Determination of Pyrophosphate By Precipitation with Cadmium and Polarographic Measurement of Cadmium in the Precipitate GCNTHER COHN AND I. hI. KOLTHOFF School of Chemistry, University of Minnesota, &Iinneapolis, Minn.
1
N THE present paper a new method for the determination of pyrophosphate in the presence of varying amounts of orthophosphate is described. The method is of special advantage in the determination of small amounts of pyrophosphate, and may find use in the determination of pyrophosphate in biological material. I n the proposed method the molar concentration of orthophosphate may be 16 times as large as the concentration of pyrophosphate, but even in the presence of 40 times the molar concentration of orthophosphate satisfactory results have been obtained. If larger samples are available, as is usually the case in the analysis of commercial phosphates, the proposed method appears not to be superior to existing methods. The effect upon the proposed method of meta- and (or) polyphosphate, which are often present in commercial phosphates, has not been investigated. Until now no satisfactory method has been described for the determination of small amounts of pyrophosphate in the presence of orthophosphate. Larger amounts of meta-, ortho-, and pyrophosphate present in the same solution can be determined satisfactorily by acidimetric titrations. Gerber and Miles (2, 3) determined the three phosphoric acids by titrating to different pH values using for the determination of orthophosphate the amount of nitric acid liberated by precipitating the phosphates with silver nitrate. Pyrophos hate is further determined according to Britzke and Dragunov (17 by precipitating disodium dihydrogen pyrophosphate with zinc sulfate and titrating the liberated sulfuric acid. In the analysis of mixtures considerable titration error results if the amount of yrophosphate present corresponds to less than 50 mg. of piosphorus pentoxide. When small amounts of pyro- and orthophosphate are present in a solution a separation has to be made by precipitation. Several authors have described the separation of pyrophosphate b precipitation with a zinc or cadmium solution. Kiehl and d a t s (6) found that the precipitate of zinc pyrophosphate was not satisfactorily crystalline but when reprecipitated the pyrophosphate could be determined gravimetrically with an accuracy of 1 per cent. Nylen (9) reported an accuracy of 1 to 2 per cent by the zinc method when working in an acetate buffer with a pH of 5. He found no coprecipitation of orthophosphate when 6 to 15 mg. of phosphorus as pyrophosphate were determined in the presence of 15.5 mg. of phosphorus in the form of orthophos hate. An accuracy of about 1 per cent was also found by Majorsky and Clark (8), who se arated pyrophosphate by precipitation with zinc sulfate a t p 2 2 . 7 to 2.8. Gerber and Miles (S)!precipitating with zinc sulfate at pH 3.0 to 3.3, observed no interference by orthophosphate but a strong interference by ammonium chloride. These authors p i n t out that the conditions for the zinc separation are not yet well enough defined to recommend this method for general application. Hull (4) in qualitative experiments found that precipitation of zinc pyrophosphate is incomplete and that large quantities of orthophosphate are co recipitated. Precipitation with cadmium in a pH 5 acetate guffer resulted in a much better separation. Wurzschmitt and Schuknecht (IO), however, stated that cadmium is not suitable for quantitative pyrophosphate determinations. They claim that titration errors in the precipitation of HIPPOI-- with zinc are caused by hydrolysis of'the ordinary zinc salts, and that they are avoided by precipitating with the complex ammonium zinc iodide. On the basis of Hull's experiments the authors have investigated the separation of pyrophosphate and orthophosphate by precipitation and determination of pyrophosphate as the cadmium salt. The precipitation was carried
out in a buffered solution, and the cadmium in the precipitate was determined polarographically. A few gravimetric determinations have also been made. The results obtained may also be of value in an improvement of the precipitation method of pyrophosphate with zinc.
Reagents Used The salt was prepared by heating SODIUM PYROPHOSPHATE. dehydrated pure disodium hydrogen phosphate (Merck) over a MBker burner to constant weight. A 0.01 M solution was prepared as stock solution. The concentration of this solution was checked by hydrating the pyrophosphate wit.h hydrochloric acid at boiling temperature and titrating the orthophosphate amperometrically with uranyl acetate (7). Two titrations gave a deviation of -0.2 and -0.4 per cent, respectively, from the theoretical value, which is within the limit of error of this method. CADMIUM CHLORIDE.A 0.1 M cadmium chloride solution was prepared from dehydrated CdC1t.21/2H~0(Mallinckrodt) which had been dried to constant weight at 110" C. The waterfree cadmium chloride is hygroscopic but attracts water sufficiently slowly to allow preparation of standard solutions by weighing. The 0.1 M solution was used for calibration measurements in the polarographic cadmium determination. CADMIUM ACETATE. Solutions 0.1 M, 0.2 M , 0.4 M , and 3 M m-ere prepared by dissolving Merck's cadmium acetate dihydrate in water. Except for the 0.1 M solution the concentrations were not adjusted accurately, since the solutions were used only a% precipitating agents. ACETATEBUFFERS. The following buffer solutions were prepared: 4.7 M sodium acetate 0.14 M acetic acid, pH 6.1; 1 -51 sodium acetate 1 M acetic acid H 4.7; 1 M ammonium ace8 M acetic acid, pH 3.6. '4hese buffer solutions are destate ignated below as 6.1, 4.7, and 3.6, respectively.
