The DETERMINATION of MAGNESIUM as MAGNESIUM AMMONIUM PHOSPHATE HEXAHYDRATE .. J. P. MEHLIG
..
Oregon State College, Corvallis, Oregon
Magnesium may be determined very conveniently and satisfactorily by weighing the precipitate of magnesium ammonium phosphate hemhydrate afgr suitable washing ?uith alcohol and ether and drying. Comparative results obtained by students using this method and the pyrophosphate method are given.
off this carbon and leave a white, quantitative residue. Worsham (2) has shown that MgNH4POcGHzO does not undergo any change in weight a t 18' to 30" and under ordinary laboratory atmospheric conditions. If placed in a desiccator over dry calcium chloride a t 18' to 30" i t will lose weight a t the rate of about one milligram in twenty-four hours for each gram of ++++++ sample. N THE usual gravimetric determination of magAdvantage has been taken (3), (4) of this behavior nesium, as well as of phosphorus, it has long been as a basis for weighing the hexahydrate as such withthe custom to ignite to constant weight the precipi- out ignition to the pyrophosphate. The precipitate of tate of magnesium ammonium phosphate, the final MgNH4P04.6Hz0,which has been obtained in the product being magnesium pyrophosphate, Mg2P1O7. usual manner, is filtered on a Gooch crucible and Not only is this a tedious, time-consuming procedure, washed free from chlorides with 1.5 M amrnouium but if a filter paper has been used in collecting the hydroxide. It is then washed with three successive precipitate, another difficulty is encountered, since, 15-ml. portions of 95Y0 alcohol and finally twice with during the process of losing its water of hydration in 15-mI. portions of ether and sucked dry a t the filter the early stages of ignition, M~NHIPOCBH~O has pump for ten minutes. The crucible is wiped with a the property of attaching firmly within its complex dry linen cloth, placed in a desiccator for twenty structure certain gaseous organic compounds which minutes, and then weighed a t once as MgNHSOrlater break down and leave graphitic carbon (1). It is 6Hz0. The crucible must undergo the same steps in its often very difficult and sometimes impossible to bum preparation.
I
Since this method may perhaps not be very widely known, it has occurred to the writer that it would be advisable not only to call attention to it, but also to present the results that have been obtaiied by its use by the students in quantitative analysis at Oregon State College during the past three years. The procedure was followed as outlined above except for the very satisfactory substitution of synthetic methanol for the 95y0 alcohol. In the accompanying table are also included student results obtained by the use of the pyrophospbate method prior to 1932. All figures are the averages of duplicate determinations and represent ordinary class work. The samples are magnesium salts, either pure or mixed with inert material.
Sompk NO.5 4 . 6 9 % M P A* PyroPhorPhola A* h'czahydralc % ME Deaiolion % ME Dcniolion 4.65 4.88 4.79 4.43 4.64 4.51 4.75 4.78
Av. 4.68
-0.04 +0.19 f0.10 -0.26 -0.05 -0.18 +0.06 +0.09 0.12
Av.
4.55 4.50 4.67 4.62 4.67 4.76 4.78 4.73 4.47 4.56 4.68 4.62 4.75 4.44 4.63 4.45 4.69 4.62
-0.14 -0.19 -0.02 -0.07 -0.02 +0.07 CO.09 +0.04 -0.22 -0.13 -0.01 -0.07 +006 -0.25 -0.06 -0.24 *O.OO
0.10
. 6 ~ 5 . 5 9 %Mgr* As Pwophospharc Dcaiolion
% M8 Sample No. I-25.30% As Bcxahvdrolc
% ME
Deviation
25.16 25.50 25.46
-0.14 +0.20 +0.16
Av. 25.37
0.17
~
-"."" -" "L
+0.04
-0.05 f0.21 -0.19 +029 -0.01 -0.06 -0.20 -0.26 0.16
Av. 5.50
M8** As Pnoghorpholc % MK Denialion
8.19 8.19 8.24 8.34 8.17 8.24 8.09 8.08 8.14 8.20 8:47 Av. 8.21
As Pno$hmphob
..
Av. 9.47
0.11
. 7--8.38%
M8t
0.05
-0.19 -0.19 -0.14 -0.04 -0.21 -0.14 -0.29 -0.30 -0.24 -0.18 +0.09 0.18
Snmgk No. 8-11.55% Mg** As E~xohydrola Ar Pnophospharc % MI Dmiolia % ME Deaiolion
9.84 9.79 9.93 9.98 9.91 9.87 9.90 9.95 9.88 9.83 9.79
Av. 9.88
? .,
11.62 11.39 11.52 11.75 11.72 11.54 11.48 11.54 11.53 11.53 11.70 11.49 11.50 Av.. 11.56
-
+0.07 -0.16 -0.03 +0.20 +0.17 -0.01 -0.07 -0.01 -0.02 -0.02 +0.15 -0.06 -0.05 0.08
11.56 11.30 11.561 11.52 11.55 11.73 11.27 11.44 11.44
Av. 11.49
+0.01 -0.25 +0.01 -0.03 -0.00 +O. 18 -0.28 -0.11 -0.11 0.11
* h ~ ~ e t o r determination 's (pyrophorphate method).
t C a l ~ l a t e dvalue for Mg(NOdr6H~0.
Av. S o m p ! ~No. Hcxohyd"o1c Deainlion -0.12 -0.15 -0.12 -0.15 -0.08 -0.10 -0.12
Av.
~
25.25 25.51 25.11 25.59 25.29 25.24 25.10 25.04 Av. 25.27
Sample No. 2-9.48%
Ar Rcxohrdrnls
ME*
Ar PwoDhosDhal~ % M8 Dcaialion
f Calculated value for MgsOc7H10. ~etermioationof commercial laboratory ouppbing samples (ppraphosphate method).
**
Inspection of the table shows that the results obtained by the hexahydrate method compare very favorably with those obtained by the pyrophosphate method. I t is a noteworthy fact too that students are much more successful with the hexahydrate method in getting satisfactory results the first time they attempt a determination. Dick (5) recommends a similar determination of
arsenic as magnesium ammonium arsenate hexahydrate, and Dick and Rudner have shown (6) that magnesium may also be determined as magnesium ammonium arsenate hexahydrate. SUMMARY
The determination of magnesium as magnesium ammonium phosphate hexahydrate gives results that compare very favorably with those obtained by the pyrophosphate method.
The hexahydrate method is more rapid, less tedious, and more easily carried out, and there is no black residue. Success is more often achieved by students on initial attempts with the hexahydrate method than is the case with the pyrophosphate method. The hexahydrate method is strongly recommended, not only for magnesium, but also for phosphorus.
LITERATURE CITED
ammonit& bhosphate ( ~ h ~ ~ & ~ 0 d 3 ~ e 0amethod )-and of determining phosphorus as magnesium ammonium phosphate (MgNH4PO~6H20),"Dissertation, Columbia Universitv. 1923. (3) FALES,H. -A,. '%organic quantitative analysis," The
(5) DICK,J., '~ravimetric methob for deterkininh arsenic as magnesium ammonium arsenate,"iM.. 93, a 9 3 3 (1933). A,, "Gravimetric determination of (6) DICK,J. AND RUDNER, maenesium as maenesium ammonium arsenate." i6id.. 96. 245-8 (1934). "
