The Hydrogen-ion Concentration, etc., of Sea-Water. 



55 



ungreased stopcock was immersed in the KCl and the reading was 

 taken in the usual manner. 



The calomel electrodes were made from mercury redistilled in 

 Hulett's still (and plated on platinum so as to be portable), KCl was 

 recrystallized many times in quartz beakers, and calomel was made 

 by the electrolytic method of Lipscomb and Hulett. The 0.1 n KCl 

 calomel electrodes were made at 20° and hence have a very slight 

 error at 10° and 30° due to volume change of the KCl solution in them. 

 The KCl was weighed with standard weights and the weighings reduced 

 to vacuo. A saturated KCl calomel electrode was often used as an 

 intermediate on account of its lower resistance, but each time was 

 compared against the 0.1 n electrode. The saturated electrode often 



Fig. 19. — Capillary electrometer of increased 

 sensitivity. The large bulb is inclined 

 toward the observer by bending the neck 

 while the glass is soft, so that the elec- 

 trometer will lie flat on the stage of a 

 microscope. It is attached in this posi- 

 tion by means of rubber bands and the 

 microscope is tilted back until the tube 

 is horizontal. Wires are hooked in the 

 little rings, which are platinum loops 

 fused through the glass. The dotted rec- 

 tangle represents a cover-glass mounted 

 on the capillary with Canada balsam. 

 The capillary must be the smallest ob- 

 tainable and of thin enough walls to 

 admit of an 8 mm. objective and x 20 

 ocular in the microscope. A few specks 

 of dust in the ocular will serve to locate 

 the zero-point, and a micrometer scale is 

 not necessary. 



varied about 1 millivolt, although the readings were never taken in less 

 than 3 hours after the room was brought to constant temperature, 

 after a maximum fluctuation of about 2 degrees. 



Two special potentiometers made by Leeds and Northrup were used. 

 One was standardized by the Bureau of Standards and the other 

 compared with it. One of these is now at Tortugas and the other in 

 Minneapolis. The special features are the time-saving arrangement 

 and key which automatically short-circuits the capillary electrometer 

 when not in use. 



We had difficulties with the ordinary forms of capillary electrom- 

 eters, but found the slight modification shown in figure 19 satisfac- 

 tory. It was always necessary to make sure the electrometer was 

 not polarized before taking a reading. The sensitivity was made 

 sufficient by using the smallest capillary tubing obtainable and a 

 microscope of very high power, especially in the ocular. The electrom- 

 eter was attached to the stage of a tilted-back microscope with rubber 

 bands. If polarized, it is tilted sidewise until a drop of mercury 



