ANALYTICAL CHEMISTRY
1052 it) and wrapping the wick around this metal tube, the authors have been able to burn methylnaphthalene without smoking. Attempts have been made to burn bottoms samples in this lamp. So far they have been unsuccessful, and the results have indicated that more radical lamp modifications would be needed.
ACKNOWLEDGMENT
The assistance of J. Grider with most of the experimental work is gratefully acknowledged. W. T. Harvey of the Sun Oil Company, Marcus Hook, Pa., very kindly contributed the results of the carbon-hydrogen train analyses.
SUMMARY
A modified lamp design is presented, allowing analysis of highboiling organic liquids Accuracy of the determination of hydrogen is of the order of *0.02 to 0.037, hydrogen, absolute. Variations in this design are suggested, which should permit the use of only one lamp for the determination of the hydrogen and carbon contents of all organic liquids from those volatile at room temperature to those of boiling point greater than 500" C.
LITERATURE CITED
(1) Am. SOC. Testing Materials, D90-46T. (2) Hindin, S.G., and Grosse, A . V., IND. ENG:CHEM.,ANAL.ED., 17, 767-9 (1945). (3) Javes, A. R., J. Inst. Petroleum Tech., 31, 129-53 (1945). (4) Simmons, M. C., ANAL.CHEM.,19, 385-9 (1947).
RECEIVED March 16. 1948.
Determination of Phosphorus Pentoxide in Phosphate Rock JAMES L. KASSNER, HOWARD P. CRAMMER, AND 3IARY ALICE OZIER School of C h e m i s t r y , Metallurgy, a n d Ceramics, University, Ala. Perchloric acid may be used for the dehydration of silica in phosphate rock, prior to the determination of phosphorus pentoxide. A new mixed indicator comprised of phenol red and bromothymol blue gives a sharp color change at pH 7.5. When this mixed indicator is used, the phosphorus pent-
I
N THE determination of phosphorus pentoxide in phosphate
rock, it is customary to separate the phosphate from the interfering substances by precipitating it as ammonium molybdiphosphate (9). The most common procedures for the treatment of the ammonium molybdiphosphate are the alkalimetric method, in which the nitric acid-free precipitate is dissolved in an excess of a standard solution of sodium hydroxide and the excess caustic is titrated Kith standard nitric acid in the presence of phenolphthalein; and the gravimetric method, in which the ammonium molybdiphosphate is dissolved in dilute ammonium hydroxide, citric acid is added, the phosphate is doubly precipitated with magnesia mixture, and the resulting magnesium ammonium phosphate hexahydrate is ignited to the pyrophosphate. Hillebrand and Lundell (3) have shown through the cooperation of representative analysts that the results obtained by the alkalimetric method when phenolphthalein is used as the indicator are approximately 0.3% higher than those obtained by the double precipitation as magnesium ammonium phosphate. In order to avoid this error, it is common practice among commercial laboratories to determine the titer of the alkali empirically by analyzing a Bureau of Standards phosphate rock rather than by using its stoichiometrical measurement. An improved procedure developed in this laboratory eliminates the empirical nature of the present alkalimetric method. A mixed indicator has been developed which gives a sharp color change a t the stoichiometric end point; the molybdate solution has been stabilized; and the conditions for precipitation have been established which give an ammonium molybdiphosphate precipitate of uniform composition. Data are presented which show that the precision and accuracy of the improved procedure are both very good. The maximum deviation in thirty consecutive samples mas 0.1 mg., and the average deviation 0.05 mg. of phosphorus pentoxide. REAGENTS AND STANDARD SOLUTIONS
Indicator. Prepare the phenol red and bromothymol blue solutions by triturating 0.1000 gram of each of the indicators in an agate mortar with an excess of sodium hydroxide. When the
oxide titer of the alkali may be determined with potassium acid phthalate. Free molybdic acid does not separate at the boiling point when molybdate solutions containing citric and nitric acids of prescribed composition are used. At the boiling point, the yellow precipitate separates almost immediately.
