mercury by reacting with the tetraphenylarsonium chloride titrant. However, as mercury forms a more insoluble precipitate with tetraphenylarsonium chloride than does cadmium, its solubility product is exceeded first and consequently the cadmium does not interfere when as much as 5 mmoles of cadmium is present per millimole of mercury On the other hand, larger amounts of cadmium repress the solubility of the cadmium-tetraphenylarsonium chloride preciDitate, b y virtue of the common ion effect, t o the point where both cadmium and mercury precipitate upon addition of the tetraphenylarsonium chloride titrant. The uranyl ion typifies the second type of interference. Elimination of interference is largely dependent upon the ability of the instrument to suppress the diffusion current of the nonreacting ion and yet measure the change in the diffusion current due to the removal of the ion being titrated. With the polarograph used in this study, the current due to 0.1 mmole of uranium can be suppressed in the titration of 0.01 mmole of mercury at a potential of -0.4 volt. At an applied voltage
of zero, 0.9 mmole of uranyl ion per 0.01 mrnole of mercury can be tolerated. A t this potential, the uranyl ion does not exhibit a diffusion current; therefore, a large quantity can be tolerated. In the presence of more than 0.9 mmole of uranium per 0.01 mmole of mercury, the titration of the mercury with tetraphenylarsonium chloride is erratic. The effect of stannic ion was studied as representative of the third class of interfering ions, which reacts with the titrant and also produces a diffusion current a t the titration potential. A mole ratio of stannic ion to mercury of 5 to 1 can he tolerated without effect. A t higher mole ratios, the error in the mercury determination is pronounced. The interfering ions that were tested form more insoluble tetraphenylarsonium chloride precipitates than other ions of that class. Therefore, the interference from other ions will probably be of the same order or less than that of the ion tested. ACKNOWLEDGMENT
The authors acknowledge the assistance of AI. A. M a r k in the preparation of this report.
LITERATURE CITED
Kelley, M. T., bliller, H. H., ANAI.. CHEM.24, 1895 (1952j Kolthoff. I. M.. Johnson, R. A., J Electrochem. Soc. 98, 231 (1951). Kolthoff, I. &I Lingane, .,,, J. J., “Polarography, 2nd ed., vol. 11, pp. 511, 577, Interscience, New York, 1952. (4) Lamprey. H., thesis, University of Michigan, 1935. (5) Rudden, C. J., “Analytical Chemistry of the Manhattan Project,” let ed., pp. 399-402, NcGraw-Hill New York. 19.50. (6) Sandell, E. B., ‘ Colorimetric Determination of Traces of Metals,” 2nd ed., pp. 441-52, Interscience, New York, 1950. (7) Snell, F. D., Snt41, C. T., “Colorimetric Methods of Analvsis.” 3rd ed., pp. 63-78, Van Xostcand, S e w York, 1953. (8) Willard, H. H., Smith, G. SI.,ISD. ENG. CHEK., ANAL.ED. 11. 186 (1939). (9) Ibid., p 269. RECEIVEDfor review August 27, 1956. Accepted November 17, 1956. Division of Analytical Chemistry, 130th hleeting, ACS, Atlantic City, N. J., Se tember 1956. Work carried out under Eontract KO. W-7405-eng-26 at Oak Ridge National Laboratory, operated by Union Carbidc Nuclear Co., a division of Union Carbide and Carbon Corp., for the Atomic Energy Commission.
Precipitation of Zinc Phosphates from Solutions of Sodium Ortho-, Pyro-, and Triphosphate 0.T. QUIMBY
and
H. W. McCUNE
Miami Valley laboratories, The Procfer and Gamble Co., Cincinnati 3 I , Ohio
,This study, undertaken to evaluate zinc precipitation methods of determining pyrophosphate and triphosphate, involved precipitation of zinc orthophosphate, pyrophosphate, and triphosphate, alone and in mixtures; coprecipitation of triphosphate with pyrophosphate at pH 3.8 was determined with radio-tagged phosphates. The triphosphate interference with pyrophosphate precipitation in the Bell method often causes the results for both species to be significantly in error; nevertheless, it may be possible to modify this method to yield trustworthy rapid determination of pyrophosphate in such samples as commercial triphosphate. Increasing the pH from 3.8 to near 5 will bring about nearly complete precipitation of orthophosphate, pyrophosphate, and triphosphate, but gives no promise of
248
ANALYTICAL CHEMISTRY
providing a trustworthy indirect method for one of the three species in their mixtures. Only in special casese.g., intermediate pH’s and low orthophosphate level-will it be practical to precipitate all of the pyrophosphate and triphosphate without contamination by orthophosphate. A new phase Zn?PzOj.3Hz0 was encountered.
