A “Reference” Precipitated Tricalcium Phosphate Hydrate PREPARATION AND IDENTIFICATION
I
A concentrated sucrose solution 1s Since industrial and reagent tricalcium NDUSTRIAL precipitated tricalsaturated with calcium oxide, clarified, phosphates show such variance in cium phosphates are characterized and neutralized by the slow addition chemical, structural, and fertilizer by marked variance in chemical, strucof concentrated H3P0, requisite for properties, a process is proposed for the tural, and fertilizer properties (9). the formation of Caa(P0&. The syspreparation of a tertiary precipitate of Workers in phosphate research have tem is agitated vigorously during the accordant composition and uniform been handicapped by this variance, and addition and continuously for 4 hours properties as a reference material in they should be able t o obtain or prepare t h e r e a f t e r . T h e p r e c i p i t a t e is chemical and biochemical research. a reference precipitate of definite comwashed free of sucrose by either The process prescribes the slow addiposition and uniform characteristics. centrifugation or filtration and dried tion of concentrated HaPo, to a chilled There is no acdepted or sponsored a t low temperature. The several lime-saturated concentrated sucrose procedure for making the tertiary phosfactors that affect the process will be solution, prolonged agitation, filtration phate of calcium. At least three procmentioned and pertinent precautions or centrifugation of the precipitate, esses had been employed in the manuwill be prescribed. and low-temperature drying. Chemfacture of the several lots that were ical and x-ray examinations demonexamined, and two of the purported terstrated that the calcium content of the P R E P A R A T I O N O F REACTAXTS. PhO8tiaries proved to be dicalcium phosphoric A c i d , 85’% C.P. Determine aqueous solution of sucrose occurs as phate. The pertinent conclusion by exact PpOs content and express as per the sucrate rather than the hydroxide. cent by weight. (In titrimetric analyHpdge et al. (6) that “the composition Accordance in composition and reprosis, the aliquot should contain 1 2 0 of the precipitated phosphates is seen ducibility of product were established mg. of Pz06.) to depend upon the mode of precipitaSucrose Solution of Lime. Dissolve by chemical and x-ray determinations. 450 grams of pure sugar in 2 liters of tion rather t h a n t h e amount of reactDifferentiation between the “refercold, carbon-dioxide-free, d i s t i 11e d ants” can be amplified by the observaence’, tricalcium phosphate hydrate water. Ignite 150 grams of finely tion that certain properties of the preand hydroxyapatite is provided through ground calcite or marble for 3 hours a t 1000” C. in an electric furnace, cipitates are affected also by temperat w o simple chemical tests in comparistirring at least three times during the ture prevalent during periods of sons of their 900’ C. calcines. 1-3 hour ueriod: cool in a desiccator precipitation, protracted digestion, and grind- t o pass an 80-mesh sieve. washing, and drying. Into the 2 liters of sugar solvent in an . appropriate flask introduce 75 grams of the burnt lime; stopper The primary objective of this paper is to prescribe a process by and shake vigorously and again hourly, six times; allow t o which a tricalcium phosphate of accordant composition and uniclarify overnight or centrifuge and determine CaO content in per form properties can be prepared as a standard reference for cent by weight. chemical and biochemical researches. Although intended priWashing Solution. Saturate 10 liters of carbon-dioxide-free distilled water with tricalcium phosphate and protect from marily for the preparation of small quantities, the process can be atmosphere. scaled to industrial batch operations to obtain a product certifiable as reagent grade ( 7 ) . PRECIPITATION. Introduce 800 grams of the sucrose solution X second objective is to fulfill the expectation “that a simple of lime into a 1-liter disk-covered beaker, partially immersed in a and rapid chemical test would be proposed for the identification bath of iced water. A small hole in the center of the disk is proof industrial tertiaries” (9). Hence, a simple technique has been vided to accommodate the shaft of a rotating blade so that the evolved whereby the proposed reference tricalcium phosphate system can be agitated under cover during the addition of the hydrate can be distinguished from hydroxyapatite without resort acid. Add concentrated &Po4 dropwise, and stir vigorously to x-ray identification. and continuously until the addition amounts to a n excess of 0.25% of the stoichiometric requirement of Ca3(P04)2. MainPREPARATION tain the solution-suspension a t proximately 5” C. during 4 hours The present procedure is distinct from the processes that emof additional stirring, and either centrifuge or cover closely and ploy soluble neutral calcic salts and phosphates and from the proallow to stand overnight. Siphon the supernatant, dilute cedure whereby a n aqueous system of monocalcium phosphate the remaining solution with an equal quantity of “washing and calcium sulfate is ammoniated, according to the equation solution”, and filter with low suction on a 12-inch Buchner funnel. (10,1 1 ) : Wash free of sucrose by means of successive small portions of the Caa(POa)z-saturated washing solution, allowing full removal of CaH4(POn)z 2CaS04 4XH3 = Cas(PO,)n 2(NHJzSO4
+
+
+
W. H. MACINTIRE, GEORGE PALMER, AND H. L. MARSHALL1 The University of Tennessee Agricultural Experiment Station and The Tennessee Valley Authority, Knoxville, Tenn. 1 Present
address, Phosphate Mining Company, Nichols, Fla.
