Patricia W. Liepins for collecting many “of the data. LITERATURE CITED
(1) Clarke, F. E., ANAL.CHEM.22, 553 (1950). (2) Dole, V. P., Stall, B. G., Schwartz, I. L., Proc. SOC.Exptl. Biol. Med. 77,412 (1951). ( 3 ) Dubskj., J. V., Trtilek, J., Mikrochemie 12,315 (1933).
(4) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,’’ 3rd ed., Macmillan, New York, 1952. Krumholz, P., Krumholz, E., J f o n atsh. 70, 431 (1937). Krumholz, P., Watzek, H., Zbtd.. 70,437 (1937). Nelson, N., J . Biol. Chem. 153, 373 (1944). I
,
Rdberts; I., IND. ENG. CHEJI., ANAL.ED.8, 365 (1936).
Schales, O., Schales, S., J . Biol. Chem. 140, 879 (1941).
Schwartz, I. L., Thaysen, J. H., Dole, D. F., J . Exptl. Med. 97, 429 (1953).
Vosburgh, W. C., Cooper, 0. R., J . Am. Chem. Soc. 63, 437 (1941). Yoe, J. H., Jones, A. L., IND. ENG. CHEJI., k A L . ED. 16, 111 (1944). RECEIVED for review July 24, 1957. &4cceptedJanuary 18, 1958.
Spectrophotometric Determination of Phosphorus in Polyethylene Terephthalate GEORGE TELEP and ROBERT EHRLICH -Textile Fibers Department, E. 1. du Pont de Nemours & Ca., Inc., Kinston,
b A semimicro- and a micromethod have been developed for the determination of small quantities of phosphorus in polyethylene terephthalate. The methods are applicable to all difficultly oxidizable or slightly soluble materials. They consist of an oxidation with perchloric acid and color development b y the heteropoly blue procedure using hydrazine sulfate as the reducing agent. Accurate measurements can be made on as little a s 5 y of phosphorus per 50 ml. in the semimicromethod and on 0.1 y of phosphorus per 50 ml. in the micromethod.
p
terephthalate, the polymer from which Dacron (trade-mark for Du Pont’s polyester fiber) polyester fiber is made, may contain small added quantities of phosphorus. This paper reports the development of fairly rapid methods for the determination of phosphorus in such yarns and polymers. oLmTnYmm
REAGENTS, APPARATUS, AND PROCEDURE
Reagents. Sodium molybdate dihydrate, ACS reagent grade, 2.5% solution in 10N sulfuric acid. Ammonium molybdate tetrahydrate, analytical reagent grade, 1% solution in 2.5N sulfuric acid. Hydrazine sulfate, C.P. grade, 0.15% solution in distilled water. Reducing Solution. SEMIMICROMETHOD. Immediately before use, 25 ml. of the 2.5% molybdate solution and 10 ml. of the hydrazine sulfate solution are mixed in a 100-ml. volumetric flask and diluted t o the mark with distilled water. MICROMETHOD. Immediately before use, 25.0 ml. of the 1% molybdate solution and 10.0 ml. of the hydrazine sulfate solution are measured from burets into a 1146