S
published methods for the determination of triphosphate and pyrophosphate involve the use of zinc ions for the precipitation of one or both of these phosphates (9, 4, 6, 7 , 16, 19, 66, 27, SI). I n some of the supporting experiments on which the methods are based, and in the studies of condensed phosphate precipitations b y other metal ions such as manganese(I1) ( I Y ) , and EVERAL
tris(ethy1enediamine) cobalt( 111) (61), there is evidence that pyrophosphate and triphosphate are harder to separate from each other than orthophosphate or trimetaphosphate. Recent work ( $ I ) , as well as the present study, indicates that pyrophosphate and triphosphate usually coprecipitate. Furthermore, large amounts of pyrophosphate inhibit the precipitation of triphosphate and vice versa. It is therefore pertinent to examine the basis for these methods. Common ring phosphates (trimeta and tetrameta) were not studied because their zinc salts are soluble in water. EXPERIMENTAL
Preparation of tagged and inactive sodium phosphates has been described
(24). Other materials it-ere reagent grade except ap mentioned in the text. Phosphoius was determined by potentiometric titration between end points ( I ) on completely hydrolyzed samples. Zinc, which would interfere mith the procedure, was removed by passing a solution of the zinc phosphate dissolved in the minimum amount of 10% hydrochloric acid and then diluted, through a column of Amberlite IR-120H ion exchange resin. After the column had been nadied x i t h water, the zinc Raq eluted with 10yGhydrochloric acid and determined bv precipitation with 8-quinolinol ( 5 3 ) . Sodium TI as determined by precipitation n ith magnesium ur:inj 1 acetate (34) niter interfering zinc and phosphates had been removed b y a n adaptation of a published method (22) The sample was dissolved and the contlensed phosphates were hydrolyzed to ortliophoqphate. Magnesium carbonate was then used to precipitate a n y excess phosphate not removed by the zinc (already in the sample) on a d j u s b ing to the methyl orange end point I\ ith ammonia. I n precipitation experiments sodium triphosphate mas weighed as the hexahydrate and sodium pyrophosphate as the anhydrous salt. The p H of the solution was adjusted to the desired value, first n ith concentrated and then with dilute hydrochloric acid. (All p H measurements were made with a Beckman Model G meter with glass and calomel electrodes previously standardized with borate buffers of known pH.) A mechanical stirrer and the p H electrodes remained immersed in the solution throughout the precipitation. A fen drops of almost saturated ZnSOa.7H20 solution nere added. If no precipitate formed, a few drops of 0.25N sodium hydroxide solution were added to start precipitation. When precipitation had started, the remainder of the zinc reagent was added dropn ise and sodium hydroxide solution added to restore the desired p H value. Aftet the p H had remained conptant for 15 minutes, the precipitate was filtered in a sintered-glass crucible and washed with water. A quick-drving procedure of washing with alcohol follon ed b y ether was sometimes used before the anhydrous precipitate was prepared by igniting a portion at 400” C. The sample for chemical analysis could not be ignited, for most precipitates were rendered difficultly soluble. It was weighed a t the same time as the sample for ignition and its weight was corrected by the determined water content. RESULTS
Precipitation of Single Zinc Phosphates. As conditions of complete
precipitation were being sought, precipitations were never made from solutions more acid t h a n p H 3.8. A t this p H or higher no acid zinc phosphate 1%-asdetected. This was known for orthophosphate from t h e comprehensive study of the Zn0-H3P04-Hz0 system b y Eberly, Gross, and Crowell
Table 1.
pH of Pptn. 6.5 6.0 5.5 5.0 5.0 4.5 3.8
Yield of Zinc Phosphates Yield of Anhydrous Precipitate, 7c ZndPOh ZnzPz07
a,b
101.4 99.7 99.1 I
.
.
...
95.4 ...
e
iYaZn&’aOlo e
E
100.O b 103.2*.* 101.1e 99.5bsa 99.9f 97.9b
101.j d 101.8* 100.0b
99.7b 100.5f 100.4b
...
...
98:Sb 98. gb 99, gb*# 99.3f 98. 7b
Dilution 125 ml./g. Na phosphate. 10 wt. 7G excess ZnSO1. c Dilution 50 ml./g. 40 wt. yo excess of ZnSOc. Average of 2 to 4 results, others single values. f 100 wt. 7G excess ZnSOa. a
b
6
Table 11.