+
+
+
Solubility of Cadmium Pyrophosphate Successful results depend in the first place upon the insolubility of cadmium pyrophosphate in the precipitation medium. Therefore, the solubility of cadmium pyrophosphate was determined under various conditions by measuring polarographically the cadmium concentration of saturated cadmium pyrophosphate solutions. It was found that the solubility was negligibly small in water, but was appreciable in 0.1 X potassium chloride and in 0.1 M potassium chloride plus various concentrations of buffer 6.1. It increased with increasing Concentration of buffer 6.1. Since the precipitation of pyrophosphate has to be carried out in a buffer solution in order to prevent the precipitation of orthophosphate, it, ie necessary to suppress the solubility by using a large excess of cadmium in the precipitation. The precipitate can be washed with water, since the buffer and the excess of cadmium are removed simultaneously.
Gravimetric Determinations The feasibility of the quantitative determination was checked by a few gravimetric determinations. Pyrophosphate was precipitated at room temperature from buffer 3.6 with an excess of cadmium acetate. After standing overnight the precipitate was filtered through a Jena 4 G sintered-glass crucible and washed with water. The filtration must be made very carefully, because the precipitate runs through the filter if the suction is too strong. After the precipitate had been washed with water the crucible was heated to constant weight at temperatures between 220" and 290" C. At these temperatures the cadmium pyrophosphate dihydrate obviously loses its water, and the precipitate can be weighed as cadmium pyrophosphate. 886
TABLEI. GRAVIMETRIC DETERMIN4TION OF PYROPHOSPHATE CADMIUM PYROPHOSPHATE 0.01 M NarPtOi Used
Buffer 3.8 Used
Ml.
M1.
a87
ANALYTICAL EDITION
November 15, 1942
CdAcn Used
MI.
M
CdzPaOI Calculated Found
Ma,
Mg.
AS
ml. of buffer 3.6 were added. Under these conditions no precipitate of cadmium pyrophosphate appeared. Precipitation w m achieved by adding the same amount of cadmium acetate solution as had been used in the first precipitation. The precipitate was centrifuged and washed as described before.
Error
7c
Determination of Pyrophosphate
INSOLUTIONS FREEFROM I~TERFERINQ SUBSTANCES.Table I1 contains the results of experiments in which the Some results are given in Table I. The precipitation of cadmium pyrophosphate is quantitative under the conditions given in Table I. The deviations are probably due to slight losses in the filtration of this precipitate.
Polarographic Determination of Cadmium in the Precipitate I n preliminary experiments carried out a t a p H of 6.1 attempts were made to determine the pyrophosphate in 0.005 M solution by adding a known excess of cadmium and measuring the concentration of cadmium in the supernatant liquid. The precipitate plus supernatant liquid was allowed to stand overnight, and the residual cadmium \vas determined polarographically without filtration. The amount of cadmium added to the pyrophosphate was twice the amount required theoretically for complete precipitation. However, about 20 per cent of the pyrophosphate was not precipitated. The presence of orthophosphate in a molar ratio PO1 to Pz07of 4 to 1 did not affect the results. This was no longer true when a larger excess of cadmium was used in the precipitation a t p H of 6.1 (see Table IV). For complet'e precipitation of 25 ml. of 0.01 :M sodium pyrophosphate more than 10 ml. of 0.2 hi' cadmium acetate was required. Under these conditions the determination by subtracting the amount of cadmium left in solution from the total amount used becomes inaccurate. The determination must, therefore, be carried out by measuring the amount of cadmium in the precipitate. The conditions of precipitation are specified in the tables. In order to improve the settling of the precipitate a few drops of gelatin solution (1 mg. of gelatin per ml.) were added after precipitation