COLORED PIGMENTS. 31 



(b) ZINC YELLOW. 



(1) MOISTURE. 

 Dry 2 grains for two hours at 105 r. 



(2) INSOLUBLE IMPURITI l s. 



To 1 gram of the dry pigment add 25 cc of strong ammonium hydroxid, stir 

 well, and dilute to about 100 cc with water, stir frequently for about thirty 

 minutes, carefully breaking up all lumps, but do not heat. Filter and wash 

 with dilute ammonium hydroxid and then with water, dry residue at 105 C. to 

 constant weight, and weigh, then ignite and weigh again. If desired, dissolve 

 the residue in hydrochloric acid and analyze in the usual manner. 



(3) SOLUBLE ZINC, CHROMIUM, AND POTASSIUM. 



Heat the filtrate from the determination of insoluble impurities to boiling 

 in a graduated flask, pass in hydrogen sulphid until all of the chromate is 

 reduced and the zinc is precipitated as zinc sulphid and chromium as chromium 

 hydroxid. Fill to the mark, mix, allow to settle, draw out an aliquot of the 

 clear liquid, and determine alkalis by adding sulphuric acid, evaporating to 

 dryness, igniting, and weighing as mixed sulphates. If desired, potassium may 

 be determined as potassium platinic chlorid (K 2 PtCl ). Add hydrochloric acid 

 to the residue in the flask and determine zinc and chromium as in chrome 

 yellow. 



(4) IMPURE SAMPLES. 



The preceding methods will serve only for samples which are pure or adul- 

 terated only with substances insoluble in ammonium hydroxid. Very impure 

 samples should be examined as described under chrome yellow, page 29. 



(c) RED LEAD AND ORANGE MINERAL. 



These pigments in the pure state are oxids of lead (approximately PbsOO, 

 being probably mixtures of compounds of varying proportions of lead nionoxid 

 and lead dioxid. Moisture, insoluble impurities, and total lead may be deter- 

 mined by the methods given under chrome yellow; or, in the absence of alkaline 

 earth metals, the lead may be determined as sulphate in nitric acid solution 

 (dissolve by adding a few drops of hydrogen dioxid) by evaporating with an 

 excess of sulphuric acid until fumes of sulphuric anhydrid are evolved. Deter- 

 mine as sulphate in the usual way. 



The lead dioxid (PbO 2 ) may be determined as follows: Weigh 0.5 gram of the 

 very finely ground pigment into a 150 cc Erlenmeyer flask. Mix in a small 

 beaker 15 grams of crystallized sodium acetate, 1.2 grams of potassium iodid, 

 5 cc of water, and 5 cc of 50 per cent acetic acid. Stir until all is liquid, pour 

 into the Erlenmeyer flask containing the lead, and rub with a glass rod until 

 all of the lead is dissolved ; add 15 cc of water, and titrate with tenth-normal 

 sodium thiosulphate, using starch as indicator. A small amount of lead may 

 escape solution at first, but when the titration is nearly complete this may be 

 dissolved by stirring. The reagents should be mixed in the order given, and the 

 titration should be carried out as soon as the lead Is in solution, as otherwise 

 there is danger of loss of iodin. One cubic centimeter of tenth-normal sodium 

 thiosulphate corresponds to 0.011945 gram of lead dioxid, or 0.034235 gram of 

 red lead (Pb 3 O 4 ). Test red lead for dye and water soluble; look for carbonate, 

 nitrate, nitrite, and sulphate. 