indicator has dissolved completely, adjust the pH with nitric acid to 7.5 and dilute the solution to 250 ml. in a volumetric flask. Prepare the mixed indicator by mixing 40.0 ml. of the bromothymol blue solution with 25.0 ml. of the phenol red solution. Use 0.5 ml. of this indicator for each 100 ml. of solution a t the end point. Citromolybdate Solution. SOLUTION A. Dissolve the following reagents in 1360 ml. of water, without heating: 54 grams of ammonium nitrate, 52.6 grams of citric acid, and 68 grams of ammonium molybdate, (r\rH,)&10&.4H20. SOLUTIONB. Dilute 253 ml. of concentrated nitric acid (specific gravity 1.42) with 310 ml. of water. Prepare the citromolybdate solution by pouring solution A into solution B. Clear it as follows: Bdd filter paper pulp or a few drops of diammonium hydrogen phosphate solution, boil for about 5 to 10 minutes, allow to stand overnight; then siphon off the clear solution. (Such a solution remained clear 7 years and is still clear.) Sodium Hydroxide Solution. Prepare a carbonate-free sodium hydroxide solution (approximately 0.3 N ) by diluting a saturated solution of sodium hydroxide with carbon dioxide-free distilled water. Standardize the solution with a Bureau of Standards sample of potassium acid phthalate (4) using_ phenol_ phthalein indicator. Nitric Acid Solution. Standardize the nitric acid solution (approximately 0.1 N ) with the sodium hydroxide solution, using phenolphthalein or the mixed indicator. PROCEDURE
Wet a 1-gram sample of phosphate rock with 15 ml. of water; add 5 ml. of concentrated nitric acid and 10 to 20 ml. of 60 to 70y0 perchloric acid and heat to copious fumes of perchloric acid. Cover the container and boil gently for 20 minutes to dehydrate the silica. Cool somewhat, add 75 ml. of water, heat to boiling, and filter into a 250-ml. volumetric flask. Wash filter with hot dilute nitric acid solution and then with hot water. Add any phosphate remaining in the silica to the original solution after treating it with hvdrofluoric and nitric acids and fusing- it with sodium-carbonate.. To a 25-ml. aliquot of the sample add 80 ml. of the citromolybdate solution. Heat the resultant solution to boiling, keep a t this temperature for 2 or 3 minutes, and filter either hot or cold. By decantation wash the precipitate four or five times with neutral 1% potassium nitrate or cold water, using 20 to 25 ml. for each wash. Transfer the precipitate to the crucible and wash ten
V O L U M E 20, NO. 11, N O V E M B E R 1 9 4 8
1053
To determine how much perchloric acid could be present in the solution without vitiating the results, solutions of potassium dihydrogen phosphate (14) were prepared and analyzed after Citrovarious amounts of perchloric acid had been added (Table I). PzOa Content of 25-hf1. Aliquot KHzPOh molybdate Solution Solution Calcd. Exptl. In the analysis of samples of phosphate rock, nitric acid waa KO. KO. HCIOP value value used jointly n ith perchloric acid to eliminate the possibility of an M1. Mg. Mo. explosion ( 7 , 1 6 ) due to the presence of organic matter (13). 119 Mixed Indicator. Because sodium molybdate, ammonium molybdate, and sodium ammonium acid phosphate are formed in 159 6.0 32.46 32.43 127 the titration of ammonium molybdiphosphate with sodium 8.0 32.90 8.0 32.76 hydroxide, it seemed advisable to determine the pH curve for the 60% concentration, added to 25-ml. aliquot before precipitation. titration of the yellow precipitate and of each component. The pH measurements were made with a glass electrode, and the curves are given in Figure 1. A study of these curves shows that the phenolphthalein end point comes long after the maximum times. Place the crucible in the original beaker, and dissolve the breaks in the titration of molybdic acid with sodium hydroxide precipitate in a 0.3 N sodium hydroxide solution, using about and with ammonium hydroxide. This finding substantiates 15-ml. excess. .4dd 1 ml. of the mixed indicator, and titrate the the conclusion of Hillebrand and Lundell (4)that phenolphthalein solution with 0.1 S nitric acid until it turns yellow. Remove the is not an ideal