Critical Evaluation: Ammonium Orthophosphates and Water (IUPAC SDS Volume 66)
4. Ammonium Orthophosphates

Twenty-one publications contain solubility data for ammonium orthophosphates.1-21 The following solid phases have been reported as being in equilibrium with saturated solutions in the NH3-H3PO4-H2O system.

Formula CAS Registry Number
(NH4)3PO4·3H2O [25447-33-0]
(NH4)3PO4·2H2O NA
(NH4)3PO4 [10361-65-6]
(NH4)7H2(PO4)3 NA
(NH4)HPO4·2H2O2 [51457-70-8]
(NH4)2HPO4 [7783-28-0]
NH4H2PO4 [7722-76-1]
(NH4)3H2(PO4)4 [54285-90-4]
NH4H5(PO4)2·H2O [28037-74-3]
NH4H5(PO4)2 [28537-48-6]

There are sufficient published data to enable a critical evaluation to be made of the binary system, ammonium phosphate-water, for only two of these ammonium phosphates: NH4H2PO4 and (NH4)2HPO4. A variety of experimental methods has been used to determine the temperature dependence of the solubility of these phosphates. Some of these methods are: isothermal method;23,24,27,30,49,75 graphical measurement from a plot of density vs. composition;29 Toepler's method;47 visual polythermic method;39,53,63,66 and measurement of the temperature at which the last crystal disappears.22 Ross et al.26 do not describe their experimental method, but the solubility data reported by them are reasonable. The temperature dependence of the solubility of these compounds has been reported in many articles.23,27,29,30,39,47,49,53,63,66,75. Solubility data have also been presented as a limiting condition in the study of multicomponent systems.27,28,31-38,40-46,48,50-52,54-56,57-62,64,65,67-69,71-74,76,78,80,82,83 Thus, there are ample data available for a critical evaluation to be made.

There is only one report for the solubility of (NH4)3PO426. This may be due to the fact that this salt is strongly hydrolyzed in aqueous solution and even has a significant partial pressure of ammonia at room temperature.26 There is also only a single report for the temperature dependence of the solubility of NH4H5(PO4).22 These two solubility branches in the NH3-H3PO4-H2O system will be discussed on pp. 1313,1314. Solubility branches of the other orthophosphates will also be discussed later.

4.1 Binary Systems: Phosphate-Water

Solubility of NH4HPO4

Some articles contain obviously incorrect data.27, 34, 36, 41, 82 These data were excluded before critical evaluation was made. Furthermore, the data in one reference25 are the same as in another.24 These data were considered only once. One article53 reported the existence of a transition point at 308.5 K for the NH4H2PO4-H2O system. a-NH4H2PO4 and b-NH4H2PO4 were said to be equilibrium solid phases at this temperature. However, no other sources confirmed this observation. The solubility curve does not have a break at this temperature. Therefore, all the available data were treated together. The solubility results are summarized in Table I. During the iteration process the data in Refs. 66, 75, 80, 72, 57, 38, 54, 82, 32, 64, 45, 83, 55, 58, 56 were eliminated. A summary of the values for the parameters in Eq. (1) is given in Table II. Table III contains some solubility values obtained by using Eq. [1] (see the Preface) and the parameters given in Table II. The values in Table III are recommended values.

The (NH4)2HPO4-H2O system.

The existence of the hydrate (NH4)2HPO4·2H2O has been reported and the transition temperature between it and the anhydrous salt has been reported to be 288.2 K.59 Before making the critical evaluation some obviously incorrect data points60,71,75,78 as well as data for reportedly metastable systems39,46 were eliminated from consideration. Also, the same data were reported in two articles24,25 and these data were counted only once. The experimental data are summarized in Table IV. The data for the solubility of the hydrate (NH4)2HPO4·2H2O are relatively few and rather scattered. The only equation that gave reasonable results was the three-parameter form of the smoothing Eq. [3]

y = A/T + B ln(T/Tf) + C. (5)

The values of the parameters for the smoothing equations are given in Table V and VI. Table VII lists solubility data for (NH4)2HPO4·2H2O using the values of the parameters given in Table V. These data may be accepted as tentative. Table VIII contains solubility data for (NH4)2HPO4 calculated by using the parameters listed in Table VI. The data in Table VIII are recommended values.

