WHITE PIGMENTS. 21 



(a) TOTAL LEAD. 



Weigh 1 grain of the sample, moisten with water, dissolve in acetic acid, 

 filter, and wash, ignite, and weigh the insoluble impurities. To the filtrate 

 from the insoluble matter add 25 cc of sulphuric acid (1:1), evaporate and 

 heat until the acetic acid is driven off; cool, dilute to 200 cc with water, allow 

 to stand for two hours, filter on a Gooch crucible, wash with 1 percent sulphuric 

 acid, ignite, and weigh as lead sulphate. Calculate to total lead (PbSO 4 X 

 0.68292=Pb), or calculate to basic carbonate of lead (white lead) by multi- 

 plying the weight of lead sulphate by 0.85258. 



The filtrate from the lead sulphate may be used to test for other metals, 

 though white lead is only rarely adulterated with soluble substances; test, 

 however, for zinc, which may be present as zinc oxid. 



Instead of determining the total lead as sulphate it may be determined as 

 lead chromate by precipitating the hot acetic acid solution with potassium 

 dichromate, filtering on a Gooch crucible, igniting at a low temperature, and 

 weighing as lead chromate. 



(b) COMPLETE ANALYSIS. 



If it is desired to determine the exact composition of a pure white lead, honi 

 1 gram of the pigment in a porcelain boat in a current of dry, carbon-diox id- 

 free nitrogen, catching the water in sulphuric acid and calcium chlorid and 

 the carbon dioxid in soda lime or potassium hydroxid (1.27 specific gravity). 

 By weighing the residue of lead monoxid in the boat all the factors for deter- 

 mining the total composition are obtained. Figure the carbon dioxid to lead 

 carbonate (PbCO 3 ), calculate the lead monoxid corresponding to the lead car- 

 bonate (PbCO 3 ) and subtract from the total lead monoxid, calculate the re- 

 maining lead monoxid to lead hydroxid (Pb(OH) 2 ), calculate the water corre- 

 sponding to lead hydroxid and subtract from the total water, the remainder 

 being figured as moisture. 



This method assumes the absence of acetic acid. Thompson states that 

 acetic acid varies from 0.05 per cent in Dutch process white lead to 0.7 per cent 

 In some precipitated white leads. It is then more accurate to determine the 

 carbon dioxid by evolution ; this is especially the case when working with a lead 

 extracted from an oil paste, as the lead soap and unextracted oil will cause 

 a considerable error by the ignition method. In determining carbon dioxid by 

 the evolution method, liberate the carbon dioxid with dilute nitric acid, have a 

 reflux condenser next to the evolution flask and dry the carbon dioxid with cal- 

 cium chlorid before absorbing it in the potassium hydroxid bulbs. 



(c) ACETIC ACID. 



It is sometimes necessary to determine acetic acid. The Navy Department 

 specifications demand that white lead shall not contain " acetate in excess of 

 fifteen one-hundredths of 1 per cent of glacial acetic acid." Thompson's 

 method is as follows : 



Eighteen grams of the dry white lead are placed in a 500 cc flask, this flask 

 being arranged for connection with a steam supply, and also with an ordinary 

 Liebig condenser. To this white lead is added 40 cc of syrupy phosphoric acid, 

 18 grams of zinc dust, and about 50 cc of water. The flask containing the 

 material is heated directly and distilled down to a small bulk. Then the steam 



J. Soc. Chem. Ind., 1905, 24:487. 