indicator. The curve for the titration of the yellow crucible from the beaker, wash with water, and adjust the volume of the solution to about 200 ml. In direct sunlight, or in precipitate with sodium hydroxide does not give a sharp change front of an illuminator equipped with a fluorescent daylight in pH a t any point, indicating that the solution is rather highly Mazda lamp, back-titrate the solution with 0.3 rV sodium hydroxbuffered. It is therefore necessary for the indicator selected to ide to a light permanent purple color (Th? solution becomes practically colorless just before the end point is reached.) give a sharp color change within a very narrow pH range. A4ftera great many indicators and mixtures of indicators and DEVELOPMENT OF METHOD dyes had been tried, it was found that certain mixtures of phenol red and bromothymol blue gave a sharp change from colorleH to I n determining the optimum conditions for the recommended purple, and that the pH a t which the color change took place procedure, the method was applied to solutions prepared from could be raised by increasing the ratio of bromothymol blue to reagent quality potassium dihydrogen phosphate (14) and to phenol red. Bureau of Standards samples of phosphate rock. (The former The pH a t which the purple color could be seen varied with the gave the theoretical loss on ignition of 13.32%.) The phosphorus light conditions. When a titration was made using a titration pentoxide values of the salt were found to be 52.11, 52.19, and illuminator fitted with a fluorescent Mazda daylight lamp, or 52.1070,whereas the theoretical value is 52.18%. in direct sunlight, the color change appeared at a pH of 7.5 and Use of Perchloric Acid (22,26). Inasmuch as the advantages of the results obtained by the alkalimetric method agreed wibh using perchloric acid to remove silica (15) are well recognized, those reported by the Bureau of Standards. it seemed advisable to investigate the applicability of this proI t was discovered early in this work that successive samples of cedure to phosphate rock, prior to the determination of phosphenol red and bromothymol blue obtained on the open market phorus pentoxide. The silica was therefore removed from three were not of the same degree of purity. Further study showed 1-gram samples of Bureau of Standards phosphate rock 56a by that the desired results could be obtained with any sample of using 20 ml. of 60 to 70% perchloric acid. The percentage of phenol red or bromothymol blue, by determining a new ratio for phosphorus pentoxide was determined according to the method each sample. However, through the cooperation of Eastman outlined in the procedure, and the following results were obIlodak Company, it is now possible to prepare the new mixed tained: 32.92,32.89, and 32.90 (certified value 32.9070). indicator from successive batches of these indicators without altering the ratio for each batch. The mixed indicator has a yellow color up to pH about 7.2, is practically colorless a t 7.3, and turns a light purple a t pH about 7.5 when the color change is observed in direct sunlight or under a fluorescent daylight illuminator. In most titrations the end-point color was observed to fade rather rapidly; however, in a few cases the purple color persisted from 5 to 30 minutes or even longer. The end-point color will remain 10 days or more if 0.3 A' sodium hydroxide is added in excess of that required to dissolve the yellow precipitate (about 15-ml. excess for a sample containing approximately 30 mg. of phosphorus pentoxide). Use of Citric Acid. 4 preliminary investigation in this laboratory showed that the use of citric acid and boiling temperature promoted rapid and complete precipitation of coarse crystals of ammonium molybdiphosphate which had a very uniform composition. Lipowitz ( 6 ) was the first to show that the inconstancy of composition of the ammonium Figure 1. pH Curve molybdiphosphate precipitate was due to the A . Yellow precipitate with NaOH C. HiMoOr with "{OH occlusion of molybdic acid. He proposed the B. HzMoOd with NaOH D. HsPO4 with NHIOH t o pH 6.8, then NaOH
Table I.