Vogel's equation

Vogel, et al.51 measured the temperature dependence of the water solubility of NH4H2PO4, (NH4)2HPO4 and KH2PO4. Starting with Schroeder's Eq. [77] they derived an equation which is identical to the three-parameter form of Eq. [3], and given here as Eq. [5]. In order to test the validity and usefulness of this equation, the evaluator used the iterative procedure described earlier to determine the values of the coefficients in this equation. She used all the data collected for NH4H2PO4 and (NH4)2HPO4. The values obtained are given in Table IX.

Figure 3 and Figure 4 present graphical comparisons of the recommended values of Tables III and VIII, respectively, with the values obtained by the use of Eq. [5] with the values of the parameters given in Table IX. It is obvious from these figures that Vogel's values for the coefficients may be considered satisfactory only in the temperature range 283 K to 338 K. The reason is probably the limited number of data treated by Vogel's group.

Table I. Solubility values of NH4H2PO4 in water.
C T (K) D 100w1 S Weight (init/final) S Ref.
268.5 18.0 1/0 55
268.6 17.4 1/0 34
268.6 18.0 1/0 83
268.7 16.9 1/0 69
268.7 16.9 1/0 46
268.8 18.0 1/0 75
269.0 16.8 1/0 60
271.4 19.0 1/0 55
273.0 18.4 4/4 34,42,35,55
273.0 18.5 1/1 41
273.0 18.53 1/1 32,36
273.0 18.6 1/1 28
273.0 18.8 1/0 39
273.0 19.0 1/0 56
273.0 19.2 1/0 78
273.0 19.6 1/0 23
275.5 20.0 1/0 55
277.8 20.3 1/1 24
277.8 20.5 1/1 24
282.5 22.5 1/0 55
283.0 21.3 1/0 34
283.0 21.4 1/0 55
283.0 21.8 2/0 42,35
283.0 22.1 1/0 41
283.0 22.2 1/0 39
283.0 22.4 1/1 56
283.0 22.55 1/1 32,36
288.0 24.65 1/1 32,36
291.3 25.9 1/0 24
291.3 26.0 1/0 24
293.0 25.5 3/0 34,55,70
293.0 25.9 2/0 42,35
293.0 26.0 2/0 39,56
293.0 26.3 1/0 41
293.0 26.8 1/1 32,36
293.0 27.2 1/0 85
293.0 27.3 1/0 75
295.5 27.5 1/0 55
296.0 28.3 1/1 30
298.0 28.4 1/0 78
298.0 28.8 1/0 23
298.0 28.85 1/1 43
298.0 28.9 1/1 37
298.0 29.0 2/2 49,32,36
298.0 29.05 1/1 46
298.0 29.2 2/2 23,29
298.0 29.23 1/1 38
298.0 29.3 1/1 76
298.0 29.31 1/1 50,52
298.0 29.42 1/1 79
298.0 29.45 1/1 33
298.0 29.62 1/0 23
298.0 29.7 1/0 78
298.0 29.77 1/0 67
298.0 30.4 1/0 23
302.5 30.87 1/1 44
303.0 29.6 1/0 56
303.0 30.2 4/0 55,34,42,35
303.0 30.6 1/0 39
303.0 31.18 1/1 32,36
303.0 31.2 1/1 49
303.0 31.4 1/1 24
303.0 31.7 1/1 24
305.5 32.5 1/1 55
308.0 33.51 1/1 32,36
308.5 34.0 1/1 55
312.0 35.0 1/0 55
313.0 35.5 1/0 47
313.0 35.8 2/2 49,32,36
313.0 36.1 2/2 24,37
313.0 36.2 1/1 70
313.0 36.3 1/1 24
314.5 37.5 1/0 55
323.0 39.88 1/0 33
323.0 40.5 1/1 49
323.0 40.6 1/1 78
323.0 40.7 1/1 24
323.0 40.8 1/1 24
323.0 41.0 1/1 23
323.0 41.05 1/1 23
323.0 41.6 1/0 60
333.0 43.8 1/0 42
333.0 45.2 1/0 86
333.0 45.3 1/1 49
342.0 49.7 1/1 24
342.0 49.8 1/1 24
343.0 48.3 1/0 23
343.0 50.0 1/0 23
343.0 50.2 1/1 37
343.0 51.0 1/0 53
348.0 55.66 1/0 67
353.0 53.6 1/0 42
353.0 54.8 1/1 53
356.0 56.5 1/1 23
356.0 56.7 1/0 23
363.0 58.8 1/0 24
363.0 59.3 1/1 24
363.0 59.6 1/1 53
363.0 61.85 1/0 67
373.0 63.4 1/0 66
373.0 63.5 1/0 53
373.0 63.9 1/1 37
375.0 63.2 1/0 24
382.0 70.95 1/0 57
382.4 71.08 1/0 61
382.4 71.84 1/0 58
383.0 67.3 1/0 42
383.0 67.8 1/1 66
383.0 68.3 1/0 53
383.5 67.3 1/0 24
383.5 68.3 1/1 68
393.0 72.0 1/1 66
403.0 75.7 1/1 66
413.0 79.0 1/1 66
423.0 82.2 1/0 66
433.0 85.2 1/0 66
443.0 88.3 1/0 66
453.0 91.6 1/1 66
463.0 94.7 1/1 66