and also to the wash water. The precipitate was separated from the supernatant liquid by centrifuging and then washed by shaking and centrifuging with four 50-ml. portions of water. The precipitate was then dissolved in hydrochloric acid (1 to 4), and the solution transferred to a 100-ml. volumetric flask, enough potassium chloride being added to eliminate the migration current in the subsequent polarographic determination of the cadmium. The diffusion current was measured at 25" C. at an applied e. m. f. of 0.8, 0.9, and 1.0 volt, respectively, with a mercury pool as anode in 0.5 AI potassium chloride solution. Identical current readings were obtained at these potentials. I n order to correct for the residual current, 0.15 microampere was subtracted from the current values measured. The cadmium concentration of the solution was determined by adding successively three known amounts of 0.1 AI cadmium chloride to the solution and by measuring the increase of the diffusion current. The unknown concentration was determined by linear extrapolation from the three points measured with known concentrations, taking as zero point the diffusion current obtained lvith the unknown concentration. This procedure is more accurate than using an "internal standard", where only one known amount is added, and it is also better than using a calibration curve, since changes in the properties of the capillary or in the temperature might occur. The error in the concentrations determined in this way is of the same order of magnitude as that in ordinary polarographic work ( 2 to 3 per cent). The cadmium pyrophosphate was reprecipitated as follows: The precipitate was dissolved at room temperature in 2 to 3 ml. of hydrochloric acid (1 to 4), and immediately thereafter G $1 sodium hydroxide was added dropwise until the color of Tropeolin 00 changed from red to orange yellow. Then 5 to 10
influence of pH, buffer concentration, reprecipitation, and temperature on the precipitation of pyrophosphate was investigated. Pyrophosphate was precipitated from 25 ml. of 0.01 iV sodium pyrophosphate solution at room temperature. The precipitate was allowed to stand overnight together with the supernatant liquid. Table I1 shows that the precipitation is complete in buffer solutions with a p H ranging between 6.1 and 3.6. The error found was of the order of 2 per cent or smaller (occasional errors, not listed in Table 11, were presumably due to improper washing of the precipitate or to losses during decantation). Since a low p H is favorable for the prevention of coprecipitation of orthophosphate (Table IV), buffer 3.6 was selected as the medium in most of the experiments. From experiment 15 (Table 11) it is obvious that addition of acetic acid instead of the buffer gives quantitative precipitation, since a buffer is formed by addition of the cadmium acetate. Experiments 13 and 14 show that losses by hydration of pyrophosphate do not occur when t h e cadmium pyrophosphate is dissolved, and when the precipitation is repeated.
TABLE 11. PRECIPITATION O F CADMIUM PYROPHOSPH.iTE
No.
(25 ml. of 0.01 A4 NaaPaOi solution taken) CdAoi Cadmium in Used for Precipitate Buffer Used Precipitation Calculated Found
M1. 1
5
2 3 4 5 6
5 10 10 10 10 10 10 5 5
7 8 9 10 11 12 13 14 15 16 17 18 19 20 a
I
? ? J
10 10 10 20 10 10
(6.1) (6.1) (4.7) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) (3.6) 1 iM H Ac (3.6) (3.6) (3.6) (3.6) (3.6)
Ml. 25 25 25 25 25 20 20 20 2 2 2 2 2 2 15
2 2
25
15 15
M 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4
Millimole 0.500 0.490 0.500 0.490 0.500 0.494
3
0.500 0.500 0.500 0.500 0.500 0.500 0,500 0.500 0.500 0.500 0.500 0.500
3 3 3 3 3 0.4 3 3 0.2 0.4 0.4
0.500 0.500
0.500 0.500 0.500
0.500 0.506 0,490 0.496 0.495 0.499 0.494 0.513 0.508 0.490" 0.4135a 0.513 0.480 0.478 0.450 0.476a 0.470h
Error
% -2.0 -2.0 -1.2 0 f1.2 -2.0 -0.8 -1.0 -0.2 -1.2 4-2.6 f1.6 -2.0 -1.0 f2.6 -4.0 -4.4 -10.0 -4.8 -6 0
Reprecipitated a n d again allowed to stand overnight.