Effect of Perchloric Acid on Determination of Phosphorus Pentoxide
1054
ANALYTICAL CHEMISTRY
Table 11. Effect of Temperature and Citric Acid upon Per Cent Phosphorus Pentoxide"
Table 1V. Svnthetic Potassium Acid Phosnhate Solutions
2. e 7.5 35 53 _i -\'on-..40 None 75 35.55 50 103 None 75 49 30a 50 103 1 0 90 35.22 40 103 2.0 115 35.19 5 40 6 40 103 4.0 165 35.19 Bureau of Standards sample 120, 35.20% PzOa, b y double pptn. b Note influence of higher temperature and lack of citric acid.
.m -"
1
2 3 4
phosphorus pentoxide. Eighty milliliters of citromolybdate solution cleared with filter paper pulp were used in each run. The data are tabulated in Table 111.
PzOs Content of Aliquot Indicatedm citroKHzPOd molybdate Calcd. Exptl. S o h No. S o h . No. Aliquot value value
Table 111. Effect of Addition of Nitric Acid and Ammonium Nitrate on Completeness of Precipitation Run NHdNO HNOs Added PaOr No. Added (Sp. Gr. 1.42) Found Error Grams M1. Mo. M Q. 1 2 3 4 5
6 7
8 9
10 11 12 13
None Iione 1.0 1.0 2.0 2.0 3.0 3.0 3.0 None
None None None None None Sone None None None 6.0
31.48 31.59 31.47 31.54 31.51 31.66 31.72 31.86 31.76 31.44
-0.04 0.07 -0.05 0.02 -0.01 0.04 0.20 0.34 0.24 -0.08
None None None
6.0 8.0 8.0
31.51 31.36 31.44
-0.01 -0.16 -0.08
146 149
118
160
125
157 157
Ma.
30.
Mo.
10 10 10 5 10
11.64
5
6.47
10 5
12.46 6.23
11.71 11.65 11.60 5.91 12.92 12.97 13.03 6.53 6.54 12.53 6.23
0.07 0.01 -0.04 0.09 -0.01 0.04 0.10 0.06 0.07 0.07 0.00
5.82
12.93
-
Total Av. a
KHzPOd S o h . No.
results, a 0.9560-gram sample of Bureau of Standards phosphate rock No. 120 was dissolved as outlined in the procedure and diluted to 250 ml. A 25-ml. aliquot was used in each run. The ammonium molybdate solution used in these analyses contained 25 grams of ammonium molybdate and 80 ml. of nitric acid (specific gravity, 1.42) in 200 ml. of water. The data are given in Table 11. Phenolphthalein was used as the indicator in runs I, 2, and 3, and the mixed indicator in runs 4, 5, and 6. In runs 4, 5, and 6 the indicated amounts of citric acid and ammonium molybdate solution were substituted for the citromolybdate solution as outlined in the procedure.
Synthetic Potassium Acid Phosphate Solutions P2Os Content of 25-MI. Aliquota Citromolybdate Soln. No.
Calcd. value
Exptl. value
Mo.
Mg.
Mo.
118
150
32.33
117
149 147 146
29.09
121
152
31.88
122
155
31.68
123
156
31.04
125
157
31.13
32.32 32.40 32.35 29.06 29.16 29.11 29.07 31.92 31.87 31.75 31.67 31.77 31.02 31.06 31.08 31.03 31.18 31.13 31.16
-0.01 0.07 0.02 -0.03 0.07 0.02 -0.02 0.04 -0.01 0.07 -0.01 0.09 -0.02 0.02 0.04 -0.10 0.05 0.00 0.03
Total Av.
To show the influence of temperature and citric acid on the a
0.56 0.05
Samples filtered within 1 minute after boiling.
