COLORED PIGMENTS. 29 



2. Graphite. 



Graphite is more of a gray pigment than a black. The finer grades are 

 used as lubricants and in making lead pencils. All commercial graphite con- 

 tains a considerable quantity of ash. For analysis, determine the moisture as 

 usual, boil with hydrochloric acid, filter, dry, and weigh, then ignite ; it may be 

 necessary to use oxygen to insure complete combustion; the loss is taken as 

 carbon. If a previous test has shown the absence of carbonates, the treatment 

 with hydrochloric acid may be omitted. Graphite used as a lubricant is some- 

 times adulterated with soft coal. Good Ceylon graphite is practically unaffected 

 by hot sulphuric acid and potassium sulphate, while soft carbon is completely 

 decomposed. A fairly good quantitative determination of soft coal in graphite 

 is made as follows: 



Determine the moisture and the ash in one portion, and calculate the total 

 carbon by difference. Treat 1 gram in a Kjeldahl digestion flask with 30 cc of 

 strong sulphuric acid and 2 grams of potassium sulphate. Heat for two hours 

 at a temperature about the fuming point of the sulphuric acid, cool, add water, 

 filter, wash, dry at 105 C., and weigh; then ignite until all of the carbon is 

 burned; the loss here represents the carbon as graphite, and the difference 

 between this determination and that of total carbon represents the soft carbon. 



COLORED PIGMENTS. 

 1. Yellow, Orange, and Bed Pigments. 



The most important yellow pigment is chrome yellow, which varies in shade 

 from a light lemon yellow to a deep orange. The lighter shades generally con- 

 tain lead sulphate as well as lead chromate, while the deep orange contains some 

 basic lead chromate. Pure chrome yellow should contain only lead chromate, 

 lead sulphate, and possibly some basic lead in the darker shades. Zinc yellow 

 is zinc potassium chromate of a composition approximating the following for- 

 mula : 



(a) CHBOME YELLOW. 



(1) MOISTURE. 

 Dry 2 grams for two hours at 105 C. 



(2) INSOLUBLE IMPURITIES. 



Treat 1 gram with 20 cc of strong hydrochloric acid, cover and heat on a 

 steam bath for fifteen minutes, add 100 cc of hot water, boil (the solution should 

 be complete), filter, wash, ignite, and weigh the insoluble impurities. 



(3) LEAD. 



Evaporate the filtrate from the insoluble to about 2 cc, but not to dryness, add 

 100 cc of 95 per cent alcohol, stir, allow to stand for one hour, cool in ice water, 

 filter on a Gooch crucible, wash with 95 per cent alcohol, dry for one hour at 



Lead sulphate may be replaced by other insoluble lead compounds. A 

 chrome yellow should be considered adulterated if it contains anything besides 

 insoluble lead compounds. 



