16 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



until used. Avoid drying. Measure in millimeters the three longest 

 roots on each seed and record the average length per solution on label 

 of culture with your name. 



Measure again after four weeks. Plot the data as curves with average 

 length as ordinates and composition of mixtures as abscissae (cf. Oster- 

 hout, "Injury, Recovery, and Death in Relation to Permeability," 1922; 

 also Rubenstein, Protoplasma, 4: 259, 1928). Text p. 26. 



HYDROGEN IONS 



2. Acidity Determined by Means of Taste. — Place the following N/10 

 acids in their order of sourness: hydrochloric, acetic, and boric. Dry 

 the side of the tongue and apply a small quantity of acid with a camel's 

 hair brush. Rinse the mouth after each application. Dilute a sample of 

 each acid ten times and repeat the experiment. 



3. Acidity Determined by Hydrolysing Power. — Make an iodine spot 

 plate determination of the prepared starch solution before beginning 

 experiment. Place 3 cc. of the starch solution in each of three test tubes. 

 Add 5 cc. of the N/10 acids to the test tubes. Raise all three to boiling 

 and boil for one minute in a water bath. Remove simultaneously and cool 

 quickly in tap water. Make the spot plate test. What is the order of 

 hydrolysis? Differentiate between "total acidity" and "hydrogen ion 

 concentration." 



4. Action of Buffers. — Titrate 20 cc. of the peptone solution with N/10 

 NaOH in steps of 0.5 cc. of the NaOH. As each 0.5 cc. of the base is run 

 in (stirring well) remove 5 drops of the solution to a spot plate and add 

 indicator. Begin with Brom thymol blue using the Clark's color chart. 

 As the pH increases use Phenol red and Cresol red in turn. Determine pH 

 for each step. Plot results, pH on ordinate; cc. of base on abscissa. 



5. Repeat the procedure in 4 using 20 cc. of distilled water in place of 

 peptone solution and running the NaOH in steps of 0.2 cc. in place of 0.5 cc. 

 Plot results upon the same axes as in 1. Explain difference between curves 

 1 and 2. Also criticize the experiment. Text p. 20. 



6. Reaction between CO2 and Buffers. — Prepare an alkaline buffer 

 solution and introduce CO2 from tank. Determine the pH. Repeat 

 using distilled water. What is the pH of pure water? 



Determination of pH. — Six unknowns (designated A-F) will be deter- 

 mined by indicators, Ex. 7, and if possible by potentiometric measure- 

 ments, Ex. 8 (hydroquinone electrode). 



7. The Indicator Method for Determining pH. — There are many varia- 

 tions of this. For rough determinations the spot plate and indicator chart 

 are used alone (as in the experiment above). In more careful determina- 

 tions the spot plate is used to ascertain the approximate pH range of the 

 unknown. Then definite amounts of acid and basic buffer salts are com- 