Titration of Acid Liberated in Precipitation
Mole Ratio Ortho-Pyro- Dihydrogen 1st Tri End Point, end point,, Taken pH pH 1 . 0 0 , o oo,o.oo 4.55 .. 0.00,1.00,0.00 4.03 .. 0.00,0.00,1.00 3.89 .. 1.00,1.00,0.00 4.35 .. 1.00,0.00,1.00 4.13 3.65 .. 0.00,1.00,1.00 4.05 1.00,1.00,1.00 4.23 4.00 4.02 3.87 4.02 3.98 3.00,1.00,1.00 4.27 3.38 4.27 3.83 1.00,1.00,3.00 4.07 3.98 4.07 3.93
pH 5.13 4.38 4.53 5.52 5.66 4.82 4.23 5.37 5.56 5.58 5.41 5.58 5.56
Zinc Present 2nd End Point Found, Theo., ml. base ml. base 11.49 11.47 6.20 6.14 4.50 4.44 10.16 9.73 7.74 7.79 6.22 6.26 8.10 7.68 8.63 7.68 8.58 7.68 10.52 9.36 9.30 9.36 8.25 6.56 7.41 6.56
Error,
%
- 0.17
0.98 1.3 4.2 - 0.64 - 0.64 5.6 12 12 12 12 11
13
2.00 g. sodium phosphate, 100 nil., 12.00 g. ZnSOd
(1.9). Basic salts were not encountered
at p H 5.5 or lower. Thus in each case the normal zinc or sodium zinc salt was formed. Yield data (Table I) show that precipitation was substantially complete at p H 5.0. The p H was repeatedly adjusted after the precipitation until it remained at the “pH of precipitation.” The yields are based on the weight of anhydrous precipitate, the weight of sodium phosphate taken, and the formulas given. The identity of the precipitates was confirmed b y analysis. D a t a given are for those precipitated at p H 5.0 to 5.5. Analysis, Calculated for Zn3(P04)z: Zn, 50.8; P, 16.1. Found: Zn, 50.0; P, 16.0. Calculated for Zn2Pz07: Zn, 42.9; P, 20.3. Found: Zn, 42.9; P, 20.3. Calculated for KaZn2P3Ol0: Zn, 32.2; P, 22.8; Na, 5.66. Found: Zn, 32.2; P, 22.7; Na, 5.70. Wide variations in the excess of zinc had little effect on yield, which was essentially quantitative a t p H 5 and the concentrations used (Table I). Interference from precipitation of zinc hydroxide or a basic zinc sulfate
would not be expected. A t p H 5.5 the concentration of zinc ion would have to be four times greater than that employed before a basic sulfate would precipitate according t o calcula t‘ions based on a solubility product of 2 X lOP17. This value was determined b y p H titration of 0.020M zinc sulfate with standard sodium hydroxide solution. It falls among the published values (11, 16, 80) of 1 X lo-”, 7 X 10-la, and 4.5 X IO-”. The composition of the solid phase, Zn4(OH)&O1, agreed with that of Kolthoff and Kameda (20). The precipitate formed when alkali is added to a zinc chloride solution is more soluble. If a solution containing but one of the three phosphate species is adjusted to its dihydrogen end point, addition of the theoretical amount of zinc for precipitation of the normal salt should free two hydrogens, which can be titrated b y adding enough sodium hydroxide to return to the same pH. This is not, however, exactly consistent with the actual behavior of pyrophosphate and triphosphate. For solutions containing 2.0 grams of Na2HP04, NarPzOT, or VOL. 29, NO. 2, FEBRUARY 1957
249
NaJ?3010 in 75 nil. of water, the dihydrogen end point falls a t 4.55, 4.03, or 3.89, respectively, by titration with hydrochloric acid. As will be seen from Table 11, the end point obtained with sodium hydroxide after addition of zinc is not a t the p H of the dihydrogen end point. Severtheless, the sodium hydroxide titration to the actual end point is essentially the theoretical value for t\yo hydrogens for orthophosphate and about 1% high for pyrophosphate and triphosphate. Characterization of P u r e Zinc Phosphates. T h e x-ray diffraction pattern of t h e zinc orthophosphate and triphosphate precipitates did not change during drying t o constant weight (3 t o 5 days) a t 25' t o 28' C. in air of 45 t o 5oY0 relative humidity. Upon ignition for 2 hours at 400' C. t h e airdried samples lost weight in accord with t h e reported hydrates (Table 111)namely, 4 H z 0 for Zn3(PO& [Eberly ( I J ) , 'Thilo and Schulz ( 2 8 ) ] ,and 9 H z 0 for NaZnzP3010[Raistrick (65) and Topley (SO) 1. With pyrophosphate the reported ZnzPz07 5H20 [Bassett, Bedwell, and Hutchinson (6)]was always precipitated, except in rather acid solutions or at higher temperatures. Some samples of ZnzPz07.5H20 had lost moisture upon air-drying under the above conditions (Table IT), for the ignition loss was but 97 to 99% of that for the pentahydrate. Probably all samples would do so, a t least on the surface, for diffraction patterns taken repeatedly on a sample of zinc pyrophosphate precipitate mounted on an x-ray diffractometer showed the fresh sample of damp cryatals to be Zn2Pz07.5Hz0, but as the surface of the mounted sample dried, lines of ZnzP207 5H20 weakened and new lines appeared. As the process continued, the new lines became strong enough to identify them unequivocally a$ arising from the trihydrate, ZnzPz07. 