164
February, 1945
A . Sucrose solvent, 22.5 %
INDUSTRIAL AND ENGINEERING CHEMISTRY
E . Sucrose nolution of CaO [absence of sucrose a n d of Ca(0H)zI
C. Sucrose a n d occluded amorphous CaCOi f r o m B , after passage of COz
165
D. Calcite from CaCOa of C, after removal of sucrose
Figure 1. X-Ray Diffraction Patterns Showing Components of Sucrose Solution, Alone and Saturated with CaO, and after Removal of CaO by Passage of COn
every portion2. The washed precipitate should be exsicc a k d i n a Hemphilldesiccator until constant weight is attained, or dried in a n electric oven overnight a t 105’ C., and ground to pass 100 mesh.
content, the hydroxide content of the sucrose-lime system was replenished through hydrolysis of the sucrate. The evaporated lime-free sugar solution gave the sucrose pattern A of Figure 1, whereas the evaporated limeCOMPOSITION O F T H E SUCROSE SOLUTION OF LIME oo s a t u r a t e d sugar solution 1 AmtTtmS, As pERcrm OF 72’ R&REMEEIPT cos (Po4)+ yielded no i n t e r f e r e n c e The dissolubility of CaO maxima, as in B. Obviously, in sugar solutions has been Figure 2. Changes in pH Induced by Progression in Addithe Of the lime had shown to exceed that of tion of HsPO, to Lime-Saturated 22.5% Sucrose Solution effected conversion of the Ca(OH),, to increase with sucrose to noncrystalline calconcentration of sucrose, ciumsucrate. Another portion of the lime-saturated sugar solution and to increase with lowering of temperature (2). Dubrunfaut was subjected to a passage of gaseous COS and then evaporated. (4) found t h a t at 0” C. the molecular ratio of lime t o sucrose in a Since the residue obtained byevaporationof the COAreated limelime-saturated sugar solution was eight times t h a t a t 100’ C. sugar solution gave sucrose pattern C without lines indicative of Cameron and Bell (2) noted t h a t early workers ascribed the formulas C12HzzOL1.Ca0, 2ClzH2z011.3Ca0, and ClzH22011.3CaO to CaC03,i t is apparent that the precipitation of oalcium carbonate induced a reappearance of sucrose. When the residue from the the compound formed in sugar solutions of variant concentration. evaporated mixture of sucrose and amorphous CaC03 was washed Cameron and Patten (3) obtained t h a t solid phase by warming a free of sucrose, the occluded carbonate clustered and reglime-saturated sucrose solution and found t h a t the solid phase istered as calcite in pattern D. The pattern of sucrose in A , was amorphous and was “one of a series of solid solutions”. Because of the uncertainty as to whether the solute in limeits absenceand the absence of the pattern of Ca(OHh in B , the resaturated concentrated sucrose solution is hydroxide or sucrate, appearance of the sucrose pattern in C without pattern indication i t seemed advisable t o establish the composition of the stipulated of the observable precipitate of CaC08, and the occurrence of calcitein D point to the following conclusions: Calcium sucrate, lime-saturated 22.5% sucrose solution. This solution had a pH of 12.4 by potentiometric determination, indicative of OH equivarather than Ca(OH)z, was the predominant solute in the sugar solution of lime; the sucrate underwent progressive hydrolysis lence of 0.032 N , and contained 0.03% COZby weight. Two days and carbonatation during the passage of CO,; the precipitate of after a n aliquot of the lime-saturated sucrose solution had been spread thinly in a Petri dish and allowed to evaporate in the CaC03 was too dispersed in the re-formed sucrose to register a laboratory atmosphere, the resultant solid showed a COz content pattern; and removal of the freed sucrose allowed the peptized of 2.86%. Hence, upon the carbonatation of its initial Ca(OH)2 calcium carbonate t o acquire the calcite structure.
