204 EJyPEEIMENT STATION EECORD. 



" Place 100 cc. concentrated sulphuric acid, free from nitrates, in a 250 cc. 

 beaker set in a large porcelain casserole full of cold water. Run the sample in 

 slowly from a 10 cc. pipette to the bottom of the acid, stirring meanwhile with 

 the pipette ; this procedure is designed to prevent loss of nitric acid fumes. Run 

 in the ferrous sulphate solution slowly in a fine stream with constant stirring 

 until the solution turns from yellow to faint brown or pink. Then rinse out the 

 pipette by sucking it full of the acid and draining it and continue titrating cau- 

 tiously to the first color change. The end point is clear to 0.05 cc. and different 

 operators should agree within 0.1 cc. 



" The casserole of water serves the double purpose of cooling the acid and 

 making the end point much clearer. It is sometimes necessary to halt the titra- 

 tion and let the solution cool. The temperature should never exceed 60° C. and 

 is better kept below 40°. Let the burette stand five minutes before taking the 

 reading, as the ferrous sulphate solution drains very slowly." 



With care the error in the method does not exceed rh ot the quantity of 

 nitric acid estimated. It is not suitable for traces. Constancy of conditions is 

 important. Chlorates, bromates, iodates, chlorids, bromids, and iodids interfere 

 but nitrites do not when sulphuric acid is used as a medium. The method is 

 recommended for technical purposes and results of tests made are given. 



Molybdic acid recovery, C. G. Abmstbong {Jour. Indus, and Engin. Chem., 

 7 {1915), No. 9, p. 764, fiff- !)• — Numei'ous methods for the recovery of molybdic 

 acid were tried but discarded on account of thoir impracticability or unnecessary 

 consumption of expensive reagents. A method was finally devised as follows : 



The waste molybdic acid residues wore filtered from any phosphomolybdate 

 precipitate that it contained. This was most conveniently accomplished by 

 means of an asbestos suction filter. 



" The filtered solution is then placed in a 5-liter German flask and supported 

 inverted over a large evaporating dish on a sand bath, allowing but a small 

 amount of the solution in the dish. This method allows a largo amount of liquid 

 to be evaporated with little attention. The flask may be refilled until the pre- 

 cipitate of molybdic anhydrid, which forms in the bottom of the evaiwrating 

 dish, becomes too bulky. 



" Remove the flask and evaporate the solution in the dish until it begins to 

 foam consi(leral)ly and there is just enough solution left to cover the precipitate 

 and keep the ii-on in solution. Cool, dilute with one-half volume of cold water, 

 allow to settle, and decant. Wash the precipitate thoroughly a couple of times 

 with water by decantation to remove the iron salts and treat with enough 1 : 1 

 ammonium hydroxid to fill the dish. A dark brown precipitate will form, due 

 to precipitated iron. 



"The whole is then washed into a large flask, warmed slightly, and allowed 

 to stand a couple of hours with an occasional shaking to facilitate solution. 

 When all is in solution, or after two hours, the liquid may be filtered oK by a 

 siphon, sand, and asbestos suction filter into another flask. Arrange the suction 

 tube so that the lower portion of the solution, containing the precipitate, will be 

 the last to come upon the filter, thus preventing troublesome clogging of the filter 

 by the iron precipitate. Add 5 per cent of the original amount of ammonium 

 hydroxid to the solution to make up for that used in precipitating the iron." 



This solution contains the molybdic anhydrid as ammonium molybdate and, 

 when the specific gravity of it is taken with a hydrometer at 25° C.. by referring 

 to an accompanying curve the percentage of molybdic anhydrid present may be 

 found and the proper amount of fresh molybdic anhydrid added to bring the 

 concentration up to any required strength. The solution of ammonium molyb- 

 date may be evaporated to dryness and then roasted to 600° to molybdic anhydrid. 

 The recovery in these tests, which were carried out on large amounts of residues 



