74 SUBSTANCES SOLUBLE IN WATER. 



mercuric cyanide and 100 cc. of caustic soda (sp. gr. 1-145) in a 

 litre of water ; 0-4 gram of the cyanide = 40 cc. of solution mdi-' 

 cate 0-10 gram glucose. Knapp determines the end of the ex- 

 periment by touching a drop of the solution on filter paper with 

 a drop of sulphide of ammonium. An excess of mercury would 

 produce a brown colour ( 200). 



Instead of Knapp's solution an alkaline solution of potassio- 

 mercuric iodide may be employed for estimating glucose, as re- 

 commended by Sachsse. The sugar solution for this reagent may 

 be prepared'as directed in 83. The reagent as first recommended 

 by Sachsse 1 contained a large excess of alkali, which rendered the 

 estimation of dextrose and levulose in the presence of saccharose 

 inaccurate. Heinrich 2 therefore altered the composition by re- 

 ducing the amount of alkali to a minimum, and directed that a 

 litre should contain 18 grams of mercuric chloride, 25 of iodide 

 of potassium and 10 of caustic potash. 40 cc. indicate 0'1342 

 gram glucose. The titration is made in the same way as with 

 Fehling's solution, and the end of the experiment determined by 

 testing a drop with stannous chloride, which should not throw 

 down a grey precipitate, showing that no excess of mercury 

 remains in solution. The presence of ammonium salts does not 

 interfere with the reaction. Nessler's reagent for ammonia has a 

 composition similar to Heinrich's modification of Sachsse's solu- 

 tion, but contains a far larger quantity of caustic alkali, which is 

 necessary for the detection of ammonia ( 97). 



The appearance of the final reaction is retarded if the solution 

 contains but very small quantities of invert-sugar. It is advisable 

 to make the mercurial solution of such strength that 5 cc. indicate 

 0'0168 gram of invert-sugar. 



Glucose may also be estimated gravimetrically by using an acid 

 solution of a mercuric salt. The reagent recommended contains 

 in a litre 30 grams of mercuric oxide, 25 of concentrated acetic 

 acid and 30 of chloride of sodium. On boiling with sugar the 

 mercury is reduced and may be weighed as mercurous chloride. 3 

 5-88 parts of calomel indicate 1 part of glucose. 



1 Jahresb. f. Pharm. 1876, 375 (Journ. Chem. Soc. xxxii. 226). See also 

 Strohmer u. Klauss, Chem. Centralblatt, 1877, 697, 713 (Journ. Chem. Soc. 

 xxxiv. 246). 



. 2 Chem. Centralblatt, 1878, 409 (Journ. Chem. Soc. xxxvi. 180). 

 3 Jahresb. f. Phaxm, 1877, 340. 