b
‘
* Filtration can be expedited by first expanding the volume of the preoipitate by an equal volume of 1 2 0 -40 mesh quartz, processed by aqueoua washing before and after digestion with HCI, 1 f 9. The quartz is then screened from the precipitate. Wash-drying with acetone has been found inadmiasible.
EFFECTS FROM ADDITIONS O F H.PO, TO T H E LIME-SATURATED SUCROSE SOLUTION
CHANCESIN pH. I n planning the preparation of the “reference” tertiary phosphate, it was postulated t h a t the calcium
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the 927, addition went from pH 11.65 to 11.20; the system that received the 100% addition went from 10.27 to 5.72, whereas the one that received the 0.03-ml. excess showed a change in pH from 9.55 to 5.40. I n the preparation of the “reference” tertiary, a similar result is attained by the p r e s c r i b e d 4 - h o u r p e r i o d of agitation. IDENT~TY OF SUCCESSIVE PREC I P I T A T E S FORMED. Upon assumption that the sucrate would function as calcium hydroxide, the succession of primary, secondary, and tertiary phosphates would be indicated by the equations: 2H8P04
A . C%HP0~.23x0 upon 1 1 s neutralization of sucrose-lima solution
Figure 3.
B . rMixture of CaHPlh.2HzO a n d Cm(PD&)2 .nHzO upon. =/a neutralization of sucrose-lime solution
+ Ca(OH)2 +
C. Caa(POi)z.nHzO u p o n complete neutralization of sucrosel i m e solution
To elucidate this point, the H3P04 was added in separates of 2/3, and full stoichiometric equivalence of the CaO content of the sugar solution. The three resultant precipitates Ryere filtered immediately and subjected t o x-ray examination. The large flaky crystals, formed a t the initial stage of the addition of the acid to the sucrose solution of lime, were suggestive of the monocalcium salt. Upon agitation, however, the flakes passed quickly into a milky suspension. I n an attempt t o establish their identity, a l/12 quota of the full HaPo4 equivalence was added to another portion of the lime-saturated sucrose solution, and the resultant large flakes were filtered immediately. Under microscopic and x-ray examination they proved to be dicalcium phosphate dihydrate. Obviously the equationed successive formstions of the primary and secondary phosphates were virtually simultaneous. Pattern A of Figure 3 shows CaHP04.2Hz0as the precipitate present a t the conclusion of the initial ‘/3 addition of HsPO,. The product present after the similar addition of the second E/, portion of acid proved to be a mixture of Cas(POJz.nH20and CaHPOa.2Hz0,as shown by pattern B. The product resultant from the full addition of vias the tertiary hydrate C. The x-ray pattern of this precipitate was identical with patterns registered by parallel precipitates that were agitated 30 minutee, 2 hours, and 4 hours before filtration. Prolonging agitation, however, induces agglomeration of crystals and affords a pattern characterized by distinction of lines, as in C.
