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 falls out of the capillary. A cover-glass is mounted on the capillary with balsam.
Before using the conversion table for converting millivolts into P. a slight correction given in the following table was added to the millivolt reading, on account of the rarefaction of the hydrogen.
The above table is calculated on the hypothesis that the potential of the hydrogen electrode in a normal solution of H ions depends on the concentration and not solely on the pressure of the hydrogen. Hence the value of the hydrogen electrode against the 0.1 n KC1 calomel electrode is not 337 millivolts as used in case no correction for hydrogen pressure has been applied, but a slightly higher value. The average of the determinations listed by Clark and Lubs for temperatures up to 40° (which is the highest necessary in biological work) is 338.15, and we have rounded off this value of 338 for all temperatures between 0° and 40°.
After adding the correction from the above table, the millivolt reading was converted into P. by the conversion table in figure 20, which was cut into five sections to save space. The larger numbers (1-10) denote the numbers 338 to 950 denote millivolts, and the vertical lines temperatures, as indicated below. The double horizontal scale above is for converting the fractional part of the P. into the coefficient used in expressing the hydrogen-ion concentration, tenths of a P. being denoted by the arrows above, and the coefficient by the finely divided scale below.
The table at 10°, near bottom of page 57, serves to extend the range, and the table at 30°, just below, serves as a guide to figure 20.
In choosing the size of the apparatus for the analysis of the total in the sea-water, it was necessary to consider the fact that we had to bring the water sample to a known tension before the analysis. We found it impossible to make more than a liter of the gas mixture correctly. We made gasometers on a larger scale, but they were unsatisfactory. It was also impossible to bring a large sample of sea water to a known tension with only 1 liter of gas mixture. We finally chose the micro-apparatus used by Van Slyke in determining the total in blood-plasma. The large compartment of our appa ratus held 50 c.c., but we would recommend its increase to 100 c.c. The form we used in the first part of the work was essentially like figure 21, but later we cut off the absorption pipette P, and absorbed the by running NaOH solution directly into the gas burette B.
The following description applies to the 50 c.c. apparatus without the absorption pipette. We would recommend the substitution of a simple 2-way stopcock for the one at the top.
The apparatus was clamped upright to a vertical iron rod rising 6 feet from the floor, and a horizontal rod was placed under the lower stopcock to prevent it slipping out of the clamp. The ordinary clamps used in chemistry were used, and the vertical rod supported in the middle by attaching it to the edge of a table with a staple, and at the bottom by inserting it into a hole in a board nailed to the floor. A leveling bulb with cylindrical sides was attached by means of soft rubber suction tubing (1 mm. bore and 800 mm. long) to the lower end of the apparatus. The leveling bulb was attached by means of a swivel to a screw (of about 6 nun. pitch) and passing through a nut held in a burette clamp on the rod. Another burette clamp was attached to the rod about 800 mm. lower down, and the nut could be changed from one clamp to the other in a few seconds. The stopcocks were greased with a mixture made by dissolving pure gum-rubber (containing no sulphur) in boiling paraffin, and thinning with vaseline. This preparation was satisfactory at 10° and at 20°, but a more satis factory preparation at 30° may be prepared from sapota-tree sap.