3Hz0 (Table IT). An attempt was made to approach the composition of hydrates obtained by precipitation for all three salts b y vapor hydration and dehydration a t 26" C. and relative humidities of 33 to 8670. The met precipitates dried within about a week to 98.6 to 101% of theory for zn3(P04)2 4Hz0, ZnzPz07 5Hz0, or IiaZn2P3Olo.9 H z 0 a t all humidities and remained substantially constant thereafter. Samples partially dehydrated by heating a t 105' C. absorbed water, but came substantially to the original hydrate level only in the case of orthophosphate, which rose to 97 to 98% of theory for Zn3(PO& 4 H z 0 in 18 weeks at 62 to 86% R.H. The partially dehydrated pyrophosphate absorbed water to about 88% of the trihydrate level at 33 to 62y0R.H. and was gaining very slowly if a t all, after 18 weeks. Valid data for rehydration of the par-
250
ANALYTICAL CHEMISTRY
tially dehydrated sodium zinc triphosphate precipitate could not be obtained, as part of the triphosphate is hydrolyzed during the dehydration; this is analogous t o the decomposition of NasP3010. 6 H z 0 upon dehydration a t t2mperatures near 100' C. (25, 25, 29). As no reference to the trihydrate was found in the literature, it is presumed to be a new phase. I t s identification depends on a preparation of zinc pyrophosphate made from pure Sa4P207 under Bell conditions (S), except that the temperature of precipitation was 75' C. The wet crystals thus obtained gave the same pattern as that of the sample dried 3 days a t 75' C., showing that the phase n-as not dehydrated during drying. Table IV s h o w that the innermost line is a t a larger diffraction angle (smaller spacing) than the innermost line of the pentahydrate. This is the relation expected for a lower hydrate. Upon ignition at 400' C. the sample that had been dried for 3 days a t 75" C. lost 15.6% of volatile matter (theory for trihydrate 15.1%). Interplanar spacings calculated from x-ray diffraction powder patterns (Cuk'cu-radiation) are given in Table IV for Zn2Pz0, 5H@, ZnzPz07.3Hz0, and iYaZnzP30109Hz0. The pattern obtained for Zn3(PO& 4H20 corre-
Table 111.
sponded to ASTM data and those of Thilo and Schulz (28) for this phase, but the pattern for ZnzPz07.5Hz0 differs from the ASThI pattern, which mas never encountered. The data given for ZnzPz07.3H20 do not accord nith those of any previously known phase. The pattern for NaZnzP3010. 9 H j 0 is identical with that obtained on a sample of this salt kindly submitted by Bernard Raistrick. His sample was made by mild acid hydrolysis of pure cyclic trimctaphosphate, Na3P309, in the presence of zinc (25). Patterns for the anhydrous orthophosphate and pyrophosphate samples (ignited to 650' C.) agree with thcse given in the ASTM file. Ignition to 850' C. converted N a Z n z P 3 0 ~9oH. z 0 into a material which a t room temperature v a s amorphous to x-rays. Precipitation of Mixtures of Orth3phosphate, Pyrophosphate, and TIiphosphate. When a n attempt was made to precipitate ternary mixtures of orthophosphate, pyrophosphate, and triphosphate b y means of zinc a t p H 5.5, two complications became evident. T h e composition of t h e precipitate deviated from theory-Le., t h e phosphorus recovery was usually low and the zinc content of t h e precipitate was usually above the theoret-
Water Content
of Precipitates
(Air-dried a t 26-28' C. and 45 to 5oY0 RH) Water, yo Precipitate Theo. Found 15 67, 15 74, 15 61, 15 52, 15 71 15 73 ZndPOdz 4Hz0 22 7 5 , 22 25, 22 50, 22 12, 22 76 22 82 ZnzP?Oi 5 H ~ o 28 44, 28 54, 28 11, 28 41, 28 55 28 50 NaZn2PSOlo 9H20 Table IV.
X-Ray Diffraction Powder Pattern, Copper K, Radiation"
ZnzPzO? 5H20 12.9 VVS, 8.0 hI, 6.42 VS, 5.98 M, 5.57 S,5.01 S,4.57 W, 4.43 ?*I,4.17 IT, 3.96 11,3.73 TS, 3.61 M, 3.51 S,3.40 VW, 3.25 2.655 VW. 2.607 M. 2.539 M. 2.455 M, 2.
10.9 OVS, 6.55 M, 6.12 31,5.38 VS, 4.63 M, 4.02 S, 3.77 M,3.56 S, 3.47 VVW, 3.19 S, 3.08 S,2.941 W,2.874 VVW, 2.811 VW, 2.711 M ,2.661 M, 2.587 VVW, 2.530 M ,2.455 PIf, 2.339 M, 2.249 M,2.233 M, 2.038 S,2.000 VW, 1.954 W, 1.915 ?VI,1.905 W,1.837 &I, 1.807 VW, 1.757 VW, 1.727 W,1.700 VVW, 1.658 IT,1.629 W,1.598 W,1.565 I-S'W, 1.529 M,1.480 M, 1.460 VVW, 1.438 W,1.415 VVW, 1.406 VW, 1.385 VVW, 1.355 &I 10.0 VVS, 7.74 VS, 7.23 VS, 6.29 W,5.55 M, 5.17 W, 5.01 VS, 4.80 VS, 4.66-W, 4.35 YS, 4.19 VS, 4.02 W,3.83 S,3.69 S,3.53 VW, 3.43 VS, 3 16 VS, 2.984 VS, 2.910 W,2.845 VS, 2.789 VS. 2.673 M. 2.625 S.2!,561 M, 2.516 M, 2.463 W, 2.392 S,2.349 W, 2.322 VW, 2.274 M,2.1R3W,2161M,2.100M,2.051~,2.000~1,1.954bIJ1.915S,1.847M,1.798S, 1.775 VW, 1.745 W,1.709 VW, 1.688 W, 1.671 VW, 1.652 W, 1.631 VW, 1.617 M, 1.590 w, 1,562 M, 1.542 VW, 1.527 VVW, 1.509 W,1.487 'IV, 1.363 W,1.445 VW, 1.428 R, 1.407 W, 1.388 VW, 1.365 W,1.352 VW.
