66 CARBOHYDRATES AND THEIR [CH. 



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 / alkalis cotton fibres 

 thicken and become more cylindrical. This procedure has been em- 

 ployed 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 absolution 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 O'l 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 solutiQii 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, 

 wash well, dry thoroughly, and dissolve in strong ammonia.) Add strong hydrochloric 

 acid and the cellulose is precipitated out again. Then add water and wash the 

 precipitate until it is colourless. Test the roughly dried precipitate with a little 

 iodine and strong sulphuric acid. A blue coloration is given. 



