68 CAKBOHYDRATES - [ch. 



True or normal cellulose. Of this substance, as we have said, 

 many cell-walls are composed. The most familiar form of cellulose is 

 cotton, which consists of hairs, each being a very long empty cell, from 

 the testa or coat of the seed of the Cotton plant (Gossypium herbaceum). 

 Crude cotton (i.e. the hair cell-walls) is not quite pure cellulose, but 

 contains a small amount of impurity from which it is freed by treatment 

 first with alkali and subsequently with bromine or chlorine. All kinds 

 of cotton material, cotton-wool, and the better forms of paper (including 

 filter- paper) may be regarded as almost pure cellulose. 



Pure cellulose is a white, somewhat hygroscopic, substance; It is 

 insoluble in water and all the usual solvents for organic substances. It 

 is, however, soluble in a solution of zinc chloride in hydrochloric acid in 

 the cold, and in a solution of zinc chloride alone on warming. It is also 

 soluble in ammoniacal cupric oxide (Schweizer's reagent). 



In addition cellulose is soluble in concentrated sulphuric acid, which 

 on standing converts it first into a hydrate and then finally into glucose. 

 If, however, water is added to the sulphuric acid solution as soon as it is 

 made, the gelatinous hydrate of cellulose is precipitated. This substance 

 is termed "amyloid" since it gives a blue colour with iodine. Concentrated 

 nitric acid converts cellulose into nitrates, of which one is the substance, 

 gun-cotton. In 10% alkalies cotton fibres thicken and become more 

 cylindrical. This procedure has been employed by Mercer to give a 

 silky gloss to cotton, and the resultant product is called mercerized 

 cotton. 



Expt. 71. The colour tests and solubilities of cellulose. 



(a) Dip a little cotton- wool into a solution of iodine in potassium iodide. Then 

 put the stained wool into an evaporating dish and add a drop or two of concentrated 

 sulphuric acid. A blue coloration is given. This is due to the formation of the 

 hydrate "amyloid" mentioned above. 



(6) Dip some cotton-wool into a calcium chloride iodine solution. (To 10 c.c. of 

 a saturated solution of calcium chloride add 0*5 gm. of potassium iodide and 0*1 gm. 

 of iodine. Warm gently and filter through glass-wool.) A rose-red coloration is 

 produced which eventually turns violet. 



(c) Heat a strong solution of zinc chloride (6 pts. of zinc chloride to 10 pts. of 

 water) in an evaporating dish and add 1 part of cotton-wool. The cellulose will in 

 time become gelatinized, and if a little water is added from time to time, a solution 

 will eventually be obtained on continuous heating. 



(d) Make a solution of zinc chloride in twice its weight of concentrated hydro- 

 chloric acid and add some cotton- wool. The wool will rapidly go into solution in the 

 cold. 



(e) Add some cotton-wool to an ammoniacal copper oxide solution and note that 

 it dissolves. (To a strong solution of copper sulphate add some ammonium chloride 

 and then excess of caustic soda. Filter off the blue precipitate of cupric hydroxide, 