Data recorded are interplanar spacings ( d / n )in Angstroms and qualitative intensities, where S = strong, Yl = moderate, W = weak, and V = very. Q
ical; secondly, t h e p H no longer remained constant after restoration t o p H 5.5, b u t drifted upward with time of stirring. The data in Table V show that the phosphorus is not fully recovered in the precipitate. Xinety-three t o 99% of the phosphorus in ternary mixtures could be precipitated, whereas 99 to 100% of the phosphorus in solution of single phosphates or binary mixtures could be precipitated. For the particular composition having a 1.26: l.6S:l molar ratio of ortho- to pyroto triphosphate, variations in excess of zinc or in time or p H of precipitation failed to bring the recovery above the 97y0 level. As the zinc precipitates contained more than the theoretical amount of zinc, it was clearly evident that significant changes in precipitate composition were involved.
Table V.
The acidity titrations used for single phosphates were tried on mixtures, in the hope of obtaining the approxiniate average ionic weight on the phosphate ions present. The procedure is complicated for certain mixtures by the presence of two end points in the sodium hydroxide titration after zinc has been added. This happened for the ternary mixtures and for those binary mixtures containing orthophosphate and triphosphate (Table 11). Although the loiver end point is a very rough measure of the orthophosphate content, such separation of the orthophosphate end point from that for triphosphate in titrating mixtures would not have been predicted from the corresponding titration of the individual phosphates, where the inflection for ortho- n-as higher than for triphosphate (pH 5.1 cs. 4.5). A 1 to 1 molar mix-
Precipitation from Ternary Mixtures
(pH 5.5, 6.00 g. ZnS04.7H20,1.00 g. sodium phosphates, 50 ml.) Mole Ratio Grav. Phosphorus Taken, Tield,a Recovery,a 70Zn in hnhyd. Ppt." Ortho-Pyro-Tri % % Found Theo. 1.00,3.00,1.00 103 93.2 42.5 41 .O 1 00,2 60,3.06 103 95.7 40.0 37 6 1.00;3.00; 8.50 97.9 99.5 34.8 39.0 1.00.1.00.3.00 99.4 99.0 37.4 36.1 1 26: 1.68: l . O O b 97.0 96.0" 1 26; 1.68; l . O O b 97. 2d 1 2 6 , l . 6 8 , l .OOb Generally averages of 2-5 determinations. 6.00 g. ZnSOl 7HsO. Precipitation at pH 3.5, stood overnight, pH adjusted to 5.5. ZnS04 and NaOH solutions added alternately dropffise to maintain pH near 5.5 throughout precipitation.
* 7.50 instead of
Table VI.
Lak3.n
Bell Data Obtained by Different Analysts on Known Mixtures of Orthophosphate, Pyrophosphate, and Triphosphate
Wt. Ratio XasPtO~o Na4P20,
Total Sample Wt., Grams
A B
0.5000 0.5000
C C
0.4OOO 0.6000
A
0.5000 0.5000 0.4000 0.5000 0.5000 0.4000
B
c -4 B C C A4 B C C h B C
1/1 1/1 1/1
2/1 3/1 311 3/l 5/1 9/1 9/1 9/1
0.5000 0.5000 0.5000
0.4000
0.6000 0 5000 I
0.5000
0.4000
70Recovery Pyro as calcd. Tri by acidity from wt. Zn2PzOT titration 102,101 50, 65 97 72 26 101 102 65 104,103 74,77 97 87 102,95 82, 111 103, 103 93, 96 95 89 108 78 103 77 121, 114 86, 82 64 75 95 99 87, 86 97, 103 26, 28 108, 113 0 83 0
111
Lah A WLS running Bell analyses regularly, Lab B occasionally; prior to this study Lab C had had no experience with method. No orthophosphate in Lab C samples, 5-209T0 in others. (1
ture of orthophosphate and triphosphate reversed this picture, giving inflections a t p H 3.7 (ortho) and 5.7 (tri). The dihydrogen end point was first located b y a preliminary titration of the ternary mixture with hydrochloric acid. A second sample was then adjusted to this end point and zinc ions were added in excess. Whenever two end points were encountered in the subsequent sodium hydroxide titration, the upper end point was used in determining the average ionic weight. The data in Table I1 on mixtures come close to theory for total acidity liberated in a few cases, but several samples consumed more than the theoretical amount of sodium hydroxide. I n titrating the 1.26: 1.68: 1 molar ratio of ortho- pyro- triphosphate i t was noted that not only mas too much sodium hydroxide required to restore the p H to 5.5, but the p H of the slurry drifted upward with time of stirring thereafter. After lapse of 15 to 60 minutes significant amounts of acid m r e required to bring the p H back down to 5.5. During this period of