VISCOSITY 



531 



water. (4) The movement of very viscous fluids may be initiated by slight 

 suction applied to the short arm. (5) Clean the tubes immediately after use. 

 They are best cleaned by a strong reagent like cone. HNO3 (depending on 

 the fluid last measured), followed in quick succession by distilled water, 

 alcohol and ether. Hot air is passed through the tube to dry it. 



{a) Compare the viscosities of (i.) water, (ii.) 1 per cent, sodium ddoride 

 (iii.) colloidal iron, (iv.) 1 per cent, starch, and (v.) 1 per cent, gelatin. 



(b) Effect of inecha II ical agitation on the viscosity of a f/elatiii solution. Pre- 

 pare a 0-5 i^er cent, solution of gelatin and divide it into two parts A and B. 

 Leave A undisturbed for 12-24 hours. Cause B to be agitated for a few 

 minutes before the final experiment by sucking it up and blowing it down a 

 capillary pipette or by blowing bubbles through it. Compare the viscosities of 

 A and B. 



(c) Effect of concentration on viscosity. Prepare various concentrations of 

 gelatin and of aqueous acacia resin, e.g. 0-5 per cent., 2-5 per cent., 5 per cent., 

 10 per cent., and 15 per cent. Compare the viscosities. (The gelatin experi- 

 ments will have to be carried out at about 20-30° C.) 



(d) Effect of temperature on the viscosity of gelatin. A 5 per cent, solution is 

 divided into three tubes, and after standing overnight the tubes are put for 

 half an hour {a) into a freezing mixture, (b) into water at 40° C, and (c) into 

 water at room temperature. Compare viscosities. 



(e) Effect of electrolytes on the viscosity of gelatin. A 1 per cent, solution of 

 gelatin is taken, and the following dilutions and admixtures prepared : 



(i.) 10 c.c. 1 per cent, gelatin + 10 c.c. water. 



(ii.) 



(iii.) 



(iv.) 



(v.) 



(vi.) 

 (vii.) 



N 



9 c.c. water + 1 c.c. ^ HCl 



N 



100" 



10 c.c. — NaOH. 



10 c.c. ~ HCl 



N 



10 

 N_ 



100 



10 c.c. N NagSO^. 

 10 c.c. N NaCNS. 



10 c.c. 



NaOH. 



(/) ^Jl^'^'f ^I '^(i'f'yiitg the pH 0)1 the viscosity of acacia. Make a 5 per cent, 

 aqueous solution of acacia resin and dilute it with hydrochloric acid as under 

 to give the following concentrations of H^ in a 2-5 per cent, solution of acacia. 

 Take six test tubes and put 5 c.c. of water in each. Leave the first tube as 

 control, and add 5 c.c. 2 N . HCl to the second tube, mix and transfer 5 c.c. of 

 the mixture to the tliird tube, and so on, rejecting the 5 c.c. of the mixture 

 taken from tube 6. Now add .5 c.c. of the 5 per cent, acacia solution to each 

 tube. 



34-S 



