34 ADVENTURES IX RADIOISOTOPE RESEARCH 



W. BiLTZ^ determined the solubility of PbS by means of an ultra - 

 microscopic method: When two equivalent solutions which produce 

 a precipitate are mixed in a series of experiments at increasing dilution 

 and the mixture obtained is observed with an ultramicroscope it is noted 

 that, beyond the limit of macroscopic differences, the number of suspended 

 particles of the precipitate becomes steadily less until, at a certain 

 dilution, the mixture appears to be empty or no longer different from 

 its components. This limiting value for the disappearance of the un- 

 dissolved excess corresponds to the solubility of the substance produced. 

 i.e. lead sulphide. Biltz finds a value of 1.3 mgm/1. for the solubilit}- of 

 lead sulphide at room temperature. He remarks that the determination 

 is made more difficult, in the case of sulphides, because they form 

 colloidal solutions which are almost optically transparent at a high 

 dilution ; separate particles can of course be produced by adding salting- 

 out electrolytes with dissimilar ions but at the same time this may 

 cause an increase in solubility. The solubility determined by the ultra- 

 microscopic method is therefore probably rather too large. Correspond- 

 ingly, O. Weigel^ found that 0.86 mgm of freshly precipitated PbS dissol- 

 ved in 1 1. by calculating the solubility of PbS from the conductance on the 

 assumption that all the PbS going into solution is hydrolysed. Freshly 

 precipitated PbS, however, undergoes a transformation and after about 

 20 hr have elapsed the solubility amounts only to about 0.43 mgm/1. 

 The PbS used in our experiments was already transformed and the 

 solubility of 3 x 10"^ gm in 1 1. which we found agrees very well with 

 Weigel's value^. 



RaD is not the only radioelement which can serve as an indicator 

 for lead ; careful studies by Fleck^ demonstrate that thorium-B. 

 radium-B and actinium-B also cannot be separated from lead. The last 

 two cannot indeed be considered for our purposes but thorium-B, with 

 its half-life of 10.6 hr, might well be applied with success as an indicator 

 for lead. 



Besides lead, we know of two other elements with which a radio - 

 element can be used in practice as an indicator, viz. bismuth and thorium. 

 The former can be labelled with thorium-C or preferably with RaE,^ 

 while the latter may be labelled with uranium-X, radioactinium, radio- 

 thorium or, best of all, ionium^. 



1 W. Biltz, Z. phys. Chem. 58, 288 (1907). 



1 O. Weigel, Z. phys. Chem. 55, 293 (1907). 



2 1. BER>fFELD, Z. phys. Chem. 25 (1898) considers the PbS electrode to be a 

 reversible electrode of the second kind and calculates the lead ion concentration 

 to be 10—* at the PbS electrode at one atmosphere pressure of hydrogen sulphide 

 from the electromotive force of the cell Pb | IN Pb(N03)2 | IN NaHS | PbS. 



3 A. Fleck, Proc. Chem. Soc. 29, 7 (1913). 



4 A. Fleck, Proc. Chem. Soc. 29, 7 (1913). 

 6 F. SoDDY, Radiochemisiry . London (1911). 