drifting p H more phosphorus was prccipitated, but final recovery at a steady p H of 5.5 did not exceed 96 to 97%. T o explain all of these observations one could assume that initially the comhination of excess zinc ions with the ternary mixture of dihydrogen phosphates consumes hydrovyl ion to neutralize the liberated acidity, then near the end of the neutralization also consumes hydroxyl to form a basic salt. Presumably the basic precipitate is metastable and somewhat soluble, slowly releasing the extra hydroxyl and forming a less soluble material. As basic precipitates were not mcountered a t p H 5.5 with zinc sulfate alone or with zinc sulfate plus a single phosphate species present, zinc hydroxide or a basic precipitate of a single phosphatee.g., spencerite, Znd(PO&(OH)z 3 H 8 , a known mineral (IO)-seems to be ruled out. Presumably the basic zinc precipitate involves more than one phosphate species. I n conclusion, any p H in the range 5 to 6 appeared satisfactory from the data on precipitation of single phosphates, but only p H 5 appears reasonably satisfactory for ternary mixtures. Precipitation of Pyrophosphate from Mixtures. T h e precipitation of pyrophosphate a t p H 3.8 in t h e presence of triphosphate is t h e foundation of the Bell method (3) for determining pyrophosphate and triphosphatc. Accordingly, this precipitation mas studied in detail. Orthophosphate was omitted from many of the mixtures, for i t is not precipitated under these conditions unless present in large amounts (8, 26). I n attempting to precipitate pyrophosphate from a mixture, the ratio VOL. 29, NO. 2, FEBRUARY 1957
251
of triphosphate to pyrophosphate affects both weight of zinc precipitate and the completeness of pyrophosphate precipitation. Thus Table VI shows that the weight of zinc precipitate is usually high when the ratio of Sa5P3Ol0 to Na4P207 is 2 or less, but is usually low when this ratio is 3 to 1 or higher. Already reported (23) are tracer data showing that pyrophosphate is incompletely precipitated when triphosphate is present. K h e n the triphosphate was tagged with phosphorus-32 i t becomes evident that the zinc pyrophosphate is contaminated b y triphosphate (see p H 3.8 in Table VI1,B). Thus, the coprecipitated triphosphate often comes close to compensating for the weight of pyrophosphate not precipitated. As xould be expected, variation of p H and number of repeated precipitations affects the weight of zinc precipitate. Thus, in solutions containing either pure pyrophosphate or a mixture of pyrophosphate with triphosphate precipitation of ZnzPz07is incomplete when the p H is 3.0. I n absence of triphosphate, pyrophosphate is quantitatively precipitated a t p H 3.5 to 4.4 (Table VII,A), but this is not true of its mixtures with triphosphate at a n y p H tested in Table VI1,B. At p H values of 3.0 to 4.4 for 1 to 4 precipitations of the pyrophosphate there mas contaniination b y triphosphate. The contamination is reduced but not eliminated by lowering p H or increasing number of precipitations. At p H values just below 3.0 pyrophosphate fails to precipitate. Hence, there is little prospect of recovering a pure zinc pyrophosphate from a solution containing both pyro- and triphosphate. An additional effect of lolT-ering p H is that the pentahydrate, ZnzP207. 5Hz0, is likely to be contaminated with trihydrate as shown b y x-ray diffraction patterns. This is as expected, for many phase studies have shown that increasing acidity of the precipitation medium causes the next lower hydrate to precipitate-e.g., Flatt and Fritz (14). ANALYTICAL APPLICATIONS
Precipitation of Pyrophosphate. As yet, t h e only reliable methods of determining pyrophosphate directly in t h e presence of orthophosphate and triphosphate involve chromatography [anion exchange (6) or paper chromatography (9, 12, 1 8 ) ] or isotope dilution (24); both types are somei>-hat time-consuming. It may be, however, that a rapid and reliable method based on zinc precipitation of pyrophosphate can yet be developed. T o do this the calibration curve would have to be
252
0
ANALYTICAL CHEMISTRY
Table VII.
pH of Pptn.
Pptns. A. Sample 1
3 8
3.8
4.1 4.4
Zn?PaOi 0.500 gram Na4P207 97 1
1
2
82 1 99 8 100.7
1
99 5
2 3
4.1 4.4
3.5
=
2
3.5
3.0
Pyrophosphate Calcd. from \Tt.