Table III. Solubility of NH4H2PO4 calculated by Eq. [1].
C T (K) D 100w1 D x1 S m1 (mol/kg)
268.0 16.57 3.016 x 10-4 1.727
273.0 18.48 3.428 x 10-4 1.972
278.0 20.48 3.876 x 10-4 2.24
283.0 22.55 4.360 x 10-4 2.533
288.0 24.69 4.883 x 10-4 2.852
293.0 26.89 5.445 x 10-4 3.199
298.0 29.14 6.049 x 10-4 3.577
303.0 31.43 6.695 x 10-4 3.986
308.0 33.75 7.386 x 10-4 4.431
313.0 36.09 8.124 x 10-4 4.912
318.0 38.45 8.910 x 10-4 5.434
323.0 40.82 9.747 x 10-4 6.0
328.0 43.19 1.064 x 10-3 6.613
333.0 45.56 1.158 x 10-3 7.278
338.0 47.91 1.259 x 10-3 8.001
343.0 50.25 1.365 x 10-3 8.786
348.0 52.57 1.479 x 10-3 9.64
353.0 54.86 1.599 x 10-3 10.57
358.0 57.13 1.726 x 10-3 11.59
363.0 59.36 1.861 x 10-3 12.7
368.0 61.56 2.004 x 10-3 13.93
373.0 63.72 2.156 x 10-3 15.27
378.0 65.83 2.318 x 10-3 16.76
383.0 67.91 2.489 x 10-3 18.4
388.0 69.94 2.670 x 10-3 20.24
393.0 71.92 2.863 x 10-3 22.28
398.0 73.86 3.067 x 10-3 24.58
403.0 75.75 3.284 x 10-3 27.17
408.0 77.59 3.515 x 10-3 30.11
413.0 79.38 3.760 x 10-3 33.48
418.0 81.12 4.021 x 10-3 37.36
423.0 83.01 4.299 x 10-3 41.89
428.0 84.55 4.594 x 10-3 47.21
433.0 86.03 4.908 x 10-3 53.55
438.0 87.56 5.243 x 10-3 61.22
443.0 89.04 5.599 x 10-3 70.69
448.0 90.48 5.980 x 10-3 82.63
453.0 91.86 6.385 x 10-3 98.13
458.0 93.19 6.818 x 10-3 119.0
463.0 94.47 7.280 x 10-3 148.7

Table IV. Solubility of (NH4)2HPO4 in water.
C T (K) D 100w1+2 S Weight (init/final) S Ref.
266.3 29.6 1/1 48
266.5 29.6 1/1 40
267.5 28.0 1/1 62
273.0 29.7 1/1 24
273.0 30.3 1/1 24
273.0 30.4 1/1 40
277.0 32.0 1/1 40
281.4 34.0 1/1 40
283.0 35.0 1/1 81
283.0 38.3 1/1 24
283.0 38.4 1/1 24
283.6 35.2 1/1 40
285.2 36.0 1/1 40
288.2 39.5 1/1 62
288.8 38.4 1/0 40
289.5 39.0 1/0 40
293.0 40.7 1/0 24
293.0 40.8 3/0 24,86,6
296.0 41.1 1/1 30
298.0 41.1 1/0 65
298.0 41.4 1/1 77
298.0 41.5 1/1 72
298.0 41.6 1/1 78
298.0 42.7 2/0 29,38
303.0 40.8 2/0 86,71
303.0 42.2 1/0 24
303.0 42.8 1/1 24
308.0 43.2 1/0 65
313.0 44.4 1/0 24
313.0 45.6 1/0 24
323.0 46.5 1/0 54
323.0 47.0 2/2 24,78
323.0 47.05 1/1 45
323.0 47.2 1/0 24
333.0 48.8 2/2 24,86
333.0 49.9 1/0 24
343.0 51.0 1/1 24
343.0 51.8 1/0 24
390.0 62.2 1/1 31
395.0 64.9 1/0 31
432.0 73.0 1/0 31
453.0 82.0 1/0 31

Table VII. Solubility of (NH4)2HPO4·2H2O calculated by Eq. [5].
C T (K) D 100w1 D x1 S m1 (mol/kg)
266.3 30.9 5.749 x 10-2 3.385
268.0 31.16 5.816 x 10-2 3.427
270.0 31.58 5.925 x 10-2 3.495
272.0 32.17 6.078 x 10-2 3.591
274.0 32.89 6.268 x 10-2 3.711
276.0 33.75 6.499 x 10-2 3.857
278.0 34.77 6.779 x 10-2 4.035
280.0 35.96 7.117 x 10-2 4.252
282.0 37.3 7.508 x 10-2 4.505
284.0 38.81 7.964 x 10-2 4.802
286.0 40.49 8.495 x 10-2 5.152
288.3 42.61 9.197 x 10-2 5.621

Table VIII. Solubility of (NH4)2HPO4 calculated by Eq. [1].
C T (K) D 100w1 D x1 S m1 (mol/kg)
288.0 39.51 8.180 x 10-4 4.95
293.0 40.76 8.580 x 10-4 5.214
298.0 41.93 8.968 x 10-4 5.473
303.0 43.05 9.346 x 10-4 5.727
308.0 44.1 9.717 x 10-4 5.979
313.0 45.12 1.008 x 10-3 6.23
318.0 46.1 1.045 x 10-3 6.482
323.0 47.06 1.081 x 10-3 6.737
328.0 48.01 1.119 x 10-3 6.997
333.0 48.95 1.157 x 10-3 7.266
338.0 49.9 1.196 x 10-3 7.546
343.0 50.85 1.237 x 10-3 7.84
348.0 51.82 1.280 x 10-3 8.152
353.0 52.82 1.325 x 10-3 8.484
358.0 53.85 1.373 x 10-3 8.841
363.0 54.91 1.424 x 10-3 9.227
368.0 56.01 1.479 x 10-3 9.647
373.0 57.15 1.539 x 10-3 10.11
378.0 58.34 1.604 x 10-3 10.61
383.0 59.58 1.674 x 10-3 11.17
388.0 60.87 1.750 x 10-3 11.79
393.0 62.22 1.834 x 10-3 12.48
398.0 63.62 1.926 x 10-3 13.25
403.0 65.07 2.026 x 10-3 14.12
408.0 66.5 2.136 x 10-3 15.09
413.0 68.14 2.258 x 10-3 16.2
418.0 69.75 2.393 x 10-3 17.47
423.0 71.41 2.541 x 10-3 18.93
428.0 73.12 2.706 x 10-3 20.61
433.0 74.86 2.888 x 10-3 22.56
438.0 76.64 3.091 x 10-3 24.86
443.0 78.44 3.317 x 10-3 27.58
448.0 80.27 3.569 x 10-3 30.83
453.0 82.11 3.850 x 10-3 34.78

Figures
Solubility graph
FIG. 3. Solubility data for NH4H2PO4.
Solubility graph
FIG. 4. Solubility data for (NH4)HPO4.
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