32 ADVENTURES IN RADIOISOTOPE RESEARCH 



in the solubilities discussed above we must therefore prepare artificially 

 radiolead by the addition of relatively large amounts of radium-D 

 to lead nitrate. 



1. DETERMINATION OF THE SOLUBILIIY OF LEAD CHROMATE 



x\bout 0.2 c of emanation was allowed to decay in a closed flask over 

 distilled water and the solution thus obtained, containing about 10~^ gm 

 RaD in water, was added to a solution of approximately 10 mgm PbClg 

 in water. The lead was then quantitatively precipitated with potassium 

 chromate, filtered, washed from the filter into a stoppered bottle and 

 shaken with about 100 cm^ of distilled water in a thermostat at 25° 

 for a period of 24 hr. The mixture was immediately filtered, the first 

 portion of the filtrate being rejected because of a possible change 

 in its concentration as a result of adsorption on the filter, and 70 cm^ 

 of the remaining filtrate was evaporated to dryness on a watch- 

 ^lass-shaped nickel tray over the water bath. When equilibrium had 

 been established between the RaD and RaE the activity on the tray 

 was measured. 



The calculation was done as follows : 1 cm^ of the RaD solution used 

 for labelling the lead showed (also after establishment of equilibrium) 

 a ^-activity of 16.90 arbitrary units and, therefore, the whole solution, 

 amounting to 120 cm^, contained 2030 units. This activity had been 

 distributed on 9.69 mgm of lead chloride or 11.35 mgm of lead chromate 

 and thus one arbitrary unit of RaD was associated with 11.35/2030 = 

 = 0.00559 mgm lead chromate. The 70 cm^ of solution which had been 

 evaporated had deposited an activity of 0.15 units on the tray and thus 

 0.15 X 0.00559 = 0.000839 mgm of lead chromate must be on the tray. 

 Hence, the solubility of lead chromate at 25° C is calculated to be 

 1000 X 0.000839/70 = 0.012 mgm/1. 



A second experiment with the same solid phase also gave 1 .2 x 10"^ gm/l. 

 The first experiments, carried out with smaUer amounts of RaD and 

 accordingly with a much lower accuracy, yielded values which varied 

 between 3 x 10"^ and 6 X 10-^ gm. Lead chromate is therefore the most 

 sparingly soluble lead salt ; only the solubility of lead phosphate is 

 of the same order of magnitude. 



Apart from a rough estimate by F. KohlkauschI based on a measure- 

 ment of conductance of the saturated lead chromate solution, there are 

 no data available on the solubility of lead chr<tmat(^ ; Kohlrausch 

 estimates the solubility as 10-* gni/1. 



IF. KOHT.RAUSCH, Z. phys. Chem. 64, 159 (1908). 



