH under very nearly the same conditions have been available for study. When these are grouped according to the weight of lead disintegrated in the arc per IOO cc. of sol formed, there appears to be a maximum of concentration after 4-5 gms. lead have been disintegrated. (See Table I). The average L. W. Briggs: Diss. Columbia (1923). Q . W d a r d : Dias. Columbia (1925). M. Baeyertz: Diss. Columbia (1924).
a H.
140
H E L E S Q C I S C Y WOODARD
TABLE I Lead sols made in .ooozz S KOH a t j . 0 amps dveragc concentration a t different arcing periods Pb disintegrated per IOO cc 0.0-0.5
gme.
0 . j-1.0
;’
1.0-1.5
;!
1.5-2.0
’’
2
.o-3 .o
3.0-4.0 4.0-5 .o j
.o-6
.O
6 . o - j .o
S o determinations
Average concentration
0 . 3 3 1 ~ ;P b 0.0j4
” ”
‘’
0.106 0.146 0.167 0.169
I’
0.179
’I
’’ ”
c; deviation
0.163 o . I 60
‘’ ” ”
;’ ” ”
deviation from the average concentrations is too large for the observations to be very significant , however. Of greater significance is a series of determinations of the concentrations of single sols made at different periods during arcing. I n all sols where the arcing was continued until seven or more grams of lead had been disintegrated per I O O cc. a decrease in concentration below a previous maximum was observed, and in several sols the maximum occurred after the disintegration of 4 - j gms. per I O O cc., as in Table I. Table I1 gives the figures for three of these sols. It seems probable, therefore, that the formation of lead and silver sols is alike in that both colloids rise to a maximum of concentration during arcing, and then decrease upon further arcing. Owing to the technical difficulties in arcing for long periods between lead electrodes it was not determined whether it was possible to arc FIG.2 Silver sols made in solutions of NalS. lead sols to complete precipitation, as can Curve A = sols made in .ooo2 j be done readily with silver sols. SSa& CurveB=solsmadeIn O O I 2 j SSa?S. Some light can be thrown on the Concentrations are for total silver. mechanism of sol formation by studying the different behavior of sols made in different electrolyte solutions. Table I11 gives the results of arcing between silver electrodes in various solutions. Where the silver is reported as “total silver’’ I - j cc. samples of centrifuged sol were run into j o cc. Erlenmeyer flasks, a few drops of concentrated nitric acid were added, the mixture was boiled until solution was complete, cooled,
i\
COYPARISOX O F SILVER AND LEAD SOLS
141
and titrated against standard sodium thiocyanate. When an cnion such as the chloride or thiocyanate ion, which interfered with the solution of silver in nitric acid, was used in the arcing solution, the sol was first precipitated with solid sodium nitrate and filtered. The precipitate was washed on the filter paper, dissolved in concentrated nitric acid, titrated, and reported as “so1 silver.” Little or no solid matter remained on the filter after this procedure, and this was taken as proof that the silver existed as metallic colloid for the most part, and not as colloidal silver chloride, thiocyanate, etc., as might be supposed. For comparison, determinations of both total silver and sol silver were made on a few sols stabilized by different concentrations of XOH and SazCOs. The differences were found to be .OOI% Xg t o . 0 0 5 ~ Ag, or about 107~ of the total silvel’in each case.
TABLE I1 Lead sols made in . 0 0 0 2 2 S KOH at 7 . 0 amps Change in concentration during arcing Pb disintegrated per IOO cc. 0 . 9 1 gms. 2.32 ”
3.73 5.93 8.33
” ”
*’
I . 5 1 gms. 3 ‘43 ” 4.j6 ” 6.58 ” 8,49 ”
Conc. sol.
Pb disintegrated per 100 cc 28
”
o og4VC Pb. o 167%
4 29
”
0 224VC ”
o oj7QcPb.
o 8qgms.
0 I20c;
2
”
o 1 8 6 ’~I ~ 0 178Yc ” 0 I28Yc ”
Conc sol
6 $0 ” 8 90 ” 11 80 gms.