Composition of Precipitates Formed upon 1/3,2/3, and Full Neutralization of Saturated Sucrose Solution of Lime by Concentrated H,POa
sucrate solution would undergo continuous hydrolysis and thus provide a sustained reaction between Ca(OH)z and the slowly added acid. Therefore, pH changes induced by variant increments of concentrated H3P04 to a constant of the lime-saturated 22.5% sucrose solution were determined potentiometrically. To 100-gram portions of the chilled lime-sucrose solution, the acid was added in fractions of 17, 34,67,83,92, 100,and 100.25% of the quantity required to convert the dissolved lime to Ca3(P04)2. The resultant systems were closed, agitated vigorously, and brought to 20’ C., and their pII values were obtained within a n hour. Figure 2 shows that the effects of the 17 and 34% additions of H3PO4upon p H were not pronounced, whereas the 67% addition brought a decided decrease. Further decreases in p H were induced by the larger additions, those of 92 and 100% in particular. The sensitivity of the system in which the two solutions were brought together in stoichiometric proportion is reflected by the drop in p H from 10.27 to 9.55 that, came from a n excess of 0.03 ml. of 85’% HIPOI. The distinct alkalinity of the system that received the slight excess of acid may be attributed to a temporary occlusion of the acid and/or to a measurable hydrolyzation of the mass of tertiary precipitate. When allowed to age, however, those systems that contained 90% or more of the stoichiometric equivalence of H3POa registered decided decreases in p H values. The system that received
TABLEI. EFFECTOF CONCENTRATION
O F SUCROSE
SOLUTION OF BURNTLIME AND SYSTEM TEYPERATCRE UPON COMPOSITION FORMED BY ADDITIONOB CONCENTRATED &PO,
AND AVAIL4BILITY O F PRECIPITATES“
Concn. of Sucrose Soln.,
Temp. of System,
Precipitated Tricalcium Phosphates % of Availability by: r0Citrate-Insol. PsOh by: Agitation Agitation at 6-min. Constant agitation for at 5-min. Constant agitatiori for interval 1 hour interval 1 hour __ Single5 Single Double single Single b Single Double Single 1:lOO 1:lOO 1:lOO 1:ZOO 1:lOO 1:lOO 1:lOO 1:2OOc 20.86 19.80 5.10 8.25 51.8 54.3 88.2 81.0 19.20 18.90 3.85 7.80 55.5 56.2 91.1 81.9
Composition . O C. CaO, 70 P20s, 70 PIOa/CaO Ppt. % 26 51.36 43.30 0.843 A-1 11.25 5 50.82 42.90 0,844 B-1 11.25 A-2 22.50 25 51.28 43.10 0.840 17.65 5 50.40 43.10 0.855 13.43 B-2 22.50 a Oven-dried at 130’ C. b -4s prescribed for commercial fertilizers b y official method of A.O.A.C. C Ratios of 1 gram per 100 ml. and per 200 ml. of citrate reagent.
15.50 12.70
2.70 0.85
.. ..
59.8 68.8
63.9 70.5
93.7 98.0.
,.
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February, 1945
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CONCENTRATIOS OF THE SUCROSE SOLUTION OF LIME AND SYSTEM TEMPERATURE DURING PREClPITATION
TABLE11. “REPRODUCIBILITY” OF PRODUCT BY FOUR OPERATORS IN FOLLOWING THE PRESCRIBED PREPARATION OF PRESince the oxide of calcium is more dissolvable than the hyCIPITATED TRICALCIUM PHOSPHATE” drate, freshly burned lime was used in making the sucrose-lime Operator CaO, % P2Oh % P2Os/CaO solutions utilized in preparing the precipitates. The influence of A 49.94 42.88 0.859 lime concentration in the sucrose solution and the effect of system €3 50.50 43.00 0.851 c 50.34 42.50 0.844 temperature upon composition and filterability of the precipitates D 49.08 43.13 0.879 were initial considerations. Accordingly, stoichiometric addiTheoretical 6 51.25 43.27 0.844 8 Products dried overnight in an electric oven. 6 Caa(POdn.Hr0. tions of H3POd were made t o the lime-saturated 22.5% sucrose solution, and to one made by a 1 1 dilution, at 25” and a t 5” C. The effect of the dilution upon calcium ionization was registered TABLE 111. AMMONIUM CITRATEAVAILABILITY VALUESFOR A SERIES OF “REFERENCE” TRICALCIUM PHOSPHATE PREPARED BY by a rise from the initial p H of 12.4 to 12.5; further dilutions gave SEVERAL OPERATORS AND DRIED AT SEVERAL TEMPERATURES elevations in pH up to 12.9. % Availability by Citrate Rigestion with: ~~~i~~ The analyses of Table I indicate that the specified variance in Sample T$m&, Periodic Continuoua agitation) concentration of the lime-saturated sucrose reactant exerted no No. agitation0 Single DoubleC appreciable effect