@
ANALYTICAL CHEMISTRY
N. C.
100-ml. volumetric flask and diluted to concentrated sulfuric acid and five drops the mark with distilled water. of 72% perchloric acid. Heat on a hot Apparatus. Absorbance measureplate until dense white fumes of perments were made on a Beckman DU chloric acid appear. Continue the spectrophotometer, equipped with a fuming until all the perchloric acid has cell housing for 10-cm. absorption fumed (cessation of dense fumes and colorless solution). Cool the flask and cells, a t a wave length of 830 mp. transfer the contents quantitatively Matched 1-cm. Corex and 10-cm. borointo a 50-ml. volumetric flask using silicate glass cells, with Corex windows, about 15 ml. of distilled water. Add were used. Procedure. SEMIMICROMETHOD.20.0 ml. of reducing solution, dilute to the mark with distilled water, and heat To 1 gram of sample, containing in a boiling water bath for 15 minutes. approximately 0.25 mg. of phosphorus, Remove the flask, cool it to room temin a 250-ml. Erlenmeyer flask, add 10 ml. of concentrated sulfuric acid and perature, and redilute to the mark 2 ml. of concentrated nitric acid. if necessary. Measure the absorbance Warm the solution on a hot plate a t 830 mp using 10-cm. cells and a reagent blank as the reference solution. until oxidation begins and then add Prepare the reagent blank exactly like concentrated nitric acid dropwise the sample. until the solution remains straw yellow Calibration Curves. A standard in color. Cool the reaction mixture, phosphate solution was prepared conadd 1 to 2 ml. of 72% perchloric acid, taining 0.2132 gram of reagent grade and evaporate the solution on the hot ammonium monohydrogen phosphate plate until copious white fumes of in 500 ml. of solution. One milliliter perchloric acid are evolved. Care must of this solution is equivalent to 0.100 be exerciscd in heating perchloric acid mg. of phosphorus. Suitable diluwhich, in the presence of an excess of tions of this stock solution were used oxidizable material, may become exto obtain the calibration curves. I n plosive. the semimicromethod the color was When the fuming has been completed, developed directly from the aliquots of cool the solution, and transfer it quantithe stock solution; in the micromethod tatively to a 250-ml. volumetric flask. these aliquots were taken through the Dilute it to about 50 ml., adjust it complete procedure. The curves obey neutral to litmus with ammonium Beer-Lambert’s law over the concentrahydroxide, and dilute to the mark with tion ranges of 0.5 to 4.0 and 5 to 50 y distilled water. Pipet 25.0 ml. of this per 50 ml. of solution. solution into a 50-ml. volumetric flask and add 20 ml. of reducing solution, diluting to the mark with distilled water. DISCUSSION Develop the color by placing the flask in a boiling water bath for 15 Semimicromethod. PRELIMINARY minutes and read the absorbance in TREATMENT O F SAMPLE. Polyethyl1-cm. cells a t 830 mp on a Beckman ene terephthalate is a linear polyester DU spectrophotometer. As the reference solution, use a reagent blank which which is insoluble in all common is prepared by diluting 20 ml. of the solvents. One of the main difficulties, reducing solution to 50 ml. with distilled therefore, is the oxidation of the water and heating in the boiling water sample in an aqueous medium without bath for 15 minutes. loss of orthophosphoric acid due to MICROMETHOD. Weigh approxivolatilization. To develop an anmately 5 mg. of sample, containing alytical method dependent upon the about 0.5 y of phosphorus, into a 10-ml. determination of phosphorus as orthoErlenmeyer flask. Add L O O ml, of
phosphoric acid, it \vas essential to control the oxidation of the sample, using conditions which would minimize this loss. Kjeldahl-type and acidic mixture oxidations were investigated. Dry-ashing as well as met-ashing techniques, in which the mixture was fumed with sulfuric acid, caused significant losses of orthophosphoric acid. The addition of sodium molybdate to the oxidation mixture, for the purpose of complexing the orthophosphoric acid as it was formed, improved the results someivhat but not to the level of precision desired. Wet oxidation of the samples was attempted with a mixture of nitric and sulfuric acids to destroy the organic matter. This procedure was timeconsuming and failure to remove the excess nitric acid hindered the formation of the colored complex. Excess evaporation to remove the nitric acid contributed to low results through volatilization of the orthophosphoric acid, as was found by Robinson (S). In order to remove the nitric acid a t a loir er temperature, and thus prevent the loss of orthophosphoric acid, a method was devised using perchloric acid, analogous to that developed for iron and steel by Hague and Bright (9). The organic material was completely oxidized with a mixture of sulfuric and nitric acids, and the excess nitric acid was removed by low temperature evaporation in the presence of perchloric acid. Under these conditions the loss of phosphoric acid \vas negligible and a series of samples showed that a reproducible recovery of orthophosphoric acid could be achieved. This method of oxidation was utilized subsequently. COLORDEVELOPMENT.Orthophosphoric and molybdic acids condense to give heteropoly complex compounds, the molybdophosphoric acids (4). The reduction of these molybdophosphoric acids produces a blue color; the color intensity is proportional to the amount of orthophosphate ions incorporated in the complex. The exact constitution of these reduction products is uncertain, but their formation is reproducible (1). The optimum conditions for the development of the colored complex have been studied by many workers, and many reducing agents have been used ( I , 4). Complex formation is usually carried out in an acidic solution a t a pH less than 1. The complex color intensity is stronger in a less acidic solution but the color developed by the reagents is also much greater. The color intensity does not increase in the presence of excess reducing agent, hydrazine sulfate (1). For maximum color development, 5 ml. of the 2.5% molybdate concentration per 50 ml. of solution was sufficient for a reductant concentration of 3.0 mg. of hydrazine sulfate per 50 ml. (1). An acidity range from 0.75- to 1.25N
provided the limits within vhich reproducible results were obtained using the specified amount of molybdate. This is the same range found to apply to the color development step by Boltz and Mellon (1). Heating the sample a t various temperatures showed that 10 to 15 minutes in boiling water gave the maximum color development, with the color being stable for 18 hours. For calculation of the per cent orthophosphoric acid in an unknown, micrograms of phosphorus per 50 ml. of solution were obtained from the calibration curve and substituted in Equation 1. (y
of P/50 ml.)
