AGKICULTUKAL CHEMISTRY AGROTECHNY. 807 



nitric acid to dryness and then taken up with 10 cc. of water and 55 cc. of nitric- 

 acid. The solution is filtered through a small filter into an Erlenmeyer flask. 

 The dish and fifter are then washed with a measured amount of water until 

 35 cc. of filtrate is obtained. After adding 5 cc. of nitric acid and 1 gm. of 

 ammonium persulphate (in most cases 0.5 gm. is suflicient), the solution is 

 heated to boiling point. Then 10 drops of silver nitrate 1 : 20 is added, and 

 from the time a reddish color api>ears. boiled for 1 to I2 minutes. After cool- 

 ing, the solution is transferred to a clean, dry cylinder, made up to 50 cc. 

 with water, and compared with a standard solution of permanganate contained 

 in an identical cylinder. 



An iron content of 10 mg. per l.fKM) ce. does not affect the results. 



A note on the Hopkins and Cole modification of the Adamkiewicz test for 

 protein, Y. II. Mottram {Biochem. Jour., 7 (1912), No. 3, pp. 249-259) .—The 

 Hopkins and Cole modification of this reaction fails in the presence of small 

 amounts of oxidizing agents. " The following method of carrying out the 

 test yields excellent results: 



"One cc. of strong 'reduced oxalic' acid (Benedict's reduced oxalic will do 

 just as well) is mixed with 1 cc. of 1 per cent Witte peptone, and then 2 cc. 

 of sulphuric acid is run down the side of the test tube which is held as near 

 horizontal as convenient. The fiuids are then rapidly njixecl by shaking from 

 side to side with the test tube vertical. The quantities given are for a test 

 tube 1.5 cm. in diameter (the usual laboratory size). Subsequent addition of 

 one drop of 1 per cent ferric chlorid solution usually iucre.ises the density and 

 the blueness of the coloration. 



" The quantities given may be varied greatly and yet a good coloration be 

 obtained. One cc. or 5 cc. of sulphuric acid to 2 cc. of the mixture yields an 

 inimistakable result and the quantities may be guessed and not measured. 

 More sulphuric acid gives a bluer result than less." 



The estimation of tyrosin in proteins by bromination, R. H. A. Plimmer 

 and ELiz.\BETn C. Eaves (Biochem. Jour., 7 (1913), No. 3, pp. 297-310).— " The 

 estimation of small quantities of tyrosin — 0.01 to 0.04 gm. — can be effected by 

 J. H. Millar's " method of bromination when a more dilute solution of sodium 

 bromate is used, but it is preferable to modify his procedure by adding excess 

 of the reagent and titrating the nonabsorbed halogen with thiosulphate solu- 

 tion. u.sing potassium iodid and starch as indicator. 



" Tyrosin can not be directly estimated by bromination in the presence of 

 protein and its decomiwsition products^, since histidin and tryptophan also ab- 

 sorb bromin. Histidin can be removed by precipitation with phosphotungstic 

 acid. The absorption of bromin by tryptophan is not completely eliminated 

 after boiling with acids, so that tyrosin can not be estimated by this method 

 in solutions containing the products of acid hydrolysis of proteins which con- 

 tain tryptophan. Values for the tyrosin content of proteins, agreeing with 

 those obtained by isolation and weighing, are obtained when the bromin ab- 

 sorption of a tryptic digest is measured after an interval of about six hours." 



The separation of cystin and tyrosin, R. H. A. Plimmer (Biochem. Jour., 7 

 (1913). No. 3, pp. 311-317). — This work shows that cystin and tyrosin can be 

 separated from one another by a precipitntion with phosphotungstic acid. 

 Practically all of the tyrosin can be determined in the filtrate and washings. 

 The two amino acids may also be separated by a precipitation with mercuric 

 sulphate in 5 per cent sulphuric acid, but the cystin is incompletely precipitated 

 and the tyrosin which is recovered is impure. 



"Trans. Guinness Research Lab., 1 (1903), pt. 1, pp. 40-44. 

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