24 MASS. EXPERIMENT STATION BULLETIN 379 



COLORIMETRIC METHODS EMPLOYED 



As has been stated, colorimetric methods were employed for the determination 

 of the mineral elements, based on visual readings with a color comparator with 

 filters, such as a Duboscq. These instruments are subject to personal error and 

 are less satisfactory than a spectrophotometer but were the only apparatus 

 available. 



The fundamental theory underlying the use of "Colorimeters" to obtain maxi- 

 mum sensitivity is acknowledged by most analysts but is not properly observed 

 in practice. It requires a preknowledge of the transmission curve of the solute 

 under examination and the selection of a filter (complementary) having its 

 highest transmission in the spectral region where the solute exhibits maximum 

 absorption. Such narrow band color filters, however, are relatively dense and 

 their use is precluded in some instances by their inability to transmit sufficient 

 rays (monochromatic) for accurate measurement from the light provided. This 

 necessitates a filter of wider band and of less selectivity with probably some 

 sacrifice in accuracy. 



The color filters recommended in the text are based on actual determinations 

 made by P. A. Clifford and A. K. Klein of the Food and Drug Administration 

 on the different colored solutions employed for trace metals. The Corning Glass 

 Works supplies the various units necessar}' for the combined filters. 



The so-called C. P. chemicals used in the analyses have occasionally proved a 

 source of trouble. For instance, iron has been found in the mineral acids in 

 appreciable amounts and copper in the citric acid. Some lots of potassium 

 thiocyanate failed to produce a stable color with iron, and potassium periodate 

 to oxidize manganese salts to manganic acid. One lot of ammonium molybdate 

 was largely insoluble, and amino-naphthol-sulfonic acid gave rise to a precipitate 

 in the phosphate solution, etc. In some cases purification was possible, but in 

 others replacement was the only solution. 



The methods are based on well-known reactions and have proved entirely 

 satisfactory in the hanas of many workers in this laboratory over a long period 

 of time. Many new processes have appeared in the literature within recent 

 years, but there appears to be no particular advantage in their adoption. 



Preparation of Ash Solutions 

 Wet Combustion 



Reagents 



Sulfuric acid, sp. gr. 1.84, 95%. 



Nitric acid, sp. gr. L42, 70%. 



Water redistilled from glass. 



The acids should be free from all elements under determination, as results cor- 

 rected for impurities are less dependable. Perchloric acid, ammonium persulfate, 

 and other oxidizing agents are occasionally employed but are seldom necessary. 



Method 



Transfer 10 grams of finely ground (1 mm.) air-ory material to a 500 ml. 

 Kjeldahl flask, add 8 to 10 glass beads (6 mm.) and concentrated nitric acid, drop 

 by drop, from a separatory funnel until frothing ceases and the organic matter is 

 largely disintegrated. Add 20 to 25 ml. of concentrated sulfuric acid, heat over 

 a low flame, and continue the addition of nitric acid until the solution becomes 

 colorless. 



Cool, add 25 ml. of water, and boil until the nitric acid has been expelled. 

 Cool, add 200 ml. of water, shake occasionally, and allow to stand over night. 



