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Microdetermination of phosphorus in organic compounds by perchloric acid digestion
MICHOCHEMICAL
JOURNAL
13, 616-620 (1968)
Microdetermination of Phosphorus in Organic by Perchloric Acid Digestion L. I. DIUGUID
AND
Du-Good Chemical Laboratory,
Compounds
N. C. JOHNSON St. Louis, Missouri (8104
Receioed June 17, 196’8
A number of methods are described in the literature for the microdetermination of phosphorus in organic compounds. Only some of the more classical methods will be mentioned with their inherent limitations in regards to carrying out commercial microanalysis. Recently Malyukova and Zaitseva (1) investigated methods of decomposing organophosphorus compounds, and the fusion with sodium peroxide in the nickel Parr bomb was preferred. Other classical methods include the Kjeldahl type digestion using sulfuric-nitric digestion (Z), the Carius combustion using nitric acid in a sealed tube (3)) the Schoniger closed flask combustion (4) and fusion methods in a platinum crucible using a mixture of sodium carbonate and potassium nitrate (5). The products of the above methods of decomposing organophosphorus compounds may be determined gravimetrically as ammonium phosphomolybdate (2)) calorimetrically as molybdenum-blue complex (5)) titrametrically by the decomposition of quinoline-phosphomolybdate with sodium hydroxide and the excess sodium hydroxide is determined by titration with hydrochloric acid, etc. (5). By way of comparison some of the limitations of present methods involved in commercial work include: (i) The peroxide fusion method (Parr bomb) will decompose difficult fluorine containing phosphorus compounds, etc., but includes time consuming manipulations as compared to the Kjeldahl digestion method. Problems of contamination of silica are also encountered. (ii) The Carius method (sealed tube) is also applicable to difficult type phosphorus compounds but some phosphorus may be lost through absorption of phosphorus by the glass. Also, the method is time consuming plus the dangers of working with sealed tubes. (iii) The Schoniger closed flask is not sufficiently vigorous to quantitatively oxidize the more difficult compounds. The method involves dangers of explosions and expensive equipment. (iv) The Kjeldahl digestion method using sulfuric-nitric acid lends itself to a 616
PHOSPHORUS
IN
ORGANIC
COMPOUSDS
617
series of determinations and requires few manipulations but difficulties are encountered in the more complex type compounds to oxidize, i.e., in fluorine containing compounds, phosphonates, phosphinates and phosphonium type (P-C bonds), low and erratic phosphorus results are encountered. The above classical method and others in the literature in commercial applications have limitations such as time consuming, poor accuracy or reliability, limited applicability, tediousness, and specialized skills requiring expensive and elaborate equipment. The method used in our laboratory involves the Kjeldahl wet digestion using perchloric-nitric acid mixture and is the outgrowth of the success encountered in the microdetermination of sulfur in organic compounds (6) using perchloric-nitric acid digestion, coupled with the difficulties encountered using the classical sulfuric-nitric acid digestion method. It was realized that the more difficult type compounds, phosphonium, phosphonates (P-C bonds), etc. were not completely decomposed by the sulfuric-nitric method and a more vigorous oxidizing agent was necessary. A discussion of the useful aspects of 7&72c/, perchloric acid and the temporizing effect of nitric acid when used prior to and in conjunction with perchloric acid is given in the microdetermination of sulfur (6). The use of perchloric acid, perchloric-sulfuric acid, perchloricsulfuric-nitric acid mixtures have been used for the determination of phosphorus in the fluoro-containing phosphorus compounds, etc. on a limited scale for the calorimetric determination of phosphorus by Ingram (5)) Salvage and Dixon ( 7), and Kirsten (8). MATERIALS
AND
METHODS
Reagents: AR nitric acid (sp gr 1.42)) AR perchloric acid (70-72s ), nitric sulfuric acid mixture (9), ammonium nitrate solution (9) (2% ), molybdate reagent (2)) AR ethanol 95%; and AR acetone. Apparatus: Soltys Kjeldahl digestion flasks (30 ml), digestion rack and manifold (2).
(6), (modified),
filter tube (2), and filtration
assembly
Method. About 3-5 mg of sample is weighed and transferred into a Solty Kjeldahl flask (30 ml). Preliminary calculation of phosphorus should be made such that not more than 50 mg or less than l-2 mg of
ORGANIC
(%
phosphorodichloridate
phenylphosphonate phosphorochloridate
Triethylcitrate phosphoric acid 1,6-Hexamethylenebis (nitrilodimethylene tetraphosphoric acid hydrate
Diphenyl Diphenyl
p-Phenylene phosphorodichloridate Human hair (male, age 13) Triethylcitrate diphenylphosphate
p-Nitrophenyl
phosphorodichloridate
) &,H, ) ,P-Cl
0
fl
8
bl NICH,P(OH)z],*H,O
8 Cl A,O,“P [(HO),PCH,J2N(CH,),-
( C,H,O ( C,H,O
(%%,%p)
8-O Cl,PO -\J-
p-NO,C,H,OPCI
24.28
8.69
9.98 11.53
6.10
18.02
12.10
14.68
Theory
PERCHLORIC
C,H,O~Cl~
OPCl,
BY
Phenyl
COMPOUNDS
9.49 26.72 10.82
Formula
IN
(C,H,O),PO ( NPCl, 13 (C,H,O),P(O)OH*2H,O 0
)
OF PHOSPHORUS
1
Triphenyl phosphate Phosphonitrilic chloride, trimer Diphenyl phosphoric acid dihydrate
Compound
LMICRODETERMINATION
TABLE
P)
ACID (%
P)
24.07,24.17
8.65, 8.69
9.73, 9.77 11.51,11.49
18.07J8.15 0.02, 0.02 6.26, 6.29
12.1OJ2.26
14.85J4.86
9.53, 9.57 26.51,26.79 10.77,10.98
Found
DIGESTION
PHOSPHORUS
IN
ORGANIC
COMPOUNDS
619
final ammonium phosphomolybdate precipitate should be obtained. Three ml of concentrated nitric acid (sp gr 1.42) is added to the sample in the Kjeldahl flask, followed by 2 ml of perchloric acid (70-72%). The sample is placed on a modified digestion rack (6) and heated over a low flame until the solution is clear and then strongly until a final volume of ca. 0.2-0.3 ml is obtained (ca. 1 hour). The resulting clear solution is cooled somewhat then an additional ( 1 ml) of concentrated nitric acid is added and the solution is concentrated to ca. 0.2-0.3 ml. The solution in the Kjeldahl flask is cooled to room temperature, transferred quantitatively into a 4 x 15 cm wide mouth Pyrex tube, with several 1 ml washings of water and sufficient nitric-sulfuric acid mixture added to bring the total volume to (15 ml). The resulting mixture in the precipitation tube is heated on a hot water bath near boiling for a period of 10 minutes and the solution is removed from the bath and 15 ml of freshly filtered molybdate reagent are added from a pipette (the reagent should not be allowed to touch the walls of the precipitating tube). The tube containing the yellow ammonium phosphomolybdate is covered and kept overnight in the dark prior to filtration. The filtration is conducted essentially as described by Steyermark (2). A series of six determinations can be run simultaneously by this method and gives a high degree of accuracy ( +O.2% ). Table 1 gives a list of typical organophosphorus compounds analyzed by the above method. DISCUSSION
The use of perchloric-nitric acid digestion method has been used successfully in our laboratory over a wide range of organophosphorus compounds in commercial analysis, for over a decade, including the more difficult phosphonates, phosphinates, and phosphonium types, etc. Also, the method is adaptable to using large quantities of sample on the order of 56100 mg where the amount of phosphorus is on the order of 0.1%. However, preliminary digestion of larger samples with nitric acid should be utilized as a precautionary measure (we encountered one case of a violent reaction, using a polyhydric wax type where 100 mg of sample was used, which was accompanied by initial carbonization and blackening during the initial heating over a low flame. A safety glass is adequate protection for the analyst. For complex biological compounds and other substances containing less than 0.1% phosphorus,
620
DIIJGUID
AND
JOHNSON
a calorimetric finish may be empl.oyed using amidol to form a molybdenum blue complex (5). SUMMARY A Kjeldahl wet digestion method using perchloric-nitric acid mixture for the determination of phosphorus in organic compounds was developed and used successfully for a wide variety of compounds difficult to oxidize, including complex biological substances. The method fulfills the requirements of accuracy ( +-O.Z%), reliability, and simplicity of apparatus and lends itself to a series of determinations with little additional time required. ACKNOWLEDGMENT The authors are indebted to Dr. Joseph Baker of Monsanto Co., St. Louis, Missouri, for supplying samples of pure organophosphorus compounds and to Ida B. Phillips of this laboratory for performing the microanalyses reported in Table 1. REFERENCES 1.
MALYUKOVA, F. S., AND ZAITESEVA, A. D., Determination of phosphorus in organophosphorus compounds. Plusticheskle Massy 1966, 57. 2. STEYERMARK, A., “Quantitative Organic Microanalysis,” 2nd ed., pp. 354-365. Academic Press, New York, 1961. 8. Ooo, C. L., Microanalytical determination of phosphorus. J. Assoc. Ofic. Agr. Chemists 40, 386 ( 1957). 4. YIJ, H. Y., AND SHA, I. H., Rapid microanalysis of phosphorus in organic compounds and polymers. Chem. Bull. (Peking) 557 ( 1965). 5. INGRAM, G., “Methods of Organic Elemental Microanalysis,” pp. 341-346. Reinhold, New York, 1962. 6. DIUGU~, L. I., AND JOHNSON, N. C., ,Microdetermination of sulfur in organic Microchem. J. 12, 371376 compounds by perchloric acid digestion. (1967). 7. SALVAGE, T., AND DIXON, J. P., The calorimetric determination of phosphorus on the milligram scale. Analyst 90, 24-28 ( 1965). S. KIRSTEN, W. J., AND CARLSON, M. E., Investigation of the determination of phosphorous in organic compounds. Microchem. J. 4, 3-31 ( 1960). Y. NIEDERL, J., AND NIEDERL, V., “Micromethods of Quantitative Analysis,” 2nd ed., p, 200. Wiley, New York, 1952.
JOURNAL
13, 616-620 (1968)
Microdetermination of Phosphorus in Organic by Perchloric Acid Digestion L. I. DIUGUID
AND
Du-Good Chemical Laboratory,
Compounds
N. C. JOHNSON St. Louis, Missouri (8104
Receioed June 17, 196’8
A number of methods are described in the literature for the microdetermination of phosphorus in organic compounds. Only some of the more classical methods will be mentioned with their inherent limitations in regards to carrying out commercial microanalysis. Recently Malyukova and Zaitseva (1) investigated methods of decomposing organophosphorus compounds, and the fusion with sodium peroxide in the nickel Parr bomb was preferred. Other classical methods include the Kjeldahl type digestion using sulfuric-nitric digestion (Z), the Carius combustion using nitric acid in a sealed tube (3)) the Schoniger closed flask combustion (4) and fusion methods in a platinum crucible using a mixture of sodium carbonate and potassium nitrate (5). The products of the above methods of decomposing organophosphorus compounds may be determined gravimetrically as ammonium phosphomolybdate (2)) calorimetrically as molybdenum-blue complex (5)) titrametrically by the decomposition of quinoline-phosphomolybdate with sodium hydroxide and the excess sodium hydroxide is determined by titration with hydrochloric acid, etc. (5). By way of comparison some of the limitations of present methods involved in commercial work include: (i) The peroxide fusion method (Parr bomb) will decompose difficult fluorine containing phosphorus compounds, etc., but includes time consuming manipulations as compared to the Kjeldahl digestion method. Problems of contamination of silica are also encountered. (ii) The Carius method (sealed tube) is also applicable to difficult type phosphorus compounds but some phosphorus may be lost through absorption of phosphorus by the glass. Also, the method is time consuming plus the dangers of working with sealed tubes. (iii) The Schoniger closed flask is not sufficiently vigorous to quantitatively oxidize the more difficult compounds. The method involves dangers of explosions and expensive equipment. (iv) The Kjeldahl digestion method using sulfuric-nitric acid lends itself to a 616
PHOSPHORUS
IN
ORGANIC
COMPOUSDS
617
series of determinations and requires few manipulations but difficulties are encountered in the more complex type compounds to oxidize, i.e., in fluorine containing compounds, phosphonates, phosphinates and phosphonium type (P-C bonds), low and erratic phosphorus results are encountered. The above classical method and others in the literature in commercial applications have limitations such as time consuming, poor accuracy or reliability, limited applicability, tediousness, and specialized skills requiring expensive and elaborate equipment. The method used in our laboratory involves the Kjeldahl wet digestion using perchloric-nitric acid mixture and is the outgrowth of the success encountered in the microdetermination of sulfur in organic compounds (6) using perchloric-nitric acid digestion, coupled with the difficulties encountered using the classical sulfuric-nitric acid digestion method. It was realized that the more difficult type compounds, phosphonium, phosphonates (P-C bonds), etc. were not completely decomposed by the sulfuric-nitric method and a more vigorous oxidizing agent was necessary. A discussion of the useful aspects of 7&72c/, perchloric acid and the temporizing effect of nitric acid when used prior to and in conjunction with perchloric acid is given in the microdetermination of sulfur (6). The use of perchloric acid, perchloric-sulfuric acid, perchloricsulfuric-nitric acid mixtures have been used for the determination of phosphorus in the fluoro-containing phosphorus compounds, etc. on a limited scale for the calorimetric determination of phosphorus by Ingram (5)) Salvage and Dixon ( 7), and Kirsten (8). MATERIALS
AND
METHODS
Reagents: AR nitric acid (sp gr 1.42)) AR perchloric acid (70-72s ), nitric sulfuric acid mixture (9), ammonium nitrate solution (9) (2% ), molybdate reagent (2)) AR ethanol 95%; and AR acetone. Apparatus: Soltys Kjeldahl digestion flasks (30 ml), digestion rack and manifold (2).
(6), (modified),
filter tube (2), and filtration
assembly
Method. About 3-5 mg of sample is weighed and transferred into a Solty Kjeldahl flask (30 ml). Preliminary calculation of phosphorus should be made such that not more than 50 mg or less than l-2 mg of
ORGANIC
(%
phosphorodichloridate
phenylphosphonate phosphorochloridate
Triethylcitrate phosphoric acid 1,6-Hexamethylenebis (nitrilodimethylene tetraphosphoric acid hydrate
Diphenyl Diphenyl
p-Phenylene phosphorodichloridate Human hair (male, age 13) Triethylcitrate diphenylphosphate
p-Nitrophenyl
phosphorodichloridate
) &,H, ) ,P-Cl
0
fl
8
bl NICH,P(OH)z],*H,O
8 Cl A,O,“P [(HO),PCH,J2N(CH,),-
( C,H,O ( C,H,O
(%%,%p)
8-O Cl,PO -\J-
p-NO,C,H,OPCI
24.28
8.69
9.98 11.53
6.10
18.02
12.10
14.68
Theory
PERCHLORIC
C,H,O~Cl~
OPCl,
BY
Phenyl
COMPOUNDS
9.49 26.72 10.82
Formula
IN
(C,H,O),PO ( NPCl, 13 (C,H,O),P(O)OH*2H,O 0
)
OF PHOSPHORUS
1
Triphenyl phosphate Phosphonitrilic chloride, trimer Diphenyl phosphoric acid dihydrate
Compound
LMICRODETERMINATION
TABLE
P)
ACID (%
P)
24.07,24.17
8.65, 8.69
9.73, 9.77 11.51,11.49
18.07J8.15 0.02, 0.02 6.26, 6.29
12.1OJ2.26
14.85J4.86
9.53, 9.57 26.51,26.79 10.77,10.98
Found
DIGESTION
PHOSPHORUS
IN
ORGANIC
COMPOUNDS
619
final ammonium phosphomolybdate precipitate should be obtained. Three ml of concentrated nitric acid (sp gr 1.42) is added to the sample in the Kjeldahl flask, followed by 2 ml of perchloric acid (70-72%). The sample is placed on a modified digestion rack (6) and heated over a low flame until the solution is clear and then strongly until a final volume of ca. 0.2-0.3 ml is obtained (ca. 1 hour). The resulting clear solution is cooled somewhat then an additional ( 1 ml) of concentrated nitric acid is added and the solution is concentrated to ca. 0.2-0.3 ml. The solution in the Kjeldahl flask is cooled to room temperature, transferred quantitatively into a 4 x 15 cm wide mouth Pyrex tube, with several 1 ml washings of water and sufficient nitric-sulfuric acid mixture added to bring the total volume to (15 ml). The resulting mixture in the precipitation tube is heated on a hot water bath near boiling for a period of 10 minutes and the solution is removed from the bath and 15 ml of freshly filtered molybdate reagent are added from a pipette (the reagent should not be allowed to touch the walls of the precipitating tube). The tube containing the yellow ammonium phosphomolybdate is covered and kept overnight in the dark prior to filtration. The filtration is conducted essentially as described by Steyermark (2). A series of six determinations can be run simultaneously by this method and gives a high degree of accuracy ( +O.2% ). Table 1 gives a list of typical organophosphorus compounds analyzed by the above method. DISCUSSION
The use of perchloric-nitric acid digestion method has been used successfully in our laboratory over a wide range of organophosphorus compounds in commercial analysis, for over a decade, including the more difficult phosphonates, phosphinates, and phosphonium types, etc. Also, the method is adaptable to using large quantities of sample on the order of 56100 mg where the amount of phosphorus is on the order of 0.1%. However, preliminary digestion of larger samples with nitric acid should be utilized as a precautionary measure (we encountered one case of a violent reaction, using a polyhydric wax type where 100 mg of sample was used, which was accompanied by initial carbonization and blackening during the initial heating over a low flame. A safety glass is adequate protection for the analyst. For complex biological compounds and other substances containing less than 0.1% phosphorus,
620
DIIJGUID
AND
JOHNSON
a calorimetric finish may be empl.oyed using amidol to form a molybdenum blue complex (5). SUMMARY A Kjeldahl wet digestion method using perchloric-nitric acid mixture for the determination of phosphorus in organic compounds was developed and used successfully for a wide variety of compounds difficult to oxidize, including complex biological substances. The method fulfills the requirements of accuracy ( +-O.Z%), reliability, and simplicity of apparatus and lends itself to a series of determinations with little additional time required. ACKNOWLEDGMENT The authors are indebted to Dr. Joseph Baker of Monsanto Co., St. Louis, Missouri, for supplying samples of pure organophosphorus compounds and to Ida B. Phillips of this laboratory for performing the microanalyses reported in Table 1. REFERENCES 1.
MALYUKOVA, F. S., AND ZAITESEVA, A. D., Determination of phosphorus in organophosphorus compounds. Plusticheskle Massy 1966, 57. 2. STEYERMARK, A., “Quantitative Organic Microanalysis,” 2nd ed., pp. 354-365. Academic Press, New York, 1961. 8. Ooo, C. L., Microanalytical determination of phosphorus. J. Assoc. Ofic. Agr. Chemists 40, 386 ( 1957). 4. YIJ, H. Y., AND SHA, I. H., Rapid microanalysis of phosphorus in organic compounds and polymers. Chem. Bull. (Peking) 557 ( 1965). 5. INGRAM, G., “Methods of Organic Elemental Microanalysis,” pp. 341-346. Reinhold, New York, 1962. 6. DIUGU~, L. I., AND JOHNSON, N. C., ,Microdetermination of sulfur in organic Microchem. J. 12, 371376 compounds by perchloric acid digestion. (1967). 7. SALVAGE, T., AND DIXON, J. P., The calorimetric determination of phosphorus on the milligram scale. Analyst 90, 24-28 ( 1965). S. KIRSTEN, W. J., AND CARLSON, M. E., Investigation of the determination of phosphorous in organic compounds. Microchem. J. 4, 3-31 ( 1960). Y. NIEDERL, J., AND NIEDERL, V., “Micromethods of Quantitative Analysis,” 2nd ed., p, 200. Wiley, New York, 1952.