Table V.
precipitation of the ammonium molybdiphosphate a t the boiling point from solutions containing tartaric acid. Since that time the formation of the yellow precipitate a t the boiling point in the presence of ammonium citrate has been investigated by Pellet (10) and Graftiau (2) and in the presence of citric acid by Vincent (18). Pellet claimed that the presence of the ammonium citrate ensured the formation of an ammonium molybdiphosphate precipitate of constant composition. MacIntire, Shaw, and Hardin (8) have shown that quantitative results can be obtained at 45 a C. in the presence of small amounts of citrates.
Citromolybdate Solution.
117
Deviations
M1.
Deviations
0.73 0.04
Samples filtered within about 1 minute after boiling.
A Low results 0 Correct results
-1s the composition
of the yellow precipitate is governed by such factors as the composition of the reagents used and the presence of other substances in the solution, a series of citromolybdate solutions were prepared and used in the analysis of a potassium dihydrogen phosphate solution which contained a n equivalent of 32.72 mg. of phosphorus pentoxide for each 25-m1. aliquot. These data are shown in the ternary diagram, Figure 2. To show how the composition of the solution influenced the results, nitric acid and ammonium nitrate were added to 25-ml. aliquot8 of a potassium dihydrogen phosphate solution, which contained an equivalent of 31.52 mg. of
"0,
OOI
Figure 2.
Ternary Diagram of Citric Acid, Nitric Acid, and Ammonium Nitrate Solution
V O L U M E 20, N O . 11, N O V E M B E R 1 9 4 8 Table VI. Sample XO.
56a 120
Table \-II. Sample
1055
used in this investigation, thus making it possible to reproduce the new mixed indicator from various batches of these indicators. This research was sponsored by a grant from the Research Fund of the University of Alabama.
Results Obtained with Bureau of Standards Phosphate Rocks
Sample Gram 0.1088 0.1102 0.1100 0.0793 0.0560
PZOS Ce tified 3afalue
PZOK Exptl. Value
Fez02
AlzOs
F-~
Si02
%
%
70
70
%
70
%
32.90 32.90 35.20 35.20 35.20
32.89 32.88 35.18 35.23 35.22
2.18 2.18 0.89 0.89 0.89
2.02 2.02 0.80 0.80 0.80
3.56 3.56 3.76 3.76 3.76
11.01 11.01 7.40 7.40 7.40
0.08 0.08 0.07 0.07 0.07
_____Other ~ Materials _ _Present
Ti02
Synthetic Potassium Acid Phosphate Solutions Containing Impurities P*Osin 25-M1. Aliquot Calcd. Exptl. value MQ.
Impurities Present in 25-111. .Iliquot
value MQ.
A
31.18
B
20.92
C
10.10
D
5.53
31.14
31.16
20.94 20.95 20.92 10.03 10.12 5.52 5,82
CaO Gram 0.03
Fer01 Gram 0.005
0.005
0.1
0.03 0.03 0.06 0.03
0,005 0.010 0.005 0.005
0.005 0.010
0.1
0.005 0.005
0.1
0.03
0.005
0,005
0.1
Wash Solution. Either potassium nitrate or cold water may be used for xashing the yellow precipitate. According to Kolthoff and Sandell (5), water peptizes the precipitate and causes it to run through the filter; however, for the volumetric procedure, cold water is recommended (1). Experiments show that identical results are obtained when either potassium nitrate or cold water is used as the wash solution, provided the amount of wash solution when water is used is kept a t a minimum.
-41~0s Gram
The authors are indebted to the Eastman Kodak Company, Rochester, S . Y., for standardizing the purity of the indicators
,..
intern. chim. applicata, Rome, 1, 64 (1906).
0.OOSo 0.0005 I
.
.
...
(3)
1938. (8) (9) (10)
(11)
ACKNOWLEDGMENTS
... ...
Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis. 5th ed., p. 23 (1940). (2) Graftiau, F., Atti V I Congr. (1)
MnO Gram
Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 563, New York, John Wiley & Sons, 1929. (4) Ibid.,p. 568. (5) Kolthoff, I. M., and Sandell, E B., “Textbook of Quantitative Inarganic Analysis,” p. 371, New York, Macmillan Co., 1937. (6) Lipowitz, Pogg. Ann., 119, 135 (1860). (7) Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis,” p. 46, New York, John Wiley & Sons,
APPLICATION OF METHOD
A number of synthetic potassium acid phosphate solutions, ranging from 32 to 6.0 mg. of phosphorus pentoxide per 25-ml. aliquot, were analyzed (Tables IV and V). The data show that the results are independent of the amount of phosphorus pentoxide in the sample. The maximum deviation was 0.1 mg. and the average deviation 0.05 mg. of phosphorus pentoxide. Two Bureau of Standards samples of phosphate rock were analyzed, along with several synthetic solutions that contained certain impurities (Tables VI and 1’11).
H~SOI Gram
LITERATURE CITED
(12) (13) (14) (15)
MacIntire, W. H., Shaw, W. M., and Hardin, L. J., IND.ENG. CHEM.,AXAL.ED.,10, 143 (1938). Pauling, L., J. Am. Chem. Soc., 51, 2863 (1929). Pellet,,H., Bull. assoe. belge chim., 3, 51 (1888-89). Smith, G. F., “Pzrchloric Acid,” 4th ed., Columbus, Ohio, G Frederick Smith Chemical Co., 1940. Vincent, V.,Ann. fals., 23, 475 (1930). Wiley, H . W., “Principles and Practice of -4grioultural Analysis, Vol. 11, Fertilizers and Insecticidea,” 3rd ed., p. 63, Table VII, Easton, Pa., Chemical Publishing Co., 1931. Ibid., p. 186. Willard, H. H., and Cake, W. E., J. Am. Chem. SOC., 42, 2208 (1920).
(16)
Willard, H. H., and Diehl, Harvey, “Advanced Quantitative Analysis,” p. 8, New York, D. Van Nostrand Co., 1943.
RECEIVED October 4, 1947. Presented before the Division of Physical and CHEMICAL Inorganic Chemistry, at the 100th Meeting of the AMERICAN SOCIETY, Detroit, Mich. Report on citromolybdate aolution presented before the Division of Physical and Inorganic Chemistry, at the lOlst meeting of the AMERICAN CHEMICAL SOCIETY, St. Louis, AVO.A part of this work was presented by Howard P. Crammer in partial fulfillment of the reqoirements for the degree of master of science, 1941.
CHEMISTRY OF THORIUM Quantitative Estimation of Thorium by a Titrimetric Iodate Procedure THERALD MOELLER AND NANCY DOWNS FRITZ, University of IEZirwis, Urbana, I l l .
T
HE lack of rapid methods for the accurate estimation of thorium in the presence of yttrium and the rare earth elements has been emphasized repeatedly (1, 5, 8). Of the recorded reactions of the thorium ion, that involving its precipitation as iodate from solutions containing nitric acid appears most promising as a basis for a direct and potentially rapid method for estimating the element under these conditions. Not only does this reaction yield a thorium salt containing an ion that may be determined by familiar oxidimetric procedures, but i t also effects complete separation of thorium from yttrium and the tripositive rare earth
elements, the iodates of which are soluble in nitric acid solutions (6).
While iodate precipitation is an established procedure for freeing thorium of yttrium and the rare earth elements prior to its estimation by gravimetric means ( 8 ) , reports on its adaptation to a direct titrimetric method have been fragmentary. Chernikhov and Uspenskaya (5)described a procedure in which thorium was precipitated from nitric acid solution by excess potassium iodate, and the precipitate was washed with nitric acid containing potassium iodate, treated with 95% ethanol, dried, and dissolved in
