Solubility Product of Ni-Struvite, NH 4 NiPO 4 ∙ 6H 2 O, at 25 ˚C

Solubility product of a sparingly soluble salt is an important parameter in both pure and applied physical chemistry such as determination of values of thermodynamic functions or environmental implications of components of the substance. This paper presents the determination of the solubility product of ammonium nickel phosphate hexahydrate (Ni-struvite) at 25˚C by analysis of equilibria attained from both supersaturated and undersaturated solutions, i.e. precipitation and dissolution, respectively. Writing the dissolution process as NH 4 NiPO 4 ·6H 2 O → NH 3 + Ni 2+ + 24 HPO − + 6 H 2 O, the value pK sp = 11.03 ± 0.03 is found for both precipitation and dissolution. The solubility is a little lower than that of the isomorphous Mg salt. This is to be expected from the lattice dimensions of the two phases, the crystals of Ni-struvite being slightly more compact.


Introduction
Phosphates of divalent metals often show complex structures and great variability in chemical composition. Numerous examples were given in work by Bassett and Bedwell [1]. Their first paper in the series concerns monohydrogen phosphates as well as double salts with potassium or ammonium and from 0 to 7 mol of water of crystallization per mol of salt. An important representative is the biogenic mineral struvite, MgNH 4 PO 4 •6H 2 O, typically found in guano [2] [3].
A number of years ago we studied the crystallization of phosphates of some transition metals including Co, Ni and Cu and in particular double salts with ammonium [4] [5]. In this connection we needed, among others, the solubility product of ammonium nickel phosphate hexahydrate, NH 4 (1) This compound is often named Ni-struvite, because it is isomorphous with struvite. Our way of writing the solubility product has been chosen for two reasons: 1) The activity coefficient of an uncharged species like NH 3 is close to unity, and 2) 2 4 HPO − is the most abundant phosphate species in neutral and weakly basic solution, its concentration being orders of magnitude higher than that of 3 4 PO − except at high pH. We then avoid involving the third dissociation constant of phosphoric acid, which is not known with so high precision as the two others.
Solubility products of simple salts comprising only one type of cation and one type of anion are often found in standard tables [6] and databases [7], whereas data for mixed salts are much more difficult to retrieve, if they exist at all. For unknown reasons we never published any details on the value for Ni-struvite quoted in our paper on crystal habit [4], and nobody else seems to have done so.
Recent literature, however, points to a certain interest in this and related substances in different fields as examplified by spectroscopy [8], microbiology [9] and pollution control [10] [11], where information on solubilities seems important for understanding the mechanism of incorporation of the mineral in microbes and evaluation as fertilizer struvite obtained from Ni-containing wastewater. We therefore found it worthwhile to reconsider the data and carry out a revision using new values of equilibrium constants.

Apparatus
Colorimetric measurements were carried out on a Zeiss PM QII spectrophotometer. Precipitates were examined by optical microscopy either in situ, through the bottoms of the flasks, with a Zeiss Axiovert 25 inverted microscope, or ex situ with a Zeiss Jenapol polarizing microscope.

Reagents and Solutions
All reagents and solutions were prepared from Merck analytical grade chemicals except nickel chloride solution, which was prepared from Riedel-deHaën analytical grade nickel chloride hexahydrate. The water used for precipitation and dissolution experiments was demineralized water further purified by passing through an activated carbon filter and a Silhorko mixed-bed ion-exchange column. The conductivity of the water was close to that reported for pure water. Solutions for nickel determinations are described below under Analysis.
Ni-struvite for dissolution experiments was synthesized according to Bassett

Precipitation and Dissolution Experiments
Precipitation

Analysis
Two different methods were used for determination of equilibrium concentrations of nickel. In the precipitation experiments nickel was determined by colorimetry on the dimethylglyoxime complex according to Mitchell and Mellon [13]. The reagents are, in the order added, a saturated aqueous solution of bro-  HPO − ) in Equation (1) were calculated from the known total concentrations of chloride, nickel, ammonium and phosphate and literature values for the equilibrium constants, i.e. dissociation constants of phosphoric acid and ammonium as well as stability constants of complexes of nickel ion with ammonia, phosphate and chloride [7]. The resulting system of nonlinear equations was solved by a Newton-Raphson iteration using a previously described computer program [14], which also yields pH and ionic strength I. Activity coefficients of ions were calculated from I with the Debye-Hückel equation A total of 12 precipitation and 16 dissolution experiments were carried out. Of the latter, 3 results deviated so much from the mean that it was decided to make a statistical test for extreme deviations. The test yielded significant to highly significant deviation, so these results were not included in calculation of mean and standard deviation. One of the cases concerns the experiment with dissolution in pure water, yielding a solubility product significantly lower than the average.

Calculations and Results
Probably the rate of dissolution in the absence of acidity is so low that the solution is not yet saturated at the time of sampling. Another cause could be uncertainty of the analysis, as the concentrations were very low. In the other two cases slight, but still significant deviation in the direction of higher solubility was found; this will be discussed below. In addition to the ordinary precipitation experiments we analyzed the mother liquor from Ni-struvite synthesis, so that the total adds up to 13 of each kind of experiment. In nine of the dissolution experiments only ammonium dihydrogen phosphate was added at concentrations ranging from 0.2 to 10 mM. Table 1 and Table 2 show the amounts of salts added in the ordinary precipitation experiments and in the dissolution experiments as well as equilibrium concentrations of nickel found by analyses of the saturated solutions.
In the mother liquor from the synthesis chloride concentration was 0.0504 M, and the equilibrium concentration of Ni was 2.3 µM. Ionic strength was high in this solution, I = 0.6, but otherwise the values ranged from 0.0015 to slightly below 0.1. In this range we can trust the validity of the Debye-Hückel Equation (2) for calculation of activity coefficients. Values of pK sp = −log K sp are plotted    A remarkable fact is that the result from the mother liquor, indicated with a filled symbol, does not deviate significantly from the rest in spite of the high ionic strength. As a whole, no significant dependence on ionic strength was found; regression analysis yielded a correlation coefficient << 1. The mean value with its standard deviation was found as 11.03 0.03 sp pK = × Finally we notice that the mean values for each of the two series of experiments are very similar, being 11.036 for precipitation and 11.034 for dissolution.
Thus the general criterion for equilibrium, that the same value of the equilibrium constant should be found on approach from either side, is fulfilled in this case. This is by no means trivial, as the literature has many examples of a significantly higher value of K sp from precipitation than from dissolution.

Discussion
The solubility of Ni-struvite is a little lower than that of struvite, for which we have found pK sp = 9.94 [16]. More recent research by other investigators [17] Table 3 shows calculated solubilities of the two solids in pure water as well as concentrations in a solution saturated with respect to both. The dependence of solubilities on pH is shown in Figure 2; it is supposed that pH is regulated by the addition of hydrochloric acid.   of a new phase like TNP, which could explain deviating results for the solubility product, is more easily overlooked in dissolution than in precipitation with the procedures used in the present study.
Both metal ions, Mg 2+ and Ni 2+ , are likely to be found as hexaquo species in both the solid phase and in solution except, for the latter, at high solution concentration of ammonia. Ni-struvite is isomorphous with struvite with all three axes a, b and c being shorter by 0.5% on the average [22] [23]. The electrostatic (Madelung) energy is thus slightly more negative for Ni-struvite, whence we

Conclusion
The previously published value of pK sp [4] agrees with the present one within experimental uncertainty as demonstrated by the results presented above. The solubility product may thus be useful in estimates of dissolved nickel in soil and other systems containing ammonia and phosphate. Similar studies have been attempted with the analogous cobalt salt, but the strong color of Co(II) has turned out to be a problem in the analyses, so no reliable results have yet been obtained.