upon composition of precipitates formed a t 1 130 81.85 54.27 88.22 58.86 2 130 63.87 93.76 identical temperature. The precipitates from the 22.5oJ, sucrose 3 130 55.22 56.15 91.07 68.84 4 130 70.53 98 02 solution were, however, of larger particle size and therefore more 86.59 100.00 5 120 74.70 readily filterable. Although all of the products were closc to 6 130 54.85 59.07 95 35 7 120 72.59 91.18 100.00 theoretical in their PzOs/CaO ratios, the analyses indicate that 8 105 90.95 97.10 100 00 the precipitates formed a t 25” C. were slightly more basic and At 5-minute intervals in water bath. b In electrically heated chamber. somewhat less dissolvable in ammonium citrate than those formed Value obtained when the result from a single digestion is augmented b y the amount of PzOs extracted from the residuum of the initial digestion. a t 5’ C. The probability of hydrolytic effects is less, however, in chilled systems, and a maintained system temperature of 5” C., therefore, is prescribed. there is no prescribed official procedure by which to determine t h e The patterns of Figure 4 reprezent the four products of Table I. availability of precipitated tricalcium phosphates, the citrateSince the four tricalcium phosphate hydrates registered the same insoluble content by the A.O.A.C. digestion (1) is utilized as an pattern, i t follows that their structures mere not altered meawrinverse measure of the available content. When a 1-gram charge ably by the variance in the concentration of the sucrose solution of of a precipitated tricalcium phosphate is digested one hour in lime or by the 20’ variance in system temperature. neutral ammonium citrate, with periodic agitation as prescribed REPRODUCIBILITY OF PRODUCT. The products and analyses of for the minor water-insoluble fraction of a commercial fertilizer, Table I were made by an experienced analyst in checking results the per“centage availability of the tertiary phosphate is invariably obtained by the initial operator, who had obtained a product havless than the percentage indicated by continuous agitation of t h e ing a PZO6/CaO ratio of 0.814. To ascertain the expectancy for digestate (8). I n every comparison of Table If1 a higher perreproducibility of product, the process and analyses of respective products were assigned to four operators not familiar with the centage extraction (gs much as 25% more) was obtained when prescribed technique. The analyses of Table I1 and the x-ray agitation was continuous. A single digestion of a 1-gram charge patterns of Figure 5 indicate that reproducibility of product can in 100 ml. of neutral ammonium citrate, therefore, is not an adbe attained by operators not experienced in the prescribed missible procedure for the determination of the available PzOr ’ content of a precipitated tricalcium phosphate of unidentsed manipulation or in the analysib of the reactants and product. AVAILABLEPlO6 CONTENTOF “REFERESCE”PRECIPITATE.structure, even when the digestates are agitated continuously and It has been shown that the determined PzO6/CaO ratio is not an regardless of the inclusion of a pulped filter. infallible index to the degree of availability a precipitated tertiary The ammonium citrate reagent was proposed by Freseniw, Neubauer, and Luck in 1871 (6)as an extractant for the waterwill register in chemical and biochemical tests (9). Although
+
A-I.
11.25 % at 25’ C.
Figure 4.
B - I . 11.25% at 5’ C.
A-2.
22.5% a t 25’ C.
B-2.
22.5% a t 5O C.
.
X-Ray Diffraction Patterns of Precipitates of Table I, from Sucrose Solutions of Two Concentrations at T w o Temperatures
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-Operator A
Operator R
Operator C
Operator
L)
Figure 5 , X-Ray Diffraction Patterns Showing Reproducibility of Product in Preparation of Tricalcium Phosphate Hydrate by the Four Operators of Table I1
