4. Error Due to Volatility of Phosphoric Acid.—This source of error has been made the subject of a special study by Neubauer.[46]
From the results, a table has been constructed, the use of which is recommended for phosphoric acid determinations. The source of error in this method lies exclusively in the loss of phosphoric acid by volatilization. The magnesia-covered crucible lid offers a very good control of this error, and its use is recommended to the analyst. Of course, the presence of sulfur in the gas used for ignition is liable to disturb this check.
The following course of procedure in the determination of phosphoric acid can be recommended to avoid or correct this error:
Separate the phosphoric acid in the form of the yellow precipitate and wash this latter in the usual way. Too high a heat should not be employed, nor should the solutions be allowed to stand too long lest excess of molybdic acid separate. Dissolve the phosphomolybdate in 100 cubic centimeters of cold two and five-tenths per cent ammonia and add as many cubic centimeters of the usual magnesia mixture (fifty-five grams magnesium chlorid and seventy grams ammonium chlorid dissolved in a liter of two and five-tenths per cent ammonia) as there are centigrams of phosphorus pentoxid present. Addition should not be made faster than ten cubic centimeters per minute. Stir during the addition. After the precipitation, stir briskly once more and then allow to stand at least three hours. Wash with two and five-tenths per cent ammonia till the chlorin reaction disappears, dry the filter, and introduce into a well-cleaned crucible which has been thoroughly ignited. Place the lid at an angle, carbonize the filter, and gradually raise the heat, though not higher than a medium red heat, till the pyrophosphate becomes completely white. When this happens bring the blast into action and ignite to constant weight. The weight finally accepted must not change even after half an hour’s ignition. Upon this requirement especial stress must be laid. Pure magnesium pyrophosphate does not suffer any loss even after several hours’ ignition nor does a good platinum crucible. To the weighed amount of pyrophosphate, add the correction given in the table. For example, if the weight be 250 milligrams, the correction to be added is four and two-tenths milligrams, and the correct weight is then 254.2 milligrams. Multiplication of the sum by sixty-four gives the amount of phosphorus pentoxid in the weight taken for analysis.
Correction for Phosphoric Acid Determination.
| Found, Mg₂P₂O₇ in grams. | Lost, milligrams Mg₂P₂O₇ | Found, Mg₂P₂O₇ in grams. | Lost, milligrams Mg₂P₂O₇ |
|---|---|---|---|
| 0.10 | 0.6 | 0.24 | 4.0 |
| 0.12 | 0.8 | 0.25 | 4.2 |
| 0.14 | 1.2 | 0.26 | 4.6 |
| 0.15 | 1.4 | 0.27 | 5.0 |
| 0.16 | 1.6 | 0.28 | 5.5 |
| 0.17 | 2.4 | 0.29 | 6.1 |
| 0.18 | 2.6 | 0.30 | 6.8 |
| 0.19 | 3.2 | 0.31 | 7.6 |
| 0.20 | 3.5 | 0.32 | 8.6 |
| 0.21 | 3.6 | 0.33 | 9.6 |
| 0.22 | 3.8 | 0.34 | 10.6 |
When phosphoric acid is to be estimated as pyrophosphate it must always be first separated as molybdate, even when the original solution contains no bases capable of forming insoluble phosphates, as otherwise these corrections will not be applicable.
Using these corrections the estimation of phosphoric acid becomes one of the most accurate of known analytical methods.
61. The Color of the Magnesium Pyrophosphate.—After the final ignition of the magnesium pyrophosphate, whether secured by the citrate or the molybdic method, a black or grayish tint is often noticed. This may be due to traces of organic matter brought down by the precipitate and especially to a lack of care in the initial ignition. Many devices have been proposed for the purpose of avoiding this coloration, although general experiments have shown that there is no appreciable increase in the weight of the precipitate when colored in this way.
When the precipitation is carried on according to the citrate method, Neubauer[47] proposes to eliminate this coloration by the use of ammonium sulfate. About seven cubic centimeters of a saturated solution of ammonium sulfate should be added to the solution before the precipitation by the magnesium mixture. With this precaution it is possible to obtain a perfectly white precipitate after five minutes of ignition. The lively glowing of the precipitate throughout the whole mass at the time of changing into pyrophosphate, is much more easily observed by this treatment than when the mass is gray or black. Even should the addition of the ammonium sulfate solution to one containing a large amount of lime produce a precipitate of crystalline calcium sulfate, it is of no importance inasmuch as the ammonium citrate immediately dissolves large quantities of the calcium salt.