h Precipitated a t boiling temperature a n d separated immediately
Experiments 16, 17, and 18 show that too large a concentration of buffer 3.6 causes incomplete precipitation. Low results were also obtained when the precipitation was carried out a t boiling temperature (experiments 19 and 20). For this reason the precipitation should not be carried out hot. Precipitation a t boiling temperature gives a precipitate which settles more quickly and seems to be coarser. The concentration of the cadmium acetate in the reagent is of subordinate significance, but in order to prevent coprecipitation of orthophosphate a 0.04 M cadmium acetate solution is most advantageous, as shown below. Table I11 contains experiments with different pyrophosphate concentrations, in which the time required for complete precipitation was also determined. When the pyrophosphate concentration in the reaction medium was greater than
INDUSTRIAL AND ENGINEERING CHEMISTRY
888
Vol. 14, No. 11
final concentration of cadmium and Orthophosphate is dependent on the (25 ml. of pyrophoiphste solution taken, 5 mi. of buffer 3.6 used) concentration of the cadmium acetate Molarity CdAcr Time of Cadmium in solution added. With more concenof NerPtO7 Csed in Standing after Precipitate No. Used Precipitation Precipitation Calculated Found Error trated cadmium acetate solution a Ml. M Hours Millimoles % marked precipitation occurs during the 1 0.01= 15 0.4 2 0,500 0,486 2.8 addition, due to local excess of cad2 0.01 3 3 2 0.500 0.498 - 0.4 3 0.01 4 3 1 0.500 5.06 + 1.2 mium. With more dilute cadmium 4 0.02 4 3 0.5 1.000 0.986 acetate solution the precipitate appears 5 0.01 4 3 0.25 0.500 0.501 +- 01 .. 42 - 20 . 5 slowly. Part of the precipitate formed 6 0.0025 15 0.4 Overnight 0.125 0.122 7 0.0025 4 3 Overnight 0.125 0.125 8 0,0025 4 3 Overnight 0.125 0.1225 - 2.0 upon addition of the more concentrated 9 0.0025b 15 0.4 Overnight 0.125 0.126 +- 02 .. 08 cadmium acetate solution dissolves 10 0.0025 4 3 2 0.125 0.1225 11 0.0025 4 3 1 0.125 0.1194 - 74 .. 25 again afterwards. The final precipitate 12 0.0025 4 3 0.5 0.125 0.116 13 0.0025 4 3 0.25 0.126 0.107 -14.4 is coarser than that which is obtained 14 0.001 4 3 Overnight 0.0500 0.0480 - 4.0 by precipitating with a more dilute 15 0,001 4 3 Overnight 0,0500 0.0495 - 1.0 0.0500 0.001 15 0.4 Overnight 0.0468 - 6.4 cadmium acetate solution. Hence the - 8.0 0,0500 0.0460 l6 0.001 15 0.4 Overnight 0.0503 + 0.6 concentration of cadmium acetate used 0,0500 l7 0.001 4 3 3 - 8.0 l8 0.001 4 3 1 0.0600 0.0460 19 for precipitation is of subordinate 4 3 1 0.0500 0.0482 - 3.6 0.001 2o 0.001 4 3 3 5 minutes 0,0500 0.0390 -22 significance regarding the amount of 22 21 0.001 4 3 0.25 0.0500 0.0145 -71 orthophosphate finally precipitated. In a 10 ml. of buffer 3.6 used. the presence of pyrophosphate, however, b 10 nil. of 1 . M acetic acid added instead of buffer. this is no longer true. When 3 M cadmium acetate was added to a mixture of 25 ml. of 0.01 M pyrophosphate, 0.007 M the precipitation was complete within 15 minutes 5 mi. of 1 M orthophosphate, and 5 ml. of buffer 3.6, much more orthophosphate was coprecipitated than upon addition when the concentration of cadmium was about 0.3 M (experiment 5). of 0.4 M cadmium acetate to a mixture of 25 ml. of The precipitation of a pyrophosphate solution of the initial 0.01 M pyrophosphate, 10 ml. of 1 M orthophosphate, concentration of 0.0025 M likewise yielded satisfactory reand 10 ml. of buffer 3.6. I n the first case one reprecipisults. The time required for complete precipitation was tation did not suffice to obtain an orthophosphate-free about 2 hours (experiments 10 and 11). cadmium pyrophosphate (Table IV, experiment 6), but The lowest concentration of pyrophosphate a t which preit was achieved with one reprecipitation in the latter case (Table IV, experiment 5). It is, therefore, better to use cipitation is still complete is about 0,001 M . A visible precipitate did not appear unt.il several minutes after the addi0.4 M instead of 3 M cadmium acetate for precipitation in tion of cadmium acetate, and it seemed to be of smaller the presence of