METHODS OF ANALYSIS. 61 



CATTLE-FOOD MATERIALS. 



Moisture. 

 Use the method given for determining moisture in the foliage of trees (p. 57). 



Total arsenic. 



Testing reagents. — Make a blank on the reagents and apparatus by boiling down 100 

 cc of nitric acid and 5 cc of sulphuric acid in the same kind of flask used for treating 

 the samples, diluting and boiling down again to get rid of final traces of nitric and 

 nitrous acid, again diluting and finally running for two hours in the Marsh- Berzelius 

 apparatus, described in the following section. 



Apj)aratus. — This consists of a small flask with a wide mouth, holding about 100 cc. 

 Through one hole of a rubber stopper a separatory funnel passes to the bottom of the 

 flask. Through another hole in the rubber stopper passes the exit tube, which is in 

 turn joined to a Liebig bulb containing a small amount of a solution of lead acetate 

 to absorb any selenium, tellurium, or sulphur. This in turn is joined to a calcium 

 chlorid tube, which is then attached to a long tube of very resistant glass. This glass 

 tube is drawn out at one point rather small, nearly capillary, and the heat is applied 

 about 0.5 inch before the drawn-out portion. Heat with two Bunsen burners, neither 

 of which plays upon the glass directly but upon a piece of fine wire gauze wrapped 

 around the glass. When preparing this apparatus for use, add about 3 grams of 

 arsenic-free granulated zinc to the wide-mouthed bottle and then about 30 cc of 

 arsenic-free sulphuric acid (1 to 8). Let the apparatus run for fifteen minutes, after 

 which apply heat for approximately twenty minutes 'to be sure that no arsenic is 

 present. 



The apparatus is now ready for the addition of the extract from the cattle food. 

 Kun in and allow the action to continue for from one to two hours. Compare mirror 

 thus formed with mirrors containing known amounts of arsenic, which have been 

 previously prepared with the same apparatus. The standard mirrors usually contain 

 the following amounts of arsenic: 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.07 

 mg of arsenic. These mirrors should be in sealed tubes and be remade about every 

 three weeks, as they tend to fade a little upon standing. When not in use, they 

 should be kept in a dark place. 



In case a larger amount than 0.07 mg of arsenic is obtained, cut off that part of 

 the tube where the mirror is deposited, wipe carefully, and place in a desiccator. 

 Then weigh on an assay balance capable of weighing to the fifth place. Dip the 

 tube in sodium hypochlorite solution to dissolve the arsenic, wash with distilled water, 

 then with alcohol, and finally with ether, dry, place in a desiccator, and weigh again. 

 The difference in the two weights represents the arsenic from the amount of material 

 taken. Both of these weighings must be made with the utmost care, as a small error 

 at this point will cause a large error in the results. 



Standard solution. — To prepare the standard solution, dissolve 0.5 gram of dry 

 chemically pure arsenious oxid in a sodium carbonate solution (free from arsenic) 

 by boiling. Make weakly acid with arsenic-free sulphuric acid and make up to a 

 liter. Measure out such a quantity of this solution in a liter flask as will give 0.01 

 gram of metallic arsenic and make up to the mark. Each cubic centimeter of this 

 solution represents 0.01 mg of arsenic and is used for making the standard mirrors. 



Method. — Weigh 1 to 3 grams of the finely ground cattle food, according to the 

 amount of arsenic present, and transfer to a 200 cc Jena flask. Add 25 cc of concen- 

 trated arsenic-free nitric acid and 5 cc of concentrated arsenic-free sulphuric acid, 

 allow the first violent action to subside, and then put on an iron plate and gradually 

 heat to boiling. Continue boiling till the liquid begins to darken. When this occurs 

 add concentrated arsenic-free nitric acid in quantities of 3 cc at a time, until on 

 further heating it continues colorless and fumes strongly of sulphuric acid. Cool, 



