Table II. The column of this table which gives NaOH used needs some explanation, which is also true of the corresponding tables of the other series. Gregersen 27 has pointed out that in Neumann's method carbon dioxide present in the liquids to be titrated is a source of error tending somewhat to increase the

In order to eliminate this error he recommends to overtitrate

the solution of the yellow precipitate in NaOH with from 1⁄2 c.c.

2

to I c.c. of H2SO,, to drive off now the carbon dioxide by boiling, to cool and titrate back with NaOH. This, however, in

27 Z. physiol. Chem., 53, 455 (1907).

29 The factor in this table, like in the others, was calculated to the average of the gravimetric analysis (see Table I). No. 1, e.g., has the factor 0.41171 X=0.577.

VOL. CLXXX, No. 1077-26

creases necessarily the time required for the titration of each individual analysis, aside from the fact that the liquid acidified with

2

4

H2SO, has the tendency somewhat to bump while boiling. Inasmuch as in Neumann's method it is indispensable to ascertain as to whether or not the reagents are free from phosphoric acid, the writers have found it to be more convenient to run a "blank (preferably several "blanks ") and to subtract the amount of standard alkali applied for neutralization of the "blank" from the result of the analysis proper. The figure secured for the blank analysis can advantageously be applied to a series of analyses, and it is evident that it does away with the necessity of removing carbon dioxide from the liquid to be titrated, since both the analysis and the "blank" contain practically the same amount of carbon dioxide. Thus eight blank analyses were made, in each of which there were employed 125 c.c. of distilled water, 10 c.c. of acid mixture, 75 c.c. of 50 per cent, ammonium nitrate, 40 c.c. of 10 per cent. ammonium molybdate, and one S. & S. filter of 121⁄2 cm. diameter which was washed to neutrality (with either 50 c.c. or 150 c.c. of distilled water three times). Now each flask containing one washed filter received 150 c.c. of distilled water for

n

reducing the filter to pulp, then 6 c.c. NaOH, whereupon contents were boiled for from fifteen to twenty minutes, cooled, made up to the original volume, and titrated. The blanks required for their neutralization on the average of concordant analyses 0.20 c.c.

n

2

NaOH. The latter figure, which was applied to several series of analyses, was subtracted from the standard alkali used for each individual analysis and the so corrected figure recorded in the column in question.

In glancing over the table we can readily see that the factor is to a certain degree influenced by the amount of water applied for washing the yellow precipitate. Numbers 1 to 8, the ammonium phosphomolybdate of which was washed with 150 c.c. of water, the washing being repeated four times, have the average factors of 0.578 (a), 0.576 (b), 0.577 (c); numbers 15 to 22, whose yellow precipitate was, washed with 100 c.c. of water three times, have the average factors of 0.573 (a), 0.571 (b), 0.572 (c); while numbers 23 to 30, in which the washing of the yellow precipitate with 50 c.c. of water was performed three times, gave

the smallest average factors of 0.568 (a), 0.566 (b), 0.567 (c). On the other hand, however, the numbers 9 to 14 and 15 to 22, the yellow precipitate of which was washed three times with 150 c.c. and 100 c.c. of water respectively, yielded the identical factors.

SERIES B.

Eight grammes of secondary ammonium phosphate [(NH4)2 HPO4] were dissolved in water, made up to about five litres, and the solution thoroughly shaken. In eight portions of this solution of 250 c.c. each the phosphorus was estimated according to the method of Fresenius, and in as many portions according to the method of Schmitz. The results are expressed in the following table:

Titration According to Neumann's Method.

Thirty-one portions of the above solution of 50 c.c. each were worked up according to Neumann's method. It goes without saying that the ammonium phosphomolybdate, prior to its titration, must be washed free from the adhering sulphuric and nitric acids with ice-cold water. Neumann 29 states that in order to obtain entirely neutral reaction of the filtrate the yellow precipitate has to be washed with 150 c.c. of ice-cold water, the washing being repeated from three to four times. As a matter of fact we have found that it is sufficient to wash the yellow precipitate on a 121⁄2 cm. folded filter three times with but 50 c.c. of water. The filtrate was repeatedly tried by us with sensitive neutral litmus paper and was found to be neutral.

The results in question are summarized in Table IV.

An examination of the table reveals the fact that, while the influence on the factor of the amount of water employed for washing the yellow precipitate is small, yet it is distinct. Thus, numbers 1 to 9, the yellow precipitate of which was washed with 150 c.c. of ice-cold water, the washing being repeated three times, yielded the average factors of 0.575 (a), 0.574 (b). At the same time numbers 10 to 18, in which the washing of the yellow precipitate with 100 c.c. of water was performed three times, gave the average factors of 0.574 (a), 0.573 (b); while numbers 19 to 30, whose precipitate was washed with but 50 c.c. of water three times, gave the smallest average factors of 0.569 (a), 0.568 (b). On the whole the data of this series confirm the results of Series A, showing that, within certain limits, the factor is the higher the more water is employed for washing the yellow precipitate, and vice versa. In this connection it may be worth while here to mention that of all manipulations incident to Neumann's method the washing of the yellow precipitate in accordance with Neumann's directions is an operation not only tiresome

but one requiring considerable time. The observation that it is sufficient to wash the phosphomolybdate of ammonia with three times 50 c.c. of water (instead of three or four times 150 c.c. of water) in order to obtain a neutral filtrate enables one to save more than half the time which was originally necessary for washing the yellow precipitate. We have also found that whenever the phosphoric acid is present in inorganic form it is advantageous to use, for the formation of the yellow precipitate and its washing, a round bottom flask of some 300 c.c. capacity, instead of a flask of from 500 c.c. to 750 c.c., as is ordinarily used in Neumann's method. In such cases where previous oxidation of organic matter with the acid mixture has to take place the substance is, on oxidation in the larger flask, transferred to the 300 c.c. flask with the aid of distilled water, which is, at all events, to be added before the formation of the ammonium phosphomolybdate takes place. The 300 c.c. flask is easier to handle during the repeated filtrations, and the yellow precipitate in it is more readily washed free from the adhering acids with a comparatively small amount of distilled water.

SERIES C.

A solution of 15 grammes of crystallized disodium hydrogen phosphate 30 in water which was made up to five litres was employed for the trials of this series. Of this solution eight 250 c.c. portions were analyzed gravimetrically according to Fresenius's method, eight like portions according to Schmitz's method, while five 50 c.c. portions were converted into sodium pyrophosphate by ignition. The results are recorded in the table below.

30 The crystals have in part lost their water of crystallization.