orthophosphate. particle size than the precipitates obtained from more conWhen the orthophosphate is present in a sufficiently low concentration (about 0.03 M ) , the coprecipitated orthocentrated solutions. When the precipitation from the 0.001 M pyrophosphate solution was carried out by using more phosphate is eliminated from the precipitate on aging in the dilute reagent (experiments 16 and 17) larger errors resulted. supernatant liquid. Simultaneously the coprecipitated orthoHowever, greater errors were also found in other cases with phosphate promotes the rate of recrystallization of the cadthis dilute pyrophosphate solution, which may be due to the mium pyrophosphate. The latter has a slimy appearance when precipitated in the absence of orthophosphate, and does difficulty of centrifuging the precipitate. Three hours seem to be sufficient for complete precipitation. It is recommended that the final concentration of pyrophosphate OF Cmiwm PYROPHOSPHATE IN PRESENCE OF ORTHOPHOSPHATE TABLEIV. PRECIPITATION be a t least 0.0012 M in order (25 ml. of h'arP107 solution taken, 5 ml. of buffer 3.6 used) to get quantitative precipiMolar Molarity Treatment Cadmium in CdAo of tation. Especially in the after Precipitate Used f o r NarPzOi Precipitation Precipitation Calculated Found Error Used X O presence of orthophosphate ME. M 'Ml. 1 M Millimoles ?& the concentration of pyro1.85 f280 0.500 25 0 . 2 Standing overnight 10 0.1 4:l 0.010 phosphate in the reaction 0.518 0.500 +3 6 25 0 . 2 Standing overnight 10 0.1 4:l 0.01b 3.60 +620 0.500 10 1 40:l 0 . 4 Standing overnight 20 0.01 b mixture should not be smaller 0.83 0.500 + 66 25 5 1 20:1 0 . 2 Standing overnight 0.01b than this value. 0 . 4 8 ~ 5 ~- 3 . 0 0,500 20 0 . 4 Standing overnight 10 1 40:l 0.01 b TABLE111. PRECIPITATION OF CADMIUM PYROPHOSPHATE
IN PRESENCE OF ORTHOPHOSPHATE ASD OF CALCIUM. The precipitation of cadmium pyrophosphate is much affected by the presence of orthophosphate and of calcium. Cadmium orthophosphate has a limited solubility a t a p H of 3.6. An incomplete precipitation is obtained, when 0.1 M potassium dihydrogen phosphate solution is made about 0.1 M in cadmium. The appearance of the precipitate a t the same
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0
b e
d
0.01 0.01 0.01 0.01 0 0lb 0.01 0.01
0,005 0.005
0.0025 0.0023 0.0025 0.0025b 0.0025 0 .o
m
0.0025 0.001 0.001
5
5 5 5 5 5 5 5 5 5 1;
10 5
5 5
5
J
1 0.2 0.2 0.1 0.2 0.2 0.2 0 2 0.2 0.2
0":;
0.1 0 2 0 2 0 2 0 2 0.2
20:l 4:l 4:l 2:l 4:l 4:l
4:l 8:l 8:l 16:l 32:l 16:l 16:1 16:l 16:l 16:l 40:l 40:l
4 2
15
4 15 20 20 20 20 2 2 15 25 20 20 20 15
15
3 3 0.4 3 0.4 0.4 0.4 0.4 0.4 3 3 0.4 0.2 0.4 0.4 0.4 0.4 0.4
5 ml. of buffer 6.1 used.
i o ml. of buffer 3.6 instead of :mi. used.
Reprecipitated and amain allowed to stand overnight. Twice reprecipitated,"second time in greater dilution.
Btanding overnight 0.500 Standing overnight 0.500 Standing overnight 0,500 0.500 Standing for 2 hours 0.500 Standing overnight 0.500 Shaken for 6 hours 0.500 Shaken f o r 7 hours Shaken for 7 hours 0.250 0.250 Shaken for 10 hours Standing overnight 0.125 Standing overnight 0.125 0.125 Standing overnight 0.125 Standing overnight Standing for 14 hours 0 . 1 2 5 0.125 Shaken for 14 hours 0.125 Shaken for 6 hours 0 0500 Standing overnight Standing overnight 0.0500
0.490d 0.573 0.545 0.525 0.505 0.548 0.528 0.242 0.2525 0.134 0.180 0.1275 0.113 0.095 0.127 0.1235 0.0373 0.023
- 2.0 +14.6 f 9 0 + 5 0 f 1 0 + S 6
+-
5.6
+ 31 .. 02 1 7 2
+44.0 + 2 0 - 4 s -24.0 1.6 - 1.2 -24.6 -54.0
+
November 15, 1942
not change its appearance beyond the first 2 or 3 hours of aging. On the other hand, the originally slimy and voluminous precipitate containing copreci pi t a t ed or t h o p hosp h a te was transformed i n t o well-developed crystals after about a day of aging. The mechanism of this aging effect presumably involves a recrystallization of cadmium pyrophosphate, during which process most of the coprecipitated orthophosphate is returned to the solution. This is indicated by the following observations:
ANALYTICAL EDITION
889
TABLEV. PRECIPITATION OF CADMIUM PYROPHOSPHATE IN PRESENCE OF CALCIUM
No.
