Methods for Testing Stains 295 



stains by the general methods described consisted of measuring the spectra of a 

 series of samples of recent origin and of satisfactory staining quality. Ratios of the 

 color density at the peak (D-peak) to previously recorded assay values were then 

 calculated and compared. If these ratios (Assay/D-peak) were reasonably constant, 

 the chemical assays were carefully repeated, the ratios recalculated, and the average 

 value used to serve as the necessary factor for spectrophotometric assay of the 

 stain. If the ratios proved not to be reasonably constant, either the spectrophoto- 

 metric data or the chemical assay was considered to be at fault. Spectrophoto- 

 metric assay was obviously unsuitable when representative samples of the stain 

 showed great variation in the position of the peak or other spectral characteristics. 

 In such cases the chemical assay proved to be the method of choice. 



In a few cases the titanous chloride assay was not entirely satisfactory, and it 

 was difficult to establish a figure for per cent purity. In such cases nitrogen analyses 

 were made and the per cent dye calculated on this basis. This figure in turn served 

 to standardize the spectrophotometric assay of the dye. 



It should be noted that the methods here reported are designed to provide a 

 quick chemical and spectrophotometric evaluation of commercial stains as they 

 are received for certification. As such, the standards set are based primarily on past 

 commercial samples and their satisfactory use in staining procedure. From the 

 strictly chemical standpoint it is recognized that "per cent dye" as determined by 

 titanous chloride, spectrophotometry, or color base precipitation cannot adequately 

 describe the true content of the dye specifically named. 



CERTAIN NITRO AND AZO DYES 



Qualitative and quantitative determinations of the dyes in this group and of the 

 ones following are, for the most part, the same as the general methods outlined at 

 the beginning of the chapter. Where special treatment is required it will be found 

 under the individual dye. 



Martius Yellow, C. I. No. 9 



Identification: Martius yellow is the monosodium or calcium salt of 2:4-dinitro-I- 

 naphthol. CioHsNoOsNa or (CioH5N205)2Ca. It is identified by the following 

 method: Dissolve 50 mg. of martius yellow in 250 ml. of distilled water. Dilute 

 10 ml. of this solution to 200 ml. with distilled water. Read in Beckman 

 spectrophotometer. Absorption maximum 431-440 m/x, ratio P-15/P+15 is from 

 1.00 to 1.12. 



A marked distinction is observed in the solubilities of the two salts in water, the 

 sodium salt being soluble to the extent of 4.5 parts in 100 parts of water at 26° C, 

 whereas only 0.05 parts of the calcium salt are soluble in 100 parts of water at the 

 same temperature. 



Method of Analysis: Dissolve 100 mg. of dye in 200 ml. of distilled water, add 

 10 g. of sodium acid tartrate, heat to boiling, and titrate with 0.05 N TiCU. A 

 change from orange to a straw yellow indicates the endpoint. The following data 

 are used in calculating the percentage of anhydrous dye in the original sample: 



(1) Molecular weight, CioH5N205Na 256.153 



Hydrogen equivalents per mol of dye 12. 



ml. of N/10 TiCls per gram of dye 468.467 