insoluble phosphates, other than residues of rock, that are encountered in the analysis of acidic fertilizers. The dissolvent effectiveness of 100 ml. of the citrate solution is unduly taxed and vitiated, however, when applied to a 1-gram charge of tricalcium phosphate. This was true especially of precipitates dried a t temperatures above 120” C. Decidedly higher availability values and better concordance were obtained by double digestions of 1-gram charges of the eight precipitates of Table 111. Higher availability values (as much as 100%) were obtained when the charges were subjected to continuously agitated single digestions and when decreased to 0.5 gram. The p H of the neutral animonium citrate reagent rises to 7.9 during the digestion of both 0.5gram and 1-gram charges of the “reference” tricalcium phosphate. A citrate digestion of a 1-gram charge of a tertiary precipitate and a successive digestion of the residue from that digestion would be consonant with the analytical removal of water-soluble phosphates from a fertilizer before its residue of relatively low citrate-soluble PnO5 content is subjected to the conventional citrate digestion ( 1 ) . Should the use of ammonium citrate be deemed admissible, it appears that a successive extraction should
TABLE IV. DIFFERENTIATION OF HYDROXYAPATITE AND “REFERENCE” HYDRATED TRICALCIUM PHOSPHATE, AND THEIR
CALCINES, BY AMMONIUM CITRATEA N D CITRICACIDDIGESTIONS Dissolubility of Tertiaries I n ammonium citrate‘‘ I n 2% citric acidb Before After Bifore After ignition ignition ignition ignition
%
%
%
%
Hydroxyapatite 13.2 73.6 68.6 34.6 18.0 83.9 71.6 52.8 17.7 77.4 75.8 45.9 16.0 79.2 64.5 39.0 “Reference” Precipitatec Operator 1 49.4 96.5 85.7 86.9 Operator 2 60.0 93.0 98.8 59.0 Operator 3 90.9 49.9 97.4 91.1 Operator 4 86.4 50.9 95.9 97.1 Industrial Product 6-988 52.2 55.0 99.5 87.6 Continuous agitation ,1 hour at 65’ C.; 1-grim charge per 100 ml. of neutral solution, 1.0.)specific gravity. b b Continuous agitation one hour a t room temperature: 1-gram charge per 100 ml. Prepared at.diferent peiiods by the several operators; analyses by one analyst.
9-903 S-982 9-986 9-989
(i
be made if the availability of a precipitated tricalcium phosphate is to be measured by use of a 1-gram charge per 100 ml. of the reagent. The implied recognition of the fact that the ammonium citrate digestion a t 65” C. is not admissible in the evaluation of a basic slag can be extended logically to the evaluation of a precipitated tertiary calcium phosphate. I n the determination of the available Pzo5 content of basic slag by the Wagner method ( I ) , continuous agitation a t room temperature is prescribed for the digestion in 2% citric acid. A freshly washed “reference” tricalcium phosphate precipitate was divided into four portions, and these were dried under four conditions: 3 days over sulfuric acid a t 42 mm. pressure, and overnight a t 105”, l l O o , and 120’ C. Subjccted simultaneously to 2% citric acid digestions for 1 hour a t room temperature, the four samples gave respective percentage dissolubilities of 99.8, 99.0, 99.9, and 98.3%. These analytical values are consonant with the effectiveness shown by the “reference” tricalcium phosphate hydrate when it was compared with superphosphate in pot cultures (9). After 0.5-gram and 1-gram charges had been digested one hour in the 2% citric acid reagent, the solution was still definitely acidic, the initial pH of 2.50 having changed to respective values of 3.55 and 3.85. Obviously, the 2% solution of citric acid is preferable to the ammonium citrate reagent for the determination of the “availability” of precipitated tricalcium phosphates. CHEMICAL IDENTIFICATION
A tricalcium phosphate hydrate and a hydroxyapatite cannot be differentiated by chemical analysis or by degree of solubility in the official ammonium citrate solution. Moreover, the x-ray diffraction pattern of the tertiary hydrate is ident’ical with the pattern of the hydroxyapatite. Upon ignition a t 900” C., however, the “reference” tricalcium phosphate hydrate precipitate from the sucrose solutions yields p-Cay(POa)s, whereas the hydroxyapatite yields the oxyapatite, CaloO(P04)8 , as shown by their respective patterns in Figure 6. The relatively expensive x-ray apparatus is not common equipment, however, and it seemed desirable to evolve a simple chemical technique for the identification of a precipitated tricalcium phosphate hydrate. Since the two precipitated tertiaries underwent no discernible change in physical condition a t 900” C., it was postulated that the
February, 1945
A.
169
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hydroxyapatite precipitate
B.
Caa(PO4)z hydrate preaipitate
C.
Oxyapatite oalcine from A
D.
