230 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Bang's Second Method. 



Bang l has found that if the reduction of cupric sulphate be carried out 

 in the presence of thiocyanate or chloride in carbonate solution, cuprous 

 thiocyanate or cuprous chloride is formed and is not precipitated, i.e. that in the 

 presence of excess of potassium thiocyanate or chloride the reversible reaction 



Cu 2 Cl a + K 8 CO 8 ^> Cu 2 O + 2KC1 + CO 2 



proceeds only in the right-hand direction. A cupric solution reduced by 

 glucose will contain cuprous chloride. Bang estimates the reduced copper 

 salt by titration in alkaline carbonate solution with standard iodine solution : 



CuCl + I + K 2 C0 3 = CuC0 3 + KC1 + KI. 



The solution which becomes colourless in the reduction again becomes blue 

 on oxidation, but if small amounts are used the colour is pale blue, and when 

 the iodine titration is carried out using starch as indicator, the deep blue 

 colour formed indicating excess of iodine is easily seen. 



The preparation of the reagents is given on p. 613. 



Procedure. 



Owing to the ready oxidation special precautions have to be taken 

 during the reduction by the glucose and the titration with the iodine 

 solution : cri or 0-2 c.c. (or more) gf the glucose solution are put into 

 a 100 c.c. Jena glass flask with a straight neck and no rim, and 55 c.c. of the 

 alkaline copper solution are added. 



The flask is fitted with a rubber tube 4-5 cm. long and 3 mm. thick, 

 leaving about 2 cm. projecting, and boiled for 3 minutes. Just before the 

 expiration of this time a spring clip 2 is put over the rubber and closed at 

 the end of the time of boiling. The flask is rapidly cooled under running 

 water. The clip is removed and the titration effected with 'iN, or *oiN, 

 or -04N iodine, 3 after adding 0-5-1 c.c. of the starch solution. 4 During 

 the titration the flask is only shaken gently to prevent access of air, but it is 

 advisable and preferable to pass into it a current of carbon dioxide by means 

 of a bent tube which can be fastened by a band to the flask. The amount 

 of iodine solution is proportional to the amount of glucose present. 



The number of c.c. used divided by 270 gives the amount of glucose in 

 gm. 5 



0-4 c.c. should be deducted ; this amount is generally absorbed by the 

 alkaline copper solution. 



The factor for 'iN iodine solution is '285, for 'c^N iodine solution 0-7 

 (*8 x 2-5). 



1 Biochem. Zs., 1913, 49, i. 



2 A special clip is made by Mekaniker Hill, Lund, Denmark. 



3 The latter are prepared by diluting a 'iN solution with boiled out water and can be 

 kept for three months in the dark without undergoing alteration. 



The 'OiN solution is conveniently prepared from iodide and iodate : 



KIO, + sKI + 6HC1 = 6KC1 + sH 2 O + 61 ; 



an equivalent quantity of iodine to hydrogen chloride is formed. It is made by pouring 

 i c.c. of 2 per cent, potassium iodate solution into a 100 c.c. measuring flask, adding 2 to 

 2*5 gm. of potassium iodide and 10 c.c. of *iN hydrochloric acid and filling up to the mark 

 with boiled out water. 



4 i per cent, solution of soluble starch in saturated potassium chloride solution. It 

 keeps indefinitely. 



6 26-5 c.c. *oiN iodine solution = 10 mg. glucose; 

 or 2-67 c.c. =i mg. 



