No. 630] SHORTER ARTICLES AND DISCUSSION 



metric methods of determining hydrogen ion concentration (a 

 convenient form is supplied by the Hynson. Westcott and Dunn- 

 ing Co. of Baltimore, but it is easy to improvise one out of mate- 

 rials in hand in any laboratory) a fine-pointed pipette, a medium- 

 sized test tube, a large flask, and a few pieces of glass tubing. 

 For the pipette, tubing, etc., Pyrex or Nonsol glass should pre- 

 ferably be used; if ordinary glass be employed it should be 

 coated with paraffin where it comes in contact with the solutions. 



The first step in making a determination is the preparation of 

 an indicator solution which is in exact equilibrium with the out- 

 of-door air. This may conveniently be done as follows : A two- 

 liter flask is filled with tap water, taken to an open window or 

 out of doors and all of the water except about 100 c.c. is slowly 

 poured out, great care being taken that neither the breath of the 

 operator nor any currents of air from the laboratory come near 

 it at this time. Enough of a concentrated solution of the indica- 

 tor (phenolsulphonephthalein) is added to give a color of the 

 proper intensity, the flask is stoppered and vigorously shaken for 

 several minutes. If the tap water is not nearly in equilibrium 

 with the carbon dioxide of the air, as shown by any decided 

 change in color on shaking, the solution should be poured into a 

 second flask from which water previously brought more nearly 

 into equilibrium with the air has been emptied, and shaken for 

 several minutes more. The solution, when in equilibrium wi'l; 

 air at 16° C. (where the absorbtion coefficient is approximately 

 equal to unity), and at a pressure of 760 mm. of mercury, con- 

 tains approximately 0.3 c.c. of CO, per liter, or 0.0006 mg. per 

 c.c. The exaet amount need not be determined, however, since 

 it is a constant quantity in all of the tubes used, and it is the 

 amount added to it which is significant. To secure the benefit 

 of the most sensitive part of the range of the indicator, the solu- 

 tion thus prepared should have a p H of approximately 7.6 to 

 7.8, i.e., it should have a decided pink color with very little trace 

 of orange. If the tap water is not alkaline enough to produce 

 tins result, a few drops of very weak NaOH may be added before 

 the final shaking. If the tap water is too alkaline, it may be 

 diluted with distilled water. The use of tap water « 



merely 