0 2 2 0 7
”
o 2055;; ” o 1 9 2 Pb ~ ~
0 . 1 6 0 7 ~Pb. 0 . 2 I 7 Yc ” 0 . 238TC ” o 223Yc ” 0 . 1 9 8 7 ~”
Examination of Table 111 shows the following: I t was not found possible to make sols in distilled u-ater or in solutions of silver or ammonium nitrates, and only a few transient sols of low concentration could be made in solutions of sodium nitrate and sodium acetate. I t was not found possible to make sols in solutions of acetic or sulfuric acids at the concentrations tried. Concentrated and stable sols could be made in solutions of sodium carbonate and potassium hydroxide of various concentrations. Dilute but stable sols could be made in solutions of hydrochloric acid, sodium and ammonium chlorides, sodium thiocyanate and sodium sulfide. I t appears from the above that it is the anion in the arcing solution which determines whether a silver sol will be formed or not. This is t o be expected, since colloidal silver carries a negative charge. It also appears that the effectiveness of an anion in stabilizing the colloidal silver depends on the solubility of the salt which the anion is capable of forming with ionic silver,
~
HELEX Q C I S C Y WOODARD
142
TABLE 111 Silver sols made at 8.0 amps in different arcing solutions Electrolyte
Silver reported
o ooroKiasCOa o 0019 S Na2C03 o 0030 N KazC03
Total ,I lf
,I
o 00086SKOH o 0013 pi KOH o 0021 N KOH o 001 j
o
,, ,. Sol
?i HC1
,, ,,
0015 NXH4C1
o 0014N KaC1
I, N NaCl It o 0050 S h’aC1 HzO No sols formed. o 0015 N NaCKS Sol
o
002
j
o 0002 j K o 0007 j N
Total
Ka2S*
NKazS o 0 0 2 5 N Na2S o o
00125
0005 j
N AgxOa to
o 0026 N AgKOs o oooj K NaX03 o 0014 K h’aN03 o 0 0 2 5 K NaNOa o 0014 N TiH,N03
”
0.40
”
0.40
”
o . j o gms. 0 . 7.5 !’
0 .j o
”
o . 3 j gms.
o
02j
yc
.kg
o 029
’’
034
”
0
Xg
o 03j 0 050
”
Variable
0 050
”
o .04 gms.
o .06 gms.
0 0 1 2 pc
0.10
”
0.15
”
0 012
”
0.oj
)’
0.10
”
0 008
”
0 .IO
”
”
0 014
”
0.23
”
o 23 0.40
”
0
033
”
o
008
yc Ag
o . o ; gms.
0 . 0 8 gms.
Ag
9 Ag.
0.03
gms.
o 006
o .03
”
0.05
’I
0 011
I,
0.05
”
o .06
”
0
018
”
It
o .09
”
0.10
”
0 028
”
1
”
KOsols formed. o . O Ij gms.
o .oz gms.
o
OOqjC,;
>)
0.01;
”
0.02
”
0
) *
0.010
”
0.02
’I
0
0045 0040
Total
-kg ” ”
KO sols formed.
o , 0 0 1j 3-CH3COOSa
0.0013 K CH3COOH
o , 0 0 1j S HZSO,
0.25
Conc sol at max
o . o j ~gms.
,,
KazS
rig disintegrated per 100cc. At max. conc. At total coagof 801. ulation of sol 0 . 1 7 gins. 0 . 3 5 gms. 0 . 2 0 ” 0 . 3 5 !’
Trace sol X g at S o sols formed.
0.01
gms. disintegrated per
100
cc.
KO sols formed.
*Concentrations of K a t s given are with respect to the 5: content. Ouing to hydrolysis, the Xa content was about zo?‘ higher.
the anions capable of forming moderately insoluble compounds with silver being the most effective. Thus the nitrate, sulfate, and acetate of silver are readily soluble salts, and the acetates, nitrates (with the doubtful exceptions mentioned above) and sulfuric acid were not found capable of stabilizing silver sols. Silver carbonate and hydroxide are moderately insoluble, and sodium carbonate and potassium hydroxide were found to be good stabilizing agents for colloidal silver. Silver chloride, sulfide, and thiocyanate are highly insoluble salts, and the chloride, sulfide, and thiocyanate solutions tried were found to be only fair stabilizing agents. That the formation of
I43
COMPARISON O F S I L V E R A S D LEAD SOLS
the colloid is influenced to a certain extent by the cation as well as the anion is shown by the differences in concentration and range between sols prepared in equinormal concentrations of ammonium chloride, sodium chloride, and hydrochloric acid, but the influence of the cation appears small compared to that of the anion. Since it is the anion of the arcing solution which has the most influence on the formation of silver sols, which carry a negative charge, one would expect, as lead sols carry a positive charge, that the cation would be the important ion in the solution used for the preparation of lead sols. Table IV, which has been published in part previously’ shows this to be so. The concentrations of lead sols given are those in the “plateau” of the concentration curve, when the concentration is changing only very slowly as arcing proceeds. All sols were prepared with a current of 1.4 amps in approximately equinormal solutions of different electrolytes.
