18 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



bined. The pH of these combined solutions has been carefully deter- 

 mined electrometrically. Combinations are made having a pH in the 

 region of the unknown. Like amounts of indicator (10 drops) are then 

 added to 10 cc. of the unknown and each one of the standards. The colors 

 are matched in a comparator block. Sometimes the color of the unknown 

 solution falls between that of two standards. The pH value is then 

 interpolated. 



When an unknown solution possesses a turbidity, which will prevent 

 direct matching, the so-called "principle of Walpole" is used. This will 

 be demonstrated. Text p. 20. 



8. Potentiometric Method. — With the quinhy drone electrode deter- 

 mine the pH of the unknowns. Compare with results from indicator 

 method. Text p. 21. 



The theory of indicators is discussed by Clark, "The Determination 

 of Hydrogen Ions." For mixtures of primary phosphate KH2PO4 and 

 secondary phosphate Na2HP04 and for Mcllvaine's standards (Na2HP04 

 and citric acid) see Table 1 on p. 110. The quinhydrone electrode is 

 described by Clark, 2nd Ed., p. 289 and Michaelis, "Hydrogen Ion Con- 

 centration," 1926, p. 171. For the nature of the oxonium ion OH3 

 cf. Barnes and Jahn, Quart. Rev. Biol., 9: 306, 1934. 



IV. COLLOIDS 



1. Adsorption. — (a) Add powdered charcoal to a solution of crystal 

 violet in a test tube until color disappears. Filter off charcoal and wash it 

 with acetone acidulated with a few drops of dilute HCl. Explain return 

 of color. 



(b) Dip filter paper in methylene blue solution (positive) and set aside. 

 Dip another paper in eosin (negative). Wash both in distilled water. 

 Compare permanence of stains. What charge does the paper bear? 



2. Imbibition. — (a) Place 2 cc. granulated gelatin in each of six similar 

 test tubes. 



In No. 1 place 15 cc. distilled water 



No. 2 place 15 cc. N/10 HCl 



No. 3 place 15 cc. N/10 NaOH 



No. 4 place 15 cc. N/5 NaCl 



No. 5 place 15 cc. of equal parts N/20 HCl and N/10 NaCl 



No. 6 place 15 cc. of equal parts N/20 NaOH and N/10 NaCl 

 Determine effect on imbibition by measuring rate of swelling. Shake 

 after adding liquids. Observe effect of acid and alkali, and of salts alone 

 and in presence of acid. Cf. Bayliss, p. 100. According to Pauli, dis- 

 sociated salts of protein are formed by acid or alkali and the swelling is 

 due to the affinity for water of the protein ion. 