1 2 3 4 5 6
7 8
9 10
Molarity of NadPzOy Used
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.0025 0.00254 0.0025/
(25 ml. of NarPzO7 solution taken, 6 ml. of buffer 3.6 used) Molar R a t i o CdAcz Cadmium in CaClz Ca++/ Used for Precipitate Present PxOy ---- Precipitation Calculated Found M1. M MI. M Millimole 4 3 0.500 1 5 20:l 0.463“ 4 3 0.500 5 5b 1OO:l 0.1634 4 3 0.500 2.5 1 1O:l 0.48OC 0.5 1 2:l 2 3 0.500 0.48OC 15 0.4 0,500 1 1 4:l 0.4805 2 1 8:l 20 o 4 o 500 n 470d 20 0.4 0.500 4:l 1 1 0.455d 1 1 16:l 15 0.4 0.125 0.114C 15 0.4 0.125 1 1 16:l 0.lllc 15 0.4 0.125 1 1 16:l 0.116
Error
-
%
7.4 -67.6 4.0 4.0 4.0
--
t_ i
n_
9.0 --11.2 8.8
- 6.7
Precipitate allowed t o stand for 2 hours. b Mixture slightly turbid before addition of cadmium acetate. Precipitate allowed t o stand overnight. d Precipitate allowed t o s t a n d for 2 hours, then reprecipitated and allowed t o stand overnight. 6 10 ml. of buffer 3.6 used. f 10 ml. of 1 M acetic acid used instead of buffer.
a
1. Addition of the same amount of orthophosphate 30 or 60 minutes after the precipitation of cadmium pyrophosphate also improves the crystallinenature bf the precistate, but not so much as when the orthophosphate was coprecipitated. 2. When the precipitate obtained in the presence of orthophosphate from 0.0025 or 0.001 M pyrophosphate solutions was analyzed before the crystal perfection was visible, low values were obtained.
These results might explain the different views expressed in the literature concerning the possibility of quantitatively determining pyrophosphate in the presence of orthophosphate by precipitation with cadmium. Depending upon the time of standing before filtration, too much, too little, or the correct amount of precipitate may be found. The time of standing necessary before the filtration can be considerably shortened by shaking the suspension, Table IV contains the QUANTITBTIVE DETERMINATIONS. results of determinations of pyrophosphate in presence of orthophosphate. “Standing overnight” refers to a time of standing of 18 to 30 hours after precipitation. (The importance of the time of standing is more evident from experiments given in Table VI.) The amount of coprecipitated orthophosphate increases with increasing p H of the buffer added. With a molar ratio of orthophosphate to pyrophosphate (0.01 M in the original solution) of four the error was reduced from 280 to 3.6 per cent, when instead of buffer 6.1 buffer 3.6 was used (experiments 1 and 2, Table IV). The mixture of 25 ml. of 0.01 M pyrophosphate solution and 5 ml. of 0.2 144 orthophosphate solution represents approximately the upper concentration limit a t which the coprecipita>ed orthophosphate is eliminated again on standing. For satisfactory results with this mixture addition of 10 ml. of buffer 3.6 is required (compare experiments 2 and 10 with 8, 11, and 12). Shaking the suspension did not improve the final result. When the molar ratio of orthophosphate to pyrophosphate becomes greater than four a t the initial pyrophosphate concentration of 0.01 M a reprecipitation is necessary in order to get quantitative results (experiment 5). When 3 iLf cadmium acetate was used for the precipitation, the interference by coprecipitation of orthophosphate was more pronounced (experiments 6 and 15) than with more dilute cadmium reagent (experiments 8 and 17), and the reprecipitation in presence of larger amounts of orthophosphate had to be carried out twice (experiment 6). From experiments 22 and 23 it is evident that the precipitation of pyrophosphate from an originally 0.001 M pyrophosphate solution was incomplete in the presence of a large excess of orthophosphate. For a quantitative determination in the presence of orthophosphate an initial concentration range of 0.002 to 0.01 M pyrophosphate is recommended. Since a molar ratio of
orthophosphate to pyrophosphate of 16 to 1 does not cause interference, a reprecipitation is necessary only in extreme cases. The reduction of the period of time before filtration by shaking is evident from experiments 19, 20, and 21. After 14 hours of standing the cadmium content of the precipitate was 24 per cent too low, while after 6 hours of shaking the correct composition was found. When pyrophosphate was determined in presence of calcium, a deficiency of cadmium in the precipitate was found in all experiments. Examples are given in Table V. This deficiency was presumably caused by a coprecipitation of calcium pyrophosphate only in case of a large excess of calcium and a t higher pyrophosphate concentrations (experiments 1 and 2). With an excess of calcium smaller than tenfold a t an original pyrophosphate concentration of 0.01 M no complete precipitation was achieved. This is shown by reprecipitation experiments 6 and 7, in which even somewhat greater errors were found. The time of standing of the precipitates before reprecipitating (2 hours) was sufficient for complete precipitation under the conditions of reaction in the absence of calcium. Hence, if the negative errors had been due to a slight coprecipitation