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Calibration of an Improved Indicator Method for Use on Shipboard and in Poorly Equipped Laboratories

tubes, column, sea-water, buffer and salinity

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After the excellent work of Sorensen and his associates on the indicator method it might be supposed that the last word had been spoken on this subject. But there are several disadvantages in Sorensen's method. The solutions have to be exposed to the air during the determinations and are placed in such a position that the operator's breath might blow directly into them. (This has been obviated in our method by sealing the tubes by fusing the glass, as done by Rown tree and his associates.) The salt-content of Sorensen's buffer mixtures is much less than in sea-water, and a large salt correction had to be applied, and no new indicators could be used until their salt error was determined. This has been obviated by calibrating new buffer mix tures of the same salt-content as sea-water. The use of a colorimeter that brings the centers of the tubes together in a sharp line makes it possible to compare the sealed tubes from the side even more accurately than open tubes viewed from above. The sealed tubes are gradually changed by solution of even the most resistant glass, but this change has been reduced by increasing the volume-surface ratio as well as the buffer and NaCI concentration. The selection of the brilliant sulfophthalein indicators, brought to our notice by the work of Lubs and Clark, greatly facilitates the determinations. We found that the salt error of all of these indicators so far calibrated is the same; at least no distinction could be made within the range that we had to use.

Although we made the salt-content of the buffer mixtures the same as sea-water, it is sometimes convenient to apply a slight salt correction for the slight variation in the salinity of ocean-water rather than carry a separate set of tubes for each salinity. But the magnitude of this correction seldom exceeds one-sixth of the correction that had to be applied to Sorensen's buffer mixtures. In our final set of tubes, a slight compromise was made in order to satisfy all of the principles set forth at once. Borax is the best buffer and is not very soluble; its solubility is reduced by the presence of salt. Since it was desired to have as high a concentration of borax as possible, the salinity of the buffer mixtures was made to equal that of sea-water of about the lowest salinity ordinarily met with, and the labels of the tubes were changed so as to correct for salt error of sea-water of the mean salinity that is to be encountered in an investigation, and directions were given for correcting for variation in salinity. Unless otherwise specified the

tubes are labeled for a salinity of 31.25 grams per kilogram and with these tubes salinity variation within the ordinary range of the larger oceans causes slight errors that may be allowed for.

The buffer mixtures were made of different proportions of two stock solutions. In making a set of tubes, the indicator was selected and added to portions of the two stock solutions in two automatic burettes in the proportion of 10 mg. per liter. The burettes were protected with soda-lime tubes and the condensation water was mixed up by shaking the burette occasionally. For this purpose a burette supported by the stock bottle is most convenient, since connections are not damaged in shaking. This burette is sometimes known as Squibb's automatic.

A good glass as regards solubility is pyrex, but we have not become skillful enough to use glass of so high a melting-point. On account of its color Jena glass could not be used. The tubes used were nonsol test-tubes of 24 mm. bore. We obtained a very large number of these and divided them into grades in regard to bore, with the text-tube gage shown in figure 12. Those of the same bore and the most nearly cylindrical were selected for a set. The measurements were verified with a micrometer caliper, and no variation in any diameter greater than 0.2 mm. was allowed. Since there is danger of getting into the tubes in sealing, they were first constricted in the middle to a very narrow neck, filled, and quickly sealed by applying the blast lamp to the center of this constriction, the part removed being saved for use with sea-water. The blast lamp and all other flames were in a separate room from the burettes. The seal left a point on the tube liable to be broken, but this was covered with a layer of melted chewing-gum followed by a cap of sealing-wax. In the following list of mixtures the first column gives the percentage of the acid solution having 0.3 m boric acid and 2.25 per cent NaC1, the second column the percentage of the alkaline solution of 0.075 m borax and 1.9 per cent NaC1, the third column gives the P., the fourth column the P„ of sea-water of S = 31.25 grams per kilo (C1=17.3 per kilo or 17.7 per liter) of the same color, and the fifth column the P. of sea-water of S = 37.76 (C1= 20.9 per kilo or 21.5 per liter) of the same color, the actual volume of acid solution used for a tube in the sixth column and of alkaline solution in the seventh column. 'Unless otherwise specified, the tubes are labeled according to the fourth column, and those of P. 7.4 to 7.7 sterilized.

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