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Co the Hydrogen-Ion Concentration

solution, temperature, liter, sea-water and ions

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THE HYDROGEN-ION CONCENTRATION, CO, TENSION, AND CO, CONTENT OF SEA-WATER.* Since the volume of water changes with the temperature, the most accurate method of expressing the results of water analysis is to take the kilogram of water as the unit. The older method was to express results in grams per kilogram. In volumetric analysis it is more convenient to express results in grams per liter, and this method is equally accurate provided the water is brought to a standard tempera ature, preferably 20°, for analysis, or the temperature is recorded in each case. Since the ratios of the weights of chemical elements or radicals that will enter into chemical composition with one another have been established with an accuracy more than adequate for oceano graphic work, it is still more convenient to express results in gram equivalents per liter.

A normal solution (abbreviated to n) is a solution containing in 1 liter the quantity of substance that will combine with 23 grams of sodium or 35.46 grams of chlorine. A mol-liter solution (abbreviated to in) is the molecular weight in grams (mol) of the substance con tained in a liter of the solution, and may or may not be a normal solution, and where confusion is likely to arise, m and not n should be used. For example: if enough is added to a normal solution of NaOH to make a 2 m solution of the former, the result is a 0.5 m solution of In the ordinary use of m we mean more correctly the formal concentration—i. e., the molecular weight is taken from the ordinary chemical formula, regardless of the fact that several molecular species containing the substance in question may occur in the solution.

Where a particular molecular or ionic species is intended, it should be designated. Thus a normal solution of hydrogen ions contains 1.008 grams (or for practical purposes about 1 gram) of hydrogen ions per liter, regardless of the amount of hydrogen in other chemical states. It is not always possible to reduce such a quantity to a standard temperature, since some hydrogen ions may disappear or be formed during the change in temperature. This particular case is simplified by the fact that the normal concentration of H ions per liter is not changed very much by the ordinary changes in temperature, as will be explained later. It is more convenient to abbreviate the names of positive and negative ions thus : li• = hydrogen ion, Cl' = chlorine ion.

Since concentrations per liter and per kilogram may be readily converted one into the other if the density of the solution is known, charts for computing the density of sea-water are given in figures 1 and 2. This gives the actual density in question, as it is obviously insufficient merely to know the specific gravity at some standard temperature. The direct determination of the specific gravity by means of a floating hydrometer is often vitiated by large experimental errors, and many of these instruments are made for a standard temper ature on the obsolete Reaumur thermometer scale (14 R. = 17.5 C.), which is often unattainable aboard ship or at seaside laboratories. The best practice is to titrate the chlorides of the sea-water with silver nitrate solution, using potassium chromate as indicator, and, if a constant temperature can not be maintained, to calibrate each new silver solution with standard sea-water. Since the temperature changes affect the volumes of the silver solution and sea-water about equally, they may be ignored. The concentration of sea-water was formerly expressed as the salinity or total salt-content, but it was found that the ratio of the chief salts in sea-water is remarkably constant, and the individual elements may be determined much more accurately than the total salts in one operation. Salinity is no longer deter mined directly, but is calculated from the chlorine titration according to the Sorensen-Knudsen definition. If it is desired to know the salinity (S) more accurately than can be determined with the aid of figure 1, the following conversion table (Knudsen) may be used: As already mentioned, the unit of the hydrogen-ion concentration is 1 n or about 1 gram of hydrogen ions per liter. The quantity actu ally found in sea-water is about 1 gram in 100,000,000 liters, or 0L000p0q01 n. In order to avoid the use of so much space, this number is usually expressed as which is nothing more than a minus logarithm to the base 10—in other words, a common logarithm with the sign changed. We might write it thus: —log I• =8, but Sorensen still further abbreviated it thus: = 8. It is necessary only to remember that when the hydrogen-ion concentration decreases the P. increases in the following manner: 0.1 n = P.1 0.00000001 n H' =P.8 n H' =P„14.

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