of calcium pyrophosphate in the first precipitation, results closer to the theoretical value should have been found after the second precipitation. The appearance of cadmium pyrophosphate precipitated in presence of calcium is as slimy as in precipitation from pure pyrophosphate solution. Fortunately, the results become much better when calcium and orthophosphate are present together (Table VI). I n a mixture of 25 nil. of 0.01 M pyrophosphate, 5 ml. of 0.2 M orthophosphate, and 1 ml. of 1 M calcium chloride (experiments 1 to 6 in Table VI) no longer a deficiency but a slight excess of cadmium was found, as in the absence of calcium. The bulk of the coprecipitated orthophosphate was eliminated on aging, but a slight amount remained in the precipitate, even when the precipitate was allowed to stand for 40.5 hours or when the suspension mas shaken for 5.5 hours. Khen, however, 2 ml. of calcium chloride were added instead of 1 ml., the precipitate showed the correct cadmium content after standing overnight and almost the correct cadmium content after shaking for 5.5 hours. This beneficial effect of the presence of a higher concentration of calcium apparently does not result from a compensation of errors, by which the amount of unprecipitated pyrophosphate would just be balanced by the amount of coprecipitated cadmium orthophosphate, for the following reasons: 1. The experiments in presence of calcium only show that the amount of unprecipitated pyrophosphate is practically the same
890
INDUSTRIAL A N D ENGINEERING C H E M I S TRY
Vol. 14, No. 11
hours. When the originally TABLE VI. PRECIPITATION OF CADMIUM PYROPHOSPHATE IN PREBENCE OF ORTHOPHOSPHATE voluminous and slimy precipitate has changed into wellAND CALCIUM developed crystals, the pre(5 ml. of 0.2 M KHlPOi solution and 5 ml. of buffer 3.6. Precipitated with 20 ml. of 0.4 31 CdAcz) cipitate is filtered or centriMolar Molar Cadmium in fuged and washed with water. Ratio Ratio Treatment Precipitate The cadmium in the precipiNa4P20r CaCh POL---/ Ca++/ after Cslcutate is determined polaroNo. Used Present PiO?---PIOI---Preciuitation lated Found Error graphically after dissolving in M1. M M1. M Millimole % a few milliliters of hydrochloric 1 1 4:l 4:l Standing overnight 0.500 0,5225 + 4 . 5 1 25 0.01 acid (1 to 4), adding 25 ml. of 1 1 4: 1 4:l Standing overnight 0.500 0.570 +14.0 25 0.01 2 1 1 4:l 4:l Standing for 20.5 hours0.500 0.540 + 8.0 25 0.01 3 2 M potassium chloride solu1 1 5:l 5:l Standingfor20.5hours 0.400 0.417 ++ 46 .. 30 20 0.01 4 tion, and diluting to 100 ml. 1 4:l 25 0.01 1 Standing for 42.5 hours 0.500 0,530 4: 1 5 in a volumetric flask. 4: 1 25 4:l Shakenfor 5.5 hours 0.500 0.540 0.01 1 1 ++ 80 .. 08 6 2 4:l 8:l Standing overnight 0 . 5 0 0 0.504 25 0.01 1 7 For the determination of 4: 1 2 1 8:l Standine overnirht 0.500 0 497 - 0 R 25 0.01 8 smaller amounts of pyrophos2 1 4:l 8:l Shaken Tor 5.5 h-ours 25 0.01 0 500 0 522 9 + 4:4 phate correspondingly reduced 1 Shaken for 10 hours 8:l 8:l 25 0.005 0.250 1 10 0.245 - 2.0 16: 1 25 0.0025 Standing for 17 hours 0.125 1 16: 1 1 0.004 11 -24.8 volumes are used. 16: 1 25 0.0025 1 Standing for 24 hours 0 . 1 2 5 16:l 1 0.100 12 -20.0 The precipitate may also 16: 1 1 1 16: 1 25 0.0025 Shaken for 14 hours 0.125 0.127 13 + 1.6 be weighed after drying at 1 1 16:l 16: 1 25 0.0025 Shaken for 5.5 hours 14 0.125 0.1225 - 2 . 0 1 20:l 2 40:l 20 0.0025 Standing for 14 hours 0.100 15 0.062 -38.0 250" C., but in general the 1 16:l 2 32:l 25 0.0025 Standingfor37.5hours 0 . 1 2 5 16 0.122 - 2.4 polarographic method is more 16:l 2 1 32:l 25 0.0025 Shakenfor 14 hours 0.125 17 0.123 - 1.6 convenient and preferable. 16:l 2 1 32:l 0,0025 25 S h a k e n f o r 5 . 5 hours 0 . 1 2 5 0.122 18 - 2.4 If the orthophosphate content is larger than stated above, the -precipitate, after shaking for 5 hours and with 1 ml. or with 2 ml. of calcium solution (experiments 3 and separating from the supernatant liquid, is dissolved in a few milliliters of hydrochloric acid (1 to 4) at room temperature. 5 in Table V). 2. At lower pyrophosphate concentrations the coprecipitated The solution is neutralized immediately with sodium hydroxorthophosphate is eliminated on aging in the absence of calcium, ide (6 M), using Tropeolin 00 as indicator. After neutralizabut the cadmium content of the precipitate remains the correct tion the volume is made about the same a9 in the first precipione in presence of calcium also. tation and the same amounts of buffer solution and of cadmium 3. The crystals of cadmium pyrophosphate precipitated from acetate are added as before. different pyrophosphate concentrations were always better deOrthophosphate in the mixture is determined by hydrating a veloped when both calcium and orthophosphate were present a part of the sample with acid at boiling temperature, determining than in the presence of orthophosphate alone, and the degree of the total phosphorus content and subtracting the pyrophosperfection was greater with 2 ml. than with 1 ml. of the calcium phate content'. chloride solution.