% H3P04 = (wt. of sample, mg.) (3.16) (1)
PRECISION AXD ACCURACY. A series of six determinations on a blended polymer sample containing a theoretical phosphorus content of 0.025%, calculated as orthophosphoric acid, gave an average value of 0.025%, with a standard deviation of 0.003 and a coefficient of variation of 10.8% (Table I).
Table 1.
Precision and Accuracy of Semimicromethod
Sample Weight, Grams 1.0120 1.0253 1.1143 1.2074 1.0760 1.0817
0
0.024 0.029 0.023 0.021 Av. 0.0250 u 0.003 Coeff. of variation 10.8% Theoretical value 0.025%.
Table 11.
Precision of Micromethod
Sample Weight, &Pod, Rfg. 4.694 4.756 4.792 4.i20 4.564 4.682 4.856 4.798
70
Sample lF7eight, bfg. 4.834 4.760 4.892 4.696 4.680 4.486 4.640
0.0273 0.0261 0.0264 0.0259 0.0272 0.0278 0.0223 0.0259 Av.
H3;O4, /C
0.0265 0.0227 0.0221 0.0239 0.0248 0.0255 0.0238 0,02520 Std. dev. 0.00186 Coeff. of variation 7 3 i 7 , Calculated value = 0.02507,.
of 7 2 7 , perchloric acid for approximately 5 mg. of sample. This gave nonreproducible results as well as an unstable color. Robinson (3) used perchloric acid to oxidize the organic impurities in lake water with better results than when he used nitric acid, or a combination of nitric and perchloric acids. It was decided, therefore, to use the 7270 perchloric acid as the oxidizing agent to eliminate the nitric acid. The oxidation was complete and no loss through volatilization of orthophosphoric acid occurred. COLOR DEVELOPMENT.In measuring the intensity of a heteropoly blue solution the color, once developed, cannot be diluted or concentrated because the intensity depends on the pH, the molybdenum-acid ratio, and the amount of molybdate (6, 6). K i t h a given concentration of molybdate a minimum concentration of acid is required to prevent color development in the absence of phosphate. Just above this critical concentration there is a limited range in which the intensity of the color is proportional to the phosphate content almost independently of the acidity. Further increases in acidity cause a decrease in color intensity (4). K i t h hydrazine sulfate as the reducing agent, the minimum acidity was at 0.75.V with respect to sulfuric acid, in order to prevent the reduction of molybdic acid. The limits of acidity were 0.75- and 1.ON in the final solution as compared to 0.75- to 1.25N for the semimicromethod. For reproducible sample color the acidity was controlled precisely because of the increased sensitivity provided by the use of 10-cm. absorption cells. At the pH level to which the sample was adjusted after oxidation, the p H meter was substantially insensitive to small changes in acidity. This difficulty nas overcome by using a concentration of acid with the reagents such that, on Combination with the oxidized sample, the desired acidity was reached on dilution to a fixed volume. MEASUREMEKT OF COLOR.The equation for the line of the calibration curve was A
= 0.163~
(2)
where A is the absorbance and c is the concentration in micrograms of phosphorus per 50 ml. of solution. (The value of 0.163 might vary slightly under different instrument settings.) To determine the quantity of phosphorus in an unknown sample, Equations 1 and 2 are combined. Micromethod. PRELIMINARY TREATMENT OF SAMPLE. It F a s at9% HJ’O4 = tempted, initially, t o scale down the A oxidation procedure used in the semi(sample weight, mg.) (3.16) (0.163) micromethod, using 1 ml. of concentrated sulfuric acid, 0.1 ml. of 1.941 A concentrated nitric acid, and 0.1 ml. sample weight, mg. (3) VOL. 30, NO. 6, JUNE 1958
1147
PRECISION AND ACCURACY.The analyses of fifteen samples of approximately 5 mg. each, weighed out on a semimicrobalance, at an average value of 0.0252% orthophosphoric acid, had a standard deviation of 0.00186 and a coefficient of variation of 7.37% (Table 11). T o check the accuracy of the method a n accurately weighed quantity of ferrous phosphate octahydrate was added to polymeric material, meltblended, and ground after solidification. I n a second check of the accuracy, aluminum phosphate was suspended in water, and aliquots of this suspension were analyzed. The results are shown in Table 111.
LITERATURE CITED
Table 111.
Accuracy of Micromethod
HJ’OI, % Error, Sample Found Calculated % Polymeric material 0.0265 0.0254 4.33 Suspension 0.0765 0,0802 4,84
Interferences. The relatively high acid concentration used in both methods prevents interference by silicon (1). The only other possible interfering elements are arsenic and germanium, which can be eliminated by suitable existing methods.
(1) Boltz, D. F., Mellon, M. G., ASAL.
CHEM.19, 873 (1947).
(2) Hague, J. L., Bright, H. A., J . Research Natl. Bur. Standards 26,
. 405 (1941). (3) ENG.CHEM.. , , Robinson. R. J.. IND. ASAL. ED. 1 3 , 4 6 5 (1941). (4) Snell, F. D., Snell, C. T., “Colorimetric Methods of A4nalysis,” 3rd ed., Vol. 11, p. 630-81, Van Sostrand, New York, 1949. (5) Willard, H. H., Center, E. J., ISD. ENG. CIIEM., ANAL. ED. 13, 81 11941). (6) Woods, ’J. R., Mellon, AI. G., Ibid., 13, 760 (1941). RECEIVED for review November 2, 1957. Accepted February 12, 1958.
Specific Detection of Nitrogen through Pyrolytic Oxidation in Organic Spot Test Analysis FRITZ FEIGL and JEFFERSON R. AMARAL laboraforio da ProduFclo Mineral, Minisferio da Agriculfura, Rio de Janeiro, Brazil Translated by RALPH E. OESPER, Universify of Cincinnafi, Cincinnati, Ohio
Nitrogenous inorganic or organic compounds ignited with manganese dioxide or manganic oxide yield nitrous acid which i s readily detected in the gas phase b y the familiar Griess reaction. The chemistry of this pyrolytic oxidation i s discussed. Trials with about 150 compounds demonstrated that nitrogen in all types of materials can be thus oxidized to nitrous acid. A test for nitrogen in nonvolatile organic compounds based on this finding can be completed within 1 to 2 minutes b y the spot test technique; the limits of detection are 0.02 to 0.03 y. Volatile organic compounds can be tested if suitable precautions are used. Several applications in the field of material testing are given.
M
OST MANUALS recommend that
the test for nitrogen in organic materials be made by the Lassaigne procedure, which has been in use for more than a century (8). It is based on the formation of alkali cyanide by fusion with an alkali metal and subsequent production of Prussian blue. The partial reactions leading to cyanide have recently been discussed by Kainz and his collaborators (6, 7). The procedure itself is open to a number of objections, which have been pointed out by Cheronis and Entrikin ( 2 ) . The handling and heating of metallic sodium are dangerous, especially for beginners. Certain compounds volatilize, sublime, or undergo pyrolytic decomposition prior 1148 *
ANALYTICAL CHEMISTRY
to or during the alkali fusion-e.g., hydrazo and amino compounds give off ammonia. Some materials-e.g., azo and diazo compounds-lose their nitrogen when fused with sodium, while some materials (pyrrole derivatives, proteins, and polynitro compounds) react sluggishly, so that the results are not satisfactory. Various modifications of the Lassaigne test have been proposed to avoid these difficulties (1, 4, 7,9,12-14), but their general reliability and applicability are still open to question and require much more actual testing, especially on the micro scale. The socalled lime test (formation of ammonia by heating nitrogenous organic materials with quicklime) is not universally satisfactory. Consequently, there is a real need for a safe and reliable test for nitrogen in organic materials, which can be rapidly accomplished even with a small sample. Experiments were aimed at making wider use of methods for detecting gaseous pyrolysis products as a part of the preliminary examination of organic materials in spot test analysis (3). With the aid of the Griess nitrite reaction (red color on reaction with an acetic acid solution of sulfanilic acid and l-naphthylamine) it was proved that compounds possessing a group containing nitrogen and oxygen invariably yield nitrous acid, nitrogen trioxide, