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Their physiological importance.—These two pairs of buffer mixtures occur in blood and tissue fluids and maintain the reaction of the body fluids precisely neutral. The end products of metabolism are more acid than alkaline, so that there would be a tendency to accumulation of the acid members of these buffer mixtures were this tendency not combated by the continuous excretion of the excess of these acid members, the acid phosphate through the kidneys, the carbonic acid through the lungs.

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5. COLORIMETRIC DETERMINATION OF HYDROGEN ION CONCENTRATION. Nature of indicators. The indicators used in determining the reaction of a solution are weak organic acids whose anion has a different color in solution from that of the undissociated acid. Methyl orange, for example, is pink in acid solution and yellow in alkaline solution. In alkaline solution the indicator is all present as salt; the salt is completely dissociated, and the anion gives its characteristic yellow color to the solution. In acid solution, the excess of hydrogen ions diminishes the dissociation of the weakly acid indicator just as hydrochloric acid diminishes the dissociation of acetic acid; and the undissociated acid gives its characteristic pink color to the solution.

Pu and color of indicator.- The Pu of mixtures of the pink undissociated acid methyl orange and the yellow salt of methyl orange varies slightly about a mean just as does the pu of buffer mixtures; the mean value, as in the case of buffer mixtures, is numerically equivalent to the dissociation constant. The form of the curve for Pu is similar to that of the curves for p in mixtures of acetic acid and sodium acetate and other buffers. Since, now, mixtures of the acid methyl orange and the methyl orange salt in varying proportions correspond to definite hydrogen ion concentrations; and since these mixtures show varying shades of orange likewise corresponding to the proportions of the pink acid solution and the yellow salt solution, each shade of orange corresponds to a definite hydrogen ion concentration. And this fact gives the basis for a colorimetric method of determining hydrogen ion concentration.

Indicators useful in bacteriology.-Any one indicator changes color over only a very small range of hydrogen ion concentration; and any one busser mixture acts as such over only a very small range of hydrogen ion concentration. To cover a large range would require, tlierefore, a considerable number of indicators and several pairs of buffer mixtures. In bacteriological work we are interested chiefly in values for Pit close to neutrality (pu=7), a region which is covered by the phosphate buffer mixture (p: = 5.8 to 8.2). There are several indicators for this region; brom cresol purple, which shows changes from yellow to purple between pu 5.2 and pu 6.8, and phenol

red, which shows color changes from yellow to red between pu 6.8 and Pa 8.4, are among the best.

6. TECHNIQUE OF THE DETERMINATION.

The standard solutions.-Tenth molecular solutions of KH.PO, (13.62 grams potassium phosphate, monobasic, anhydrous, Merck's reagent, to the liter) and Na, HPO, (14.21 grams sodium phosphate, anhydrous, Merck's reagent, per liter) are convenient strengths for the stock solutions. From these the following twelve standard solutions are prepared:

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The reading.–To determine the hydrogen ion cencentration of an unknown solution coming within the limits of pu=6.8 to 8.2, add to it five drops of a 0.03 per cent solution of phenol red and compare the resulting color with that obtained by adding the same amount of indicator to 5 c. c. of each of the standard phosphate solutions diluted with 10 c. c. of water. Between the limits pu=5.8 to 6.8 the indicator brom cresol purple--five drops of a saturated solutionshould be used. (The standard solutions with indicator in them will keep several weeks if tightly stoppered.)

The comparator.— The color comparison can be made in large clear glass test tubes. To overcome the effect of turbidity, such as occurs in bacteriological media, the unknown solution is diluted to a moderate extent, say to three times its volume, and the test tubes are arranged in a device called a comparator. The device consists of a block of wood containing 6 perpendicular holes large enough to carry the test tubes. Three other holes are then bored horizontally through the block from side to side, so that one can look right through each pair of test tubes in series. When the solutions are arranged as indicated in Figure 1, in each case the light reaching the eye has passed through solution containing indicator and solution containing turbidity. In the case of the unknown, one solution contains both

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turbidity and indicator; in the case of the standards the turbidity and indicator are in separate solutions.

Adjusting reaction of culture media.-Most bacteria grow best in media whose pr lies between 7.2 and 7.6. The optimum for typhoid and paratyphoid is near the lower of these two limits; that for pneumococcus, streptococcus, and menigococcus, nearer the upper of

Fig. 1:- THE COMPARATOR.

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LIGHT

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these linits. To adjust media to any desired hydrogen ion concen

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alkali is added drop by drop to five c. c. of the somewhat diluted media containing indicator until, as shown by comparison with the standards, the desired hydrogen ion concentration is reached. From the amount of alkali required for five c. c., the amount needed for the whole batch of media can then be calculated. Sterilization of the media shifts the pu about 0.2 toward the acid side. Allowance should be made for this.

COURT DECISION RELATING TO QUARANTINE.

The Supreme Court of California has decided that by virtue of statutory authority “the State board of health has power to order the quarantine of persons who have come in contact with cases and carriers of contagious diseases," and that the unauthorized removal of a quarantine placard affixed to premises by order of the State board of health is “a misdemeanor under the Public Health Act."

TYPHOID FEVER IN THE UNITED STATES. CASE RATES, JANUARY TO NOVEMBER, 1921 AND 1920, AND MEDIANS

YEARS 1913 TO 1920.

FOR THE

The accompanying table shows the number of reported cases of typhoid fever per 1.000 population by quarters, from January to June, and by months, from July to November, 1921, compared with the same periods during 1920, and the medians for 1913 to 1920, inclusive.

The median was ascertained by arraying the figures so that the greatest number was first, the next smaller number was second, and so on to the smallest number, which was placed last. The middle number of the array was then selected as the median. Data were not available for all the States for the full eight years. As many years as possible were included for each State, but no year earlier than 1913 was used. The first column shows the number of years for which figures were obtained for each State.

The estimated populations on which the rates were computed are as follows:

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Number Estimated of States. population.

1971..
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N... :913 19.0.

40 39 38

88, 475, 824 85,189, 366 83, 144, 829

For comparison the death rates per 100,000 population in the registration area for deaths are shown in the following table: Typwid fecer death rates per 100,000 population in registration area for deaths, 1913-1920,

inclusive.

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Typhoid fever-Annual case rates per 1,000 population, January to November, 1921 and

1920, and medians for the years 1913 to 1920, inclusive.

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