Summary
A corresponding behavior wm found when the pyrophosphate was present in lower concentrations. I n experiments 11 and 12 (Table VI), for example, where the precipitate was allowed to stand for 17 and 24 hours, respectively, the precipitation was very incomplete. It was complete, however, after 5.5 hours of shaking. Experiments 15 to 18 in Table VI demonstrate further the considerable gain of time by shaking the precipitate. The minimum time of shaking required for obtaining quantitative results appeared to be dependent on the intensity of shaking. I n experiments 6, 9, 14, and 18 (Table VI), which were run simultaneously, the precipitate after 4 hours of shaking was still voluminous and fluffy, but became nicely crystalline after 5 hours of shaking. In general, the precipitation is complete after the slimy precipitate has changed completely into well-developed crystals.
A method is described for the quantitative precipitation of pyrophosphate as cadmium pyrophosphate. After filtering and washing, the precipitate can be weighed in the anhydrous form when dried to constant weight at 250" C. I n general i t is simpler and more practical to dissolve the precipitate in dilute hydrochloric acid and determine the cadmium polarographically. A procedure is given for the determination of 0.002 to 0.01 M pyrophosphate solutions in the presence of from 4 to 16 times the molar concentration of orthophosphate and from 8 to 32 times the molar concentration of calcium.
Acknowledgment Acknowledgment is made to the Carnegie Corporation of New York for a grant which ehabled the authors to carry out the present investigation.
Procedure A procedure for the determination of pyrophosphate in the presence of orthophosphate and of calcium is described below. I n the absence of orthophosphate the simplest way to determine pyrophosphate obviously is by hydration. I n many practical cases orthophosphate and calcium are present in larger amounts than is the pyrophosphate. I n such instaqces, especially when the amount of pyrophosphate is small, the cadmium procedure seems to be superior to other methods described in the literature. To 25 ml. of the neutral or weakly alkaline solution, which must be 0.002 to 0.01 h' in pyrophosphate (corresponding to 7 to 35 mg. of phosphorus pentoxide), sufficient 0.2 111 orthophosphate solution is added, if necessary, t o make the final orthophospliate concentration approximately 0.03 M . Five milliliters of acetate buffer of pH 3.6 (8 izf acetic acid, 1 M ammonium acetate) tire added and 2 ml. of 1 ill calcium chloride (or less, if calcium is present). Then 15 to 20 ml. of 0.4 '$1cadmium acetate solution are added, and the mixture is gently shaken for about 5 t o 6
Literature Cited (1) Britzke, E. V., and Dragunov, S.S.,J . Chem. Ind. (Moscow), 4, 49 (1927).
(2) Gerber, A.'B., and Miles, F. T., IXD.ENG.CHEM.,ANAL.ED., 10, 519 (1938).
(3) Zbid., 13, 406 (1941). (4) Hull, D.E., J . Am. Chem. SOC.,63, 1269 (1941). ( 5 ) Jones, L. T., IND. ENG.CHEM.,ANAL.ED.,14,5 3 6 (1942). (6) Kiehl, 5. J., and Coats, H. P., J . Am. Chem. SOC.,49, 2180 (1927).
(7) Kolthoff,I. M., and Cohn, G., IND.ENG.CHEM.,ANAL.ED., 14, 412 (1942). (8) Madorsky, S.L., and Clark, K. G., IND.ENG.CHEW,32, 244 (1940). (9) Nylen, P.,2. anorg. Chem., 229, 36 (1936). (10) Wureschmitt, B and Schuknecht, W., Angew. Chem., 52, 711 (1939).
*
After completion of this study Jones ( 5 ) described work in which he precipitated small amounts of pyrophosphate in t h e presence of a large excess of orthophosphate with manganous manganese.
