THE SOLUBILITY OF LEAD SULPHIDE AND LEAD CHllOMATK 33 



2. DETERMINATION OF THE SOLUBILITY OF LEAD SULPHIDE 



For these experimenls 9.(39 nigin ol' lead chloiide (8.36 mgni in terms 

 of sulphide) were labelled with 140 cm^ of another solution of RaD 

 which contained 6G.2 arbitrary units per cm^. The lead was then quantit- 

 atively precipitated at the boil with a hot solution of NagS, the PbS 

 was filtered off, washed and shaken with distilled water as described 

 in the case of lead chromate. The filtrate, the first part of which was 

 again rejected, was completely clear and colourless ; it contained 

 415 arbitrary units of RaD per litre. In this instance one arbitrary unit 

 corresponds to 8.36/140 X 66.2 = 9.0 x 10^* mgm of lead sulphide, and 

 thus 1 1. of solution at 25° C contained 415 x 9.0 x lO"* = 0.37 mgm 

 or 3.7 X 10~^gm. The same value was obtained after filtering the solution 

 once again ; other experiments yielded the values of 300 and 320 ar- 

 bitrary units per litre, i.e. 2.70 and 2.88 X lO^^g^ j^^d sulphide per litre. 



A part of the lead present in the solution probably occurs as hydroxide, 

 owing to hydrolysis, instead of sulphide, as suggested by 0. Weigel^. 

 The very weak turbidity obtained when the completely clear saturated 

 solution, prepared by shaking water with PbS, is treated with a stream 

 of HgS supports this view^ We have therefore determined the solubility 

 of PbS in water saturated with HgS ; the solution from w^hich the PbS 

 is precipitated cannot be used directly for determining the solubilit\- 

 since a portion of the PbS passes as a colloid through the filter ; th(> 

 once-filtered PbS, on the contrary, is already freed from the small 

 particles passing through the filter and these do not recur when the 

 solution is shaken with distilled or HgS-saturated water. In the solution 

 which is saturated with H2S and PbS, the concentration of H2S is about 

 one thousand times that of the PbS. The solubility of the latter is less 

 than the value obtained in distilled water ; calculated on the basis 

 of 1 1., the arbitrary activity amounted to 148 and 173 and hence the 

 amount dissolved was 1.33 and 1.56 x 10~* gm, respectiv^ely. It is not 

 possible to decide with certainty whether there is a decrease in solubility 

 due to an increase of the S ion concentration or due to prevention of 

 hydrolysis ; the first case, however, is improbable since the decrease 

 in solubility is only very slight in proportion to the high concentration 

 of HgS. In analytical practice, moreover, this problem need not be 

 considered ; it is only of interest to know the amount of PbS which 

 is present in solution in a clear filtrate ; our experiments give an average^ 

 value for this of 3 X 10"* gm in the absence of HgS and 1.5 x 10^^ 

 in a solution saturated with HgS. If the filtrate runs turbid through 

 the filter, it is evident that the amount of PbS will be greater. In one 

 instance we observed 1 — 2 mgm/1. 



20. Weigel, Z. phy-s. (Item. 55, 293 (1907). 

 ,3 Hevesy 



