New Apparatus for Electrometric Determination of Hydrogen-Ion Concentration and for Gas Analysis

mercury, bulb, meniscus, stopcock and water

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The apparatus was completely filled with mercury and exhausted once by closing the upper stopcock and lowering the leveling bulb, in order to practically free it from air. The bulb was raised again and the air-bubble forced out of the top. The mercury in the cup C was allowed to run down until it filled only the capillary neck. The leveling bulb was lowered below B and 10 c.c. of sea-water were allowed to run quickly from the tonometer electrode into the cup C, and then quickly into the burette B, care being taken that the capillary neck of C remained full of sea-water. 1 c.c. of 2 n HC1 (i. e., acid with about the same absorption coefficient for the elementary gases as the average sea-water) was introduced into C and allowed to run down into B. A little mercury was placed in C and some of it was allowed to run down into B, so that the hole in the stopcock was filled with it and closed. If the stopcock had two holes, as in the figure, the other hole and connecting tube had already been filled with mercury in filling the apparatus with mercury. This mercury effectively sealed the middle of the stopcock, and it seldom leaked, especially if more grease was smeared over the two ends of the stopcock. The stopcocks were usually greased again for each determination. Air getting in at first makes no theoretical difference, but leakage may ruin the determina tion by occurring at the last.

By lowering the leveling bulb, the mercury meniscus was brought to the 50 c.c. mark (100 c.c. in fig. 21). The lower cock was closed and the apparatus removed from the clamp and held with the left hand at the attachment of the rubber tube and the right hand at the upper tip of the cup C, and shaken violently laterally while in the vertical posi tion, for 2 minutei exactly. In this way the sea-water was brought into approximate equilibrium with the gas pumped out of it. The appara tus was clamped again and the lower cock turned so that the sea-water passed into the trap T, the screw being adjusted so that the water meniscus was brought exactly to the upper entrance to the hole in the lower stopcock. One minute was allowed for the sea-water to drain then the lower stopcock was reversed and the leveling bulb raised so that the gas in the gas burette B was brought to atmospheric pressure. One minute was allowed for the moisture caught between the glass and the mercury to rise, and then the sea-water on top of the mercury was carefully measured and recorded. It should be about 0.05 c.c. The leveling bulb was then raised sufficiently to make the mercury meniscus rise half the height of the water column. The lower stopcock was closed and the leveling bulb brought back to the same level as the mercury in B.

The apparatus was now removed from the clamp and placed hori zontally, with the trap T upwards. By little jerks the mercury was

forced into B, thus displacing the gas and water into the wide part of the apparatus. It was shaken in this position 1 minute, and returned to the clamp, care being taken that no water worked around from T, and rose into B when the lower cock was opened again. On opening this lower cock, the mercury meniscus should not rise or fall more than a small fraction of a millimeter. The leveling bulb was now carefull3 adjusted with the screw so as to bring its meniscus on exactly the same level as the mercury meniscus in the burette B. This is not at first ai easy matter, but may be assisted by placing the leveling bulb directl: behind the burette and placing 2 specks of dust on the mercury in ii so that it is possible to bring the meniscus in B and the two specks ( dust in the leveling bulb in line.

A cylindrical bulb should be used, as a pear-shaped bulb is liable lead to error from refraction. The water meniscus is now careful read, so as to determine the total gas volume, and recorded. It is great advantage to have the graduations on B run at least half-wl around the tube so as to avoid parallax errors, as an error of 0.001 c means an error of 0.1 c.c. per liter. We used a thermometer le for reading the meniscus. About 0.5 c.c. of half-normal NaOH was placed in C and allowed to run down into B a little a time while tapping the apparatus.

Complete absorption of usually took place in about 5 at 20° or 10°, but at 30° the vaseline floated out of the stopcock gr.( and over the surface of the alkali, and it sometimes required 30 mini for complete absorption. Half-normal NaOH has about the si absorption coefficient for the elementary gases as the average water, but it may be possible to use a stronger solution without mal gas-bubbles rise from the little sea-water above the mercury. We prefer the half-normal solution, however, because it is completely neutralized and washed out at the end of an experiment by the sea water from the trap T, and it is only necessary to grease the stopcocks before another determination. In reading the gas-volume after ab sorption it is necessary to level the bulb, reckoning of the NaOH column as mercury.

The calculation of the results are a little complicated, but it was impossible to make a conversion table possessing great accuracy. We found the gas burette in one apparatus correct for the upturned meniscus, but it was therefore incorrect for measuring the water between the upturned and downturned menisci. A small correction in cubic millimeters given in the following table must be added to the volume of water above the mercury as measured by the scale on the gas burette B, depending on the total length of this scale in centimeters.

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