Sept., 1961
SOLUBILITIES OF POTASSIUM AND ANMOXIIJM TARANAKITES
1613
SOLUBILITIES OF POTA.SSI[UM ASD A3IJPOSIUM TA‘IRASAKITES BY A. W. TAYLOR AND E. L. GURNEY Division of Chemical Development, Tennessee Valley Authority, ’IYilson D a n ~Ahbalrka , Received M a y 6,1061
Tho solubilitj- products of potassium taranalrite, H6K8A16(P0,)8~18H20, and ammonium taranakite, l i b ( ?TH,)s“&(PO,), 181J20,v w e calculated from the compositions of solutions in equilibrium lyith synthetic preparations of the mlnersls Owing to the presenre of complex ions, the respective pK,, values, 178.7 and 175.5, must be regarded as apparent rathcr than true thermodynamic values. In solutions where the aluminum phosphate ion activity product. CLAIUPO,, exceeded 1 X both compounds dissolved incongruently t o form amorphous aluminum phosphate which prevented the solutions from reaching equilibrium with the taranakites in less than 200 days.
Occurrences of the taranakites as natural aluminum phosphate minerals were discussed by Bannister and Nutchinson.l Laboratory preparations of the potassium form were described by Haseman, Lehr and Smit’h2; its X-ray properties were reporbed by Smith and Brown,3 who assigned it the formula IIsK3AIS(l’O4),.18H,O on t,he basis of densit’y,unit’-,cellcontent and chemical analysis. The potassium mineral and its ammonium analog, I&(NIS4)3.-116 (PO4),.l 8H20, have been found among the compounds formed when soils are acted upon by the concentrated acidic phosphate solutions resulting from the dissolution of some phosphate fcrtilizer~.4 Information on the solubility of the tazanakites thus is important in relation to t’lie efficiency of such fertilizers. Measurements of apparent solubility products of the potassium and ammonium taranakites by equilibrium methods are presented here, together with information about the range of solution composition over which the taranakites dissolve congruently. Cryogenic measurements are reported in a cornpmioii paper.5 Measurements. Method.; .-To prepare potassium taraiiakite, a solution of 21.6 g . of aluminum in 870 nil. of 53.5% phosphoric acid was filtered and diluted to 2 1. The p R was adjusted to 3.4 with 10% potassium hydroxide solution. The resulting slurry, diluted to 3.5 l., was digested a t 50” for 24 hours in a polyethylene bottle t o yield crystals of tsranakite. The product r a s filtered, washed 10 times, and ~acuiim-dried over calcium chloride a t room tcmperatiirc. I t consisted of 40 p lamellar aggregates of hexagonal platelets with the optical properties reported for potassium taranakite.lf2 I t contained 8.675 N,9.6% A1 and 56.6% 1 ’ 0 4 . Ammonium taranaltit,e W V ~ R made the same Jvay, except t’hat the partial neutralization wa8 made with 28% ammonium hydroxide to p B 4.0. The product consisted of 50 p bexagonal aggregates of well-formed iiniaxial ( )crystals with h’, = 1.620 and N e = 1.513. It contained 4.1% KH4, 10.5% AI and 59.6% POq. Two-gram portions of the taranakites were equilibrated in 16-oz. polyethylene bottles with 500-ml. charges of dilute solutions of aluminum chloride and potassium phosphate or potassium chloride. The init,ial p H of the solutions was adjusted with HCl. The bottles were rotated end over end in a water-b’ath a t 25 k 0.05’. Aliquots of clear solution were withdi:awn for analysis at irregular intervals and (about taTice as often) pH’s were measured with a Beckman Model G meter, which mas standardized against 0.05 N potassium acid phthalate buffer a t p H 4.00. Potassium ion
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(1)
F. A. Bannister a n d G. E. Hutchinson, Mineral Mag., 28, 31
(1947). (2) J. F. Hasernan, J. Iz. Lehr a n d J. P. Smith, Soil Sci. Sac. Am. 1’7oc. (1950). 16, 76 (1951). (3) J. P. S m i t h and W.E. Brown, A m . Mineralogisl, 44, 138 (1959). (4) W. L. Lindsay a n d A. W. Taylor, Proc. 7th International Congress
Soil Sei., 1960 (in press), (5) E. P. Egan, Jr., Z. T. Wakeficld a n d IJ. B. Luff, J . Phys. C’hem., 66, 1609 (1951).
concentrations were measurrd by ff arne photometry, ammonium by a micro-Kjeldahl procedure, and phosphatc b ~ i spectrophotometry based upon molybdenum blue. Alu minum was determined either hv a fluorimetric estim~tionof the &quinolmolate complex i n a chloroform evtract or by direct spectrophotometric estimation of the same complex. The compositions of most of the colutions became constant after about 60 days. The solid phases were examined petrographically a t regular intervals. Results .--Final compositions of the solutions equilibrated with potassiiim taranakite, each based on the mcan value of a t least three analyses, are given in Table I.
When the activity of water is assuincd to l r unity, the solubility product of potassium taraiiakite, H~IC~,41~(PO4),~lS€I,0, may bc defined by thr expression PI
