WHITE PIGMENTS. 23 



To the boiling hot filtrate add slowly, with stirring, 20 cc of a 10 per cent barium 

 chlorid solution ; let stand for two hours on the steam bath, filter, wash, ignite, 

 and weigh as barium sulphate. Calculate the total sulphate (SO 4 ), deduct 

 from it the soluble sulphate (SOO, and figure the remainder as lead sulphate. 



(d) LEAD AND ZINC. 



(1) IN THE ABSENCE OF OTHER METALS. 



Dissolve 1 gram by boiling fifteen minutes with 250 cc of water and 20 cc of 

 strong nitric acid, add 5 cc of sulphuric acid, evaporate, and heat until all of 

 the nitric acid is driven off and copious fumes of sulphuric anhydrid begin 

 to come off; cool, dilute, filter on a Gooch crucible, wash with 1 per cent sul- 

 phuric acid, ignite, and weigh as lead sulphate. Deduct from this the lead 

 sulphate obtained from the sulphate determination and figure the remainder 

 to lead monoxid. Evaporate the filtrate from the lead sulphate to 100 cc, cool, 

 add 5 grams of microcosmic salt dissolved in . water, then add ammonium 

 hydroxid until the solution is just neutral (test with litmus paper). Then 

 add 2 drops of ammonium hydroxid and 1 cc of acetic acid, stir vigorously, 

 heat on the steam bath for an hour (the precipitate should assume a crystal- 

 line character and settle well at the end of that time), filter on a Gooch cruci- 

 ble, wash with hot water, ignite at first at a very low heat and finally heat t 

 redness, and weigh as zinc pyrophosphate (Zn 2 P 2 O 7 ), calculate to zinc oxid 

 (Zn 2 P 2 O 7 X 0.53412 =ZnO). Deduct the zinc oxid present as zinc sulphate and 

 report the remainder as zinc oxid. 



(2) IN THE PRESENCE OF OTHER METALS. 



I 



With a sample containing calcium or magnesium salts the lead should be pre- 

 cipitated as sulphid from hydrochloric acid solution, then dissolved in nitric 

 acid and determined as sulphate. The filtrates from the lead sulphid can be 

 used to determine the zinc. If it is desired to make a gravimetric determination 

 of zinc, boil off the hydrogen sulphid, add a little bromin water to oxidize any 

 trace of iron, then add ammonium hydroxid in excess; only in rare cases will 

 any iron appear here, but if it does, filter, redissolve, and reprecipitate. Manga- 

 nese will be present only very rarely ; if present, it can be precipitated by add- 

 ing bromin and warming. Make the zinc solution acid with acetic acid, heat to 

 a boil, and pass hydrogen sulphid into the hot solution until it is completely 

 saturated (for twenty to thirty minutes). Allow the zinc sulphid to settle for 

 five hours, filter, wash with hydrogen sulphid water, dissolve the zinc sulphid in 

 hot dilute hydrochloric acid, boil off the hydrogen sulphid, and determine the 

 zinc as pyrophosphate, as described. 



The hydrogen sulphid method is very troublesome on account of the difficulty 

 of filtering and washing zinc sulphid, and when calcium or magnesium is 

 present it is generally better to determine the zinc volumetrically. This is best 

 done by Low's method. In the absence of iron and manganese take the filtrate 

 from the lead sulphid, add ammonium hydroxid until alkaline, then hydrochloric 

 acid until just acid, then add 3 cc of strpng hydrochloric acid, dilute to about 

 250 cc, and titrate with standard ferrocyanid just as in standardizing the 

 solution. When iron and manganese are present follow the method for oxidized 

 ores as described by Low. 



a Technical Methods of Ore Analysis, p. 209. 