TABLE IV Lead sols made a t 1.4amps in different arcing solutions Electrolyte Av. conc. sol. No. deter% deviation minations
o ,00025
K HzSO4
N HCl 0 . 0 0 0 2 5 N lactic acid O , O O O Z2 N CHSCOOH o.0002 5 K xH4C1
0 .0002
j
5 N ”4x03 0 , 0 0 0 2 5 K NaCl 0 . 0 0 0 2 5 K h’aSO3 0 , 0 0 0 2 5 1\’CH3C00Xa *o .00029 ?u’ NazS 0 . 0 0 0 2 5 K NazC03 0.0002
o.ooo22KKOH
K NaHCOl
0 . 0 3 9 7 ~P b o ,038 ” 0.044 ”
5
from av.
*I9
I5
1 2 0
124
o.oj0
”
0.035 0.038
”
8 6 9
”
IO
1 2 1
6 8
1 8
”
’,
1 9
0 .I 0 2
7 7
0.098
”
5
*I3
0.I22
”
63
*I5
0 . IO0
13 0.124 0 .I
,’ ”
*20
128
*I1
1 1 7
5
*I7 * Concentration of S a 2 Sgiven is with respect to the Ka content. Owing to hydrolysis, the S content was about 157~ lower. 0.00025
0.083
”
It was previously reported that, for most of the solutions examined, “the concentration of colloidal lead stabilized by equinormal concentrations’ of different electrolytes is a function of the pH of the initial solution.” This
is confirmed in the larger series presented here when hydrogen IS the preponderating cation in the initial solution. It appears from an examination of the table that hydrogen and ammonium are poor stabilizing agents, o.oooz 5 N solutions of hydrochloric and sulfuric acids, ammonium chloride and ammonium nitrate, being able to stabilize sols containing only 0.0350.039% lead. On the other hand, sodium and potassium appeared to be ‘ H . Q. I\-oodard: J. .lm. Chern. Soc., 50, 183j (1928).
I44
HELEN QUIXCY WOODARD
much more effective stabilizing agents, sodium chloride, nitrate, acetatp, sulfide and carbonate, and potassium hydroxide, in approximately equinormal concentrations, being able to stabilize sols containing 0.10--0. I Z ? ~ lead. Lactic and acetic acids and sodium hydrogen carbonate occupied intermediate positions between the strong acids and ammonium salts on the one hand, and the sodium and potassium compounds on the other. As with silver sols, it is probable that the formation of lead sols is influenced somewhat by the ion of opposite charge to that of the sol formed. This is suggested by the differences in concentration between the sols stabilized by different sodium salts. The reproducibility of these sols is not sufficiently good for positive statements on this question, however. Among the positive ions which may be present in the initial solution, lead itself must also be considered. When the hot arcing solutions are cooled in contact with the electrodes, lead dissolves up to a concentration which varies with the state of the electrodes, rapidity of cooling, solution used, and probably other fact'ors. The concentrations average somewhat higher when solutions of acids or ammonium salts are used than with most of the sodium salts, and are still lower for sodium carbonate and bicarbonate and potassium hydroxide, the average values being in the neighborhood of o.oooz N Pb, o . O O O I S N Pb, and O . O O O I O N Pb, respectively. As, owing to the amphoteric properties of lead, it is not certain how much of the lead present in these solutions is in the anion and how much in the cation, it can not be considered unreservedly as one of the stabilizing cations. A thorough study of the effectiveness in stabilizing lead sols of mixtures of salts of lead with salts of other metals would be necessary before the influence of lead ion could be evaluated.
Summary A comparison has been made of the changes in Bredig silver sols and Bredig lead sols on prolonged arcing. The formation of Bredig silver sols has been found to be determined mainly by the negative ion in the arcing solution. The formation of Bredig lead sols has been found to be determined mainly by the positive ion in the arcing solution.