P-Caa(PO4)r f r o m calcine B
Figure 6. Differentiation between Hydroxyapatite and “Reference” Tricalcium Phosphate Hydrate from Sucrose-Lime Solution, Attained by Distinctive Patterns of 900” C. Calcines of the Two Materials
“reference” and the hydroxy precipitates could be differentiated through variance in the solubility of their calcines. In a test of this postulation, four tertiary industrial precipitates, established as hydroxyapatites, were compared with four “reference” precipitates obtained by four operators. The unheated precipitates and their 900’ C. calcines were subjected to continuously agitated digestions in ammonium citrate a t 65” C. and to continuously agitated citric acid digestions at room temperature. The comparisons in Table IV also included a n industrial “precipitated tricalcium phosphate” that had been identified as such. The results show that, by groups, the five commercial products were dissolved t o a less extent than the four “reference” precipitates in the citrate reagent, although the dissolubility range was considerable within the groups. It is again evident, however, t h a t degree of dissolubility in ammonium citrate is not an infallible criterion for differentiation of the two types of tertiary precipitates. The industrial precipitate of low citrate solubility was however, almost completely dissolved by the 2% citric acid reagent. A maximal solubility of 18% of the charge was shown by the oxyapatite calcines from the four hydroxyapatites, whereas dissolubility values of 50 to 60% were shown by the beta calcines of the “reference” precipitates obtained from the sugar solution. The tertiaries of both groups show citric acid sohbilities sdbstantially higher than the corresponding values accorded by ammonium oitrate. Unheated,jthe four hydroxyapatites show dissolubility of about SO% by citric acid digestion against 95% values for the four “reference” precipitates and for the industrial hydrated precipitate. Although the calcines of oxyapatite and those of 8-tricalcium phosphate were both less dissoluble than their respective parent precipitates, the disparity between the degrees of dissolution shown by the citric acid digestions of the two groups of calcines continues t o be substantial and serves to differentiate the parent precipitates. It seems safe t o conclude that a precipitated tricalcium phosphate can be designated 85 a hydroxyapatite when a 1-gram charge of 900” C. calcine registers a dissolubility of less than 20% by a continuously agitated 1-hour digestion in the ammonium citrate reagent at 65” C. In contradistinction, a tertiary precipitate is identified as a tricalcium phosphate hydrate when a like charge of its calcine shows dissolubility of aa much as 50% by the stipulated citrate digestion.
As stated, however, the 2y0 citric acid reagent has distinct advantages. It is prepared more easily, and digestion is made a t room temperature. A further advantage is t h a t by the use of a n admissible aliquot of the citric acid digestate, the analytical precipitation by molybdenum trioxide can be made directly. When an analytical charge of a precipitated tertiary is heated at 900’ C. and then shows approximately 90% availability by a 1-hour continuously agitated digestion in 2% citric acid a t room temperature, the product is identified as a normal tertiary hydrate. Every “reference” precipitate prepared by the presently prescribed process has met that test. ACKNOWLEDGMENT
The foregoing studies were conducted at The University of Tennessee Agricultural Experiment Station in collaboration with The Tennessee Valley Authority, Departments of Chemical Engineering and Agricultural Relations. Acknowledgment is accorded F. D. Oldham, a former associate, by whom the tricalcium phosphate hydrate was first prepared from the lime-sugar solution of calcium oxide. LITERATURE CITED
Assoc. of Official Agr. Chem., Methods of Analysis, 5th ed., p. 39
(1940). Cameron, F. K., and Bell, J. M., U. S. Bur. of Soils, Bull. 49, 32 (1907). Cameron, F. K., and Patten, H. E., J. Phys. Chem., 15, 67-72 (1911). Dubrunfaut, A. P., Compt. rend., 32, 333 (1851). Fresenius, R., Neubauer, C., and Luck, E., Z . anal. Chem., 10, 132 (1871). Hodge, H. C . , Lefevre, M. L., and Bale, W. F., IND. ENG.CHEM., ANAL.ED., 10, 156 (1938). MacIntire, W. H., U. S. Patent 2,095,994 (1937). MacIntire, W. H., Marshall, H. L., and Meyer, T. A,, J . Assoc. O$cial AQT.Chem., 27, 272-83 (1944). MacIntire, W. H., Winterberg, S. H., Marshall, H. L., Palmer, George, and Hatcher, B. W., IND.ENG.CHEM.,36, 547-52 (1R44). I .
Ro&W. H., and Rader, L. F., Jr., J. Assoc. Oficial A g r . Chem., 23, 235 (1940). Ross, W. H., Jacob, K. D., and Beeson, K. C., I b i d . , 15, 230 (1932). PRESENTED before the Division of Fertilizer Chemistry at t h e 108th Meeting of the AMERICAN CHEMICAL SOCIETYin New York, N. Y ,
