COLORED PIGMENTS. 



35 



(a) PRUSSIAN BLUE. 



Under the name Prussian blue are included all ferrocyanid blues such as 

 Antwerp blue, Chinese blue, TiirnmiU's blue, etc. These blues are all ferric 

 ferrocyanids, ferrous ferricyanids, or double iron potassium salts of hydro- 

 ferrocyanic or hydroferricyanic acids. The analysis of these blues, as is gen- 

 erally the case with pigments, does not necessarily give results which can be 

 used to grade samples, the strength and color tests being most important. Most 

 text books say that Prussian blue is ferric ferrocyanid, Fe 4 [Fe(CN)e]8, but 

 this substance is not known commercially. Commercial Prussian blue is a 

 mixture of Williamson's blue, KFe[Fe(CN) 6 ], with other iron-alkali cyanids 

 and often with aluminum-iron cyanids, altogether a most complex substance. 



(1) MOISTURE. 



For the determination of moisture dry 2 grams for two hours at 100 C. 

 blue should contain less than 7 per cent of moisture. 



(2) INSOLUBLE IMPURITIES. 



Dry 



Ignite 1 gram in a porcelain dish at a low temperature. The ignition should 

 be carefully carried out at a temperature just high enough to decompose the 

 last trace of blue, but not high enough to render the iron insoluble, or very 

 difficultly soluble in hydrochloric acid. Cool, add 15 cc of hydrochloric acid, 

 digest for an hour on the steam bath covered with a watch glass, evaporate to 

 a sirup, add water, boil, filter from the insoluble, wash, ignite, weigh, and 

 determine the nature of the insoluble, probably barium sulphate. In pure Prus- 

 sian blue solution should be complete. 



(3) TOTAL IRON. 



Decompose as in the foregoing determination, reduce, and determine the iron 

 in the ordinary way. There should not be less than 30 per cent, calculated on 

 the dry pigment. 



(4) TOTAL NITROGEN. 



Determine on a 1 gram sample by the official Gunning method, digesting for 

 three hours. 6 The percentage of Prussian blue may be obtained with suffi- 

 cient accuracy for commercial purposes by multiplying the percentage of nitro- 

 gen by 4.4 and by multiplying the percentage of iron by 3.03. Eight samples 

 of pure Prussian blue examined by Parry and Coste gave the mean results 

 from which these factors are calculated. The following table shows the accu- 

 racy with which these factors give the percentage of Prussian blue in the eight 

 samples : 



Parry and Coste's determination of the percentage of Prussian blue in the dry 



matter of eight samples. 



a Parry and Coste, The Analyst, 1896, 21 : 225-230. 



6 U. S. Dept. Agr., Bureau of Chemistry, Bui. 107, Revised, p. 7. 