No. of
3.0
B. Sample
Completeness of ZnzPz07 Precipitation vs. pH yo Recovery of
99 1 99 1 99 $1 (39 7
1 1 =
I
+
+
0.3311 Gram SahP33*010 0.1655 Gram Sa4P20i 0.0034 Gram Xa13P308 Recovery 4; s ~ z ~ % ~ ~ ~of Pyrophosphate o ~Ppt. ~ Calcd. from Wt. Zn in Ppt. by Corr. for count of Uncorr. for P32 tri content tri content 1 15.6 49 41 2 7.6 29 27 16.0 81 71 1 9.4 86 78 2 1 81 20.5 102 16.5 101 84 1 2 12.3 97 85 4 9.6 91 82 1 20.8 115 91 88 26.3 119 1
prepared in a m-ay to correct for the triphosphate interference. This is exactly what was done by Weiser (SW)in the precipitation of triphosphate b y C ~ ( e n ) ~ + +ions-Le., + he corrected for the interference of pyrophosphate with the precipitation of triphosphate by Co(en)&ls a t p H 3.5. Precipitation of Two or Three Phosphates. Only in special cases (26) can both triphosphate and pyrophosphate be precipitated quantitatively b y zinc without a n y interference from orthophosphate. Assuming t h e availability of reliable specific methods for two of the three species, the amount of the third could then be calciilated from the weight of zinc precipitate, provided all three species were precipitated completely. This is particularly unpromising, because of the tendency toward incomplete precipitation and variability of composition. LITERATURE CITED
Sndrens, J. T. R., J . ,4m. 011 Chenzzsts’ SOC.31, 192-5 (1954). Bassett, H., Bedmell, W.L., Hutchinson, J. B., J . Chem. SOC 1936, 1412-29. (3) Bell, R. N., Bs.4~.CHEJI.19, 97-100 (1917). ( 4 ) Bell, R. N., Wreath, A . R., Curless, W. T., Ibzd., 24, 1997-8 (1952). (5) Beukenkamp, J., Rieman, Wm. I11 Lindenbaum, S., Zbid., 26, 505-12 (1954).
(6) Britzke, E. V., Dragunov, S. S., J . Chem. Ind. ( X o s c o w ) 4, 49-51 (1927). (7) Caje, W.R., Can. Chem. Process Ind. 32, 741-5 (1948). (8) Campbell, D. O., Kilpatrick, h‘l. L., J . Am. Chem. SOC.76, 893-901 (1954). (9) CroTYther, Joan, ASAL. CHEM. 26, 1383-6 (1954). (10) “Dana’s System of RIineralogy,” 7th ed., Vol. 11, p. 931, Wiley, New York, 1951. (11) Dietrich, H. G., Johnston, J., J . Am. Chein. Soc. 49, 1119-31 (1927). (12) Ebel, J. P., Bull. SOC. chim. 1953,9981000.
(13) Eberly, S. E., Gross, C. V., Crowell, 11’. S.,J . Am. Chem. SOC.42, 14339 (1920). (14) Flatt, R., Fritz, P., Helv. Chim. Acta 33, 2045-56 (1950). (15) Fultori, J. IT., Sviinehart, D. F., J . Am. Chem. SOC. 76, 864-7 (1950). (16) Heinerth, E., Fette u. Se$fen55, 165-9 (1953). (17) Jones, L. T., IND.EKG. CHEM., ANAL. ED. 14, 536-12 (1942). (18) Karl-Kroupa, E., i l s . 4 ~ .CHEY.28, 1091-7 (1956). (19) Iiiehl, S. J., Coats, H. P., J . Am. Chem. SOC.49, 2180-93 (1927). (20) Kolthoff, I. M., Iiameda, T., Ibid., 53, 832-42 (1931). (21) AlcCune, H. IT,, Arquette, G. J., h . 4 L . CHEM. 27, 401-5 (1955). (22) Overman, 0. R., Garrett, 0. F., I s n . ENG. CHEX., ilsa~.ED. 9, 972-73 (1937). (23) Qiiimby, 0. T.,’ J . Phys. Chem. 58, 603-18 (1954).
(21) Quimby, 0. T., Mabis, -4.J . , Lampe, H. \y,, A N A L . CHEJI. 26, 661-7 (1954). (25) Raistrick, B., Sci. J . Roy. Coll. Sci. 19, 9-27 (1949). (26) Raistrick, B., Harris, F. J., Lome, E. J., Analyst 76, 230-5 (1951). (27) Schmidt, H., Demald, K.,Fette u. Sezjen 5 5 , 19-20 (1953 ),
Thilo, E., Schulz, I., 2. anorg. allg e m . Chem. 265, 201-8 (1951). Thilo, E., Seeman, H., Ibid., 267, 65-75 (1961). Topley, B., Quart. Rem. (London) 3, 346-68 i1949). Travers, A4., Chu, Y. IHelv. Chim. Acta 16, 913-7 (1933). Weiser, H. J., Jr., ANAL.C m x 28, 477-81 (1956).
(33) Welcher, F. J., “Organic Analytical Reagents,” 1-01, I, p. 281, Van Sostrand, Iiew York, 1947. (34) Willard. H. H.. Diehl. H.. “Advanced
Qualitative :hal&,’”p. 262, Van Sostrand, Sew York, 1943.
RECEIVED for review May 26, 1956. Accepted Sovember 23, 1956.
Extraction of Nitroguanidine Propellants with Pentane-Methylene Chloride Azeotrope JAMES 0. WATTS and HARRY STALCUP Research and Development Department,
,The use of methylene chloride as the extracting solvent for nitroguanidine propellant formulations results in a significant loss of the nitroguanidine component through solubility, particularly in the presence of moisture. An azeotrope of pentane-methylene chloride ( 2 to 1 by volume) reduces the loss to an insignificant amount, even in the presence of an excessive quantity of water. Extractable materials are removed b y the azeotrope with the same speed and efficiency as with methylene chloride.
which is used in many explosives laboratories for the extraction of nitroglycerin, plasticizers, and stabilizers from propellant powders, introduces a solubility problem when applied to nitroguanidine propellants. Previous investigations (3) have established that the loss of nitroguanidine by extraction with methylene chloride is a function of extraction time and introduces a n error in Cordite r\’ pori-der analysis unless a correction is made. The purpose of this investigation was to find a suitable extracting solvent for Cordite N type powders that would reduce the loss of nitroguanidine during extraction t o a minimum and still effect the efficient and rapid removal of the nitroglycerin and stabilizer components. An azeotropic solution of pentane-methylene chloride (2 to 1 by volume) ( 2 ) met these requirements with respect to nitroguanidine pol+-ders containing nitroglycerin as a plasticizer. ETHYLENE
CHLORIDE,
PROCEDURE
Apparatus and Reagents. Soxhlet extraction assembly, with cycling time of 1 minute. Extraction thimble (25 X 50 mm.)
U. S.
Naval Powder Facfory, Indian Head, Md.
glass, with sintered-glass bottom of coarse porosity. Heat source, hot water bath or steam hot plate. Pentane. technical, Eastman Organic chemicals, 500-gram bottles, sealed. Methylene chloride, Fisher certified reagent. Preparation of Extracting Solvent. T h e pentane and methylene chloride are mived in t h e ratio 2 t o 1 by volume. T h e pentane container must be cooled in a refrigerator before opening. Sample Size. T h e weight of nitroguanidine (13.40y0 nitramine nitrogen, oven-dried for 1 hour a t 105’ C.) which was placed in t h e extraction thimble of t h e Sovhlet assembly was 1 f 0.01 gram. Procedure. A 1-gram sample of nitroguanidine is placed in a glass extraction thimble having a sinteredglass bottom. T h e thimble is plugged iyith borosilicate glass wool and then dried t o a constant n-eight in a n oven a t 105’ C. T h e glass thimble is inserted into a n all-glass Soxhlet extractor having a siphoning cup of 25t o 40-ml. capacity. A 250-ml. extracting flask containing 50 ml. of p e n t a n e m e t h y l e n e chloride azeotrope is attached to a Soxhlet extractor. The
extractor is then attached to a water condenser, the siphoning cycle adjusted to 1 minute, and the sample extracted over a steam bath for 4 hours. Upon completion of the extraction the thimble is removed, and the azeotrope remaining in the thimble is allowed to drain out. The thimble is then dried for 1 hour a t 105” C., transferred t o a desiccator for cooling for 1 hour, removed from the desiccator, and weighed. The same procedure is follorved when using the pentane-methylene chloride azeotrope saturated with 2 . 5 grams of water per 50 ml. and methylene chloride saturated with 2.5 grams of water per 50 ml. DISCUSSION
Experiments in this laboratory and a t the Naval Ordnance Test Station (3) h a r e indicated that the presence of water appreciably increases the solvent action of the methylene chloride on nitroguanidine. The solubility loss of nitroguanidine by methylene chloride azeotrope evtraction is compared in Table I. The effect of excess water in the extracting solvents is also indicated. The presence of excess water during extraction with the azeotrope does not
Table 1.
Loss of Nitroguanidine in Methylene Chloride vs. Pentane-Methylene Chloride Azeotrope (2 to 1 by Volume) ( U g . of nitroguanidine extracted from 1-gram samples) K e t Solvent, 2.5 Gram H20/50 RI1. Dry Solvent, 0.027, HzO Replica CHzC12 Pentane-CHaCle CHzClz Pentane-CHzCl2 9 9 0 4 1 6.2 0.3 2 7.0 9 8 0.4 7 9 0 2 3 5.0 0.5 6 5 0 2 4 5.2 0.7 8.2 0.2 5 7.8 0.3 6 6.1 9.1 0.2 0.7 Av. 6 . 2 0.5 8.6 0.2
All condensers were initially dried, but were not protected from atmospheric moist,ure during the extraction.
VOL. 29, NO. 2, FEBRUARY 1957
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