STUDIES ON T H E CHEMISTRY OF GOLD BY VICTOR LEKHER AND C . H. KAO
It has been shown by one of us1 that chloride of gold can be extracted from an acidified aqueous solution by certain aliphatic esters, It has since been found that this property differs widely among individual members and is independent of the solubility of the organic compound in water and of water in the organic compound. Ethyl acetate, which is somewhat soluble in water, gives a better extraction of gold chloride than isobutyl propionate which is practically insoluble. On the other hand, diethyl malonate which is insoluble in water gives an excellent extraction. In view of these facts, the whole subject has been studied by determining the partition coefficient of chloride of gold in water and esters under well defined conditions. Procedure. Hydrochlorauric acid was prepared by solution of gold in aqua regia with excess of hydrochloric acid and evaporation over calcium chIoride. The resulting product as shown by Lengfeld has approximately the formula HAuC14. 3Hz0. Standard solutions of this salt were prepared, their gold content being determined by analysis. The esters used were all prepared by us. 2 0 cc portions of the chloride of gold solution and 2 0 cc of the ester were shaken in a thermostat at zoo. The system was allowed to stand only SO long as was required to allow the two liquids to separate which was determined to be in most cases, five minutes. On long standing most of the esters hydrolyse and in some of them reduction takes place with the precipitation of metallic gold. The two liquids were then separated by a separatory funnel. The gold in the aqueous layer was determined by precipitation by sulfur dioxide, that in the ester by difference. Attempts have been made to obtain the molecular weight in various esters, but have been unsuccessful, metallic gold being produced. A study has been made of the distribution data of HAuC14 between water and a large number of esters, and a distribution of the second order has been shown to exist in all cases. The following were used; methyl acetate, methyl propionate, methyl butyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, isopropyl formate, isopropyl acetate, isopropyl propionate, isopropyl butyrate, isobutyl formate, isobutyl acetate, isobutyl propionate, isoamyl formate, isoamyl acetate, isoamyl propionate, isoamyl butyrate. The acetates in general are better extracting agents than any other members while the ethyl esters are in general better than any other type. As the number of carbon atoms increases the extractive power decreases. Thus it has been found that ethyl acetate gives the best extractions. Lenher: J. Am. Chem. Soc., 35 546 (1913). Lcngfeld: J . Am Chern. Soc. 26, 324 (1901).
STUDIES ON THE CHEMISTRY O F GOLD
127
Influence of various amounts of hydrochloric acid o n the partition coefficients. The effect of hydrochloric acid in varying amounts from 17~ to 30yGhas been determined. The presence of hydrochloric acid is a very important factor on the partition coefficient. The separation of gold chloride into the ester layer successively increases until at approximately I 0% concentration the maximum extraction is reached. Further addition of hydrochloric acid beyond this point of 10% has very little influence on the partition coefficient. At 30% concentration of HC1, the esters and water are miscible. Therefore considering all of the factors involved, it appears that ethyl acetate in 10% hydrochloric acid is the most efficient extraction agent for gold chloride. To test the general applicability of the use of ethyl acetate as a means of purification of gold chloride, and separation from the chlorides of other metals, a number of experiments have been conducted to that end. The procedure is to treat a 10% hydrochloric acid solution of the particular chloride in a separatory funnel with an equal amount of ethyl acetate and shake vigorously. The two layers are allowed to settle after which they are separated. The ester layer is washed twice with small amounts of 1oy0hydrochloric acid. The combined aqueous solutions are again extracted with ethyl acetate until the ethyl acetate is colorless, which requires about four extractions. The gold is then determined by evaporation of the ester and precipitation by sulfur dioxide. Gold Chloride alone Au taken Au found
Number of Determinations 2
- . 000 I
0.0443 0.3986
0,0444 0,3987
2
Average Error
- . 0001
6
Separation f r o m Sodium Chloride NaCl NaCl Error AU taken found taken ,584ggm .58.50 k.0004 ,1148
5
Separation f r o m Potassium Chloride KC1 KC1 Error AU Au Error taken found taken found .5326 .5323 --.0004 .1148 ,1147 - . 0001
Number of Determinations
4
4
Au found
Error
,1149
- ,0005
Separation f r o m Magnesium Chloride MgClz MgClz Error AU taken found taken
Au found
Error
,2992
,1378
,1377
- ,0001
Separation f r o m B a r i u m Chloride BaCL BaClz Error Au taken found taken
Au found
Error
,4937
.I375
,2995
.4935
+.0004
f.0004
'1378
,0003
I 28
VICTOR LENHER AND C. H. KAO
Number of Determinations 3 2
Separation f r o m Ferric Chloride FeC13 FeC13 Error Au Au Error taken found taken found .4218 .4218 i.0003 ,1148 ,1146 A .0002 .210g .210j --.0004 .1148 1144 - ,0004 '
Separation f r o m Aluminum Chloride AlC13 AlC13 Error Au Au taken found taken found .4332 .4333 =t.o006 .1378 . I375
Error
- .0003
4
Separation f r o m Strontium Chloride SrC12 SrClz Error Au taken found taken ,3243 .3240 --.0003 .0763
5
Separation f r o m Calcium Chloride CaCL CaCL . Error Au taken found taken .5322 ,5328 j=.ooo4 , 1 2 5 8
3
Separation f r o m Chromium Chloride CrCI3 CrC13 Error Au Au Error taken found taken found ,1609 ,1606 - .0003 ,1761 ,1762 ,0001
5
Separation f r o m Manganous Chloride MnCl? MnClz Error Au Au taken found taken found ,5085 .so82 - ,0003 ,1378 .I375
4
Separation f r o m Cobalt Chloride CoClz CoClz Error AU Au Error taken found taken found .5816 ,5812 - ,0004 ,1148 ,1146 - . 0 0 0 2
4
Separation from Nickel Chloride Error NiClz NiCl2 Error Au Au taken found taken found .so55 .so55 i=.ooo4 ,1148 ,1146 - .0002
Au found .0759
Error
Au found
Error
. I255
- .0003
- ,0004
+
Error
- .0003
4
Separation f r o m Z i n c Chloride ZnCl? ZnClz Error Au Au taken found taken found .4038 .4035 +.OOOj .I148 1147
4
Separation f r o m Mercuric Chloride Error HgClz HgClz Error Au Au taken found taken found .so20 .so21 f.oooz .1148 .1146 - .0002
Number of Determinations
'
Error - .OOOI
STUDIES ON T H E CHEMISTRY O F GOLD
Separation f r o m Copper Chloride CuCL CuClz Error Au taken found taken
Error
,4712
&.0002
.4710
ic.0002
Au found ,1148 .I147
Separation f r o m Cadmium Chloride CdCL CdCL Error Au taken found taken .494j ,4942 ic.0003 . I148
4
3
Number of Determinations 4
Au Error found .1146 f . 0 0 0 2
Separation f r o m Lead Chloride PbCL PbCL Error Au taken found taken .0406 ,0403 =k.o003 .1375
Au found
Error
,1373
- .0002
Separation f r o m Bismuth Chloride BiCI3 BiCL Error Au taken found taken ,2763 ,2759 h.0004 .1375
Au found
Error
Separation f r o m Antimony Trichloride SbCL SbCL Error Au taken found taken 4
129
,2548
,2545
--.ooo3
.137j
,1373
Au Error found .1374 -.OOOI
Separation f r o m Arsenic Acid H3Aso4 H3Aso4 Error Au taken found taken
Au found
.3654
,1375
.3652
j=.oooq
.1378
Separation from Stannic Chloride SnCL SnCL Error Au taken found taken .3894 .3894 dz.0005 .I378
i.0002
Error =k.o003
Au found
Error
,1377
=tt.0o03
Conclusions It has been established that auric chloride can be separated completely from all of the more common chlorides in a solution containing IO% free hydrochloric acid by means of extraction with ethyl acetate. Ethyl acetate is the most efficient of the esters for this separation. The method is particularly useful in separating gold chloride from the alkaline earth metals, since the more commonly used sulfur dioxide will form with them insoluble sulfates.
