532 



ILL USTRA TI VE EXPERIMENTS 



Acacia per cent., 2-5 in all tubes. 



Determine the viscosity of the mixture in each tube. The lowest viscosity 

 will be found in tubes 4 and 5. If a new series of tubes is prepared with 

 concentrations of acid lying between these values, e.g. 0-25 N to 0-125 N, the 

 lowest reading will be obtained with about 0-2 X . HCl. 



32. Determination of the Isoelectric Point of a Protein (p. 91). 



(a) Preparation of solutions of definite {temporary) pH. Seven clean, dry 

 tubes are treated as under. 



Place 2 c.c. of distilled water in tube 1 and 9 c.c. in each of the other tubes. 

 To tube 1, add 16 c.c. N/10 acetic acid. Mix and transfer 9 c.c. of the mixture 

 to tube 2. Mix and transfer 9 c.c. to the next tube, and so on, rejecting the 

 9 c.c. withdrawn from tube 7. 



Determination of the isoelectric point of casein. Put 1 c.c. of a casein sol 

 (q.v.) into each of seven clean, dry test tubes. Add the contents of acid 

 tube 1 {pB. 3-8) to the first casein tube, tube 2 (pB 4-1) to the second casein 

 tube, etc. Shake each of the tubes and record your observations on a chart 

 as below. — = no change, = opalescence, + = precipitate. 



Tube 4 (pB = 4-7) shows the greatest change, and so it is inferred that the 

 isoelectric point of casein lies near that value. 



(b) Determination of the isoelectric point of gelatin. Make up a series of 

 buffer solutions of pB 4, 4-2, 4-4, 4-6, 4-8, 5, 5-5 from the phthalate series 

 given in Experiment 25. To each of a series of seven clean boiling tubes 

 containing 10 c.c. of these buffers, add 1 gm. of powdered gelatin and 1 c.c. 

 of M/128 potassium ferrocyanide solution. To another similar series, add 

 I c.c. of M/128 copper acetate solution. After the gelatin has been in contact 

 with the ferrocyanide or the copper solution at a definite pB for about an 

 hour, pour off the supernatant fluid, wash several times with cold water to 

 remove any metal salt not combined with the gelatin and dissolve the gelatin 

 by adding warm water to each tube and immersing the tubes in warm water 

 (40° C). The volume is made up to 50 c.c. Allow to stand for two or three 

 days. The tubes of the first series, in which the pB was less than 4-7, turn 

 blue because the gelatin forms tlie cation of a salt in which Fe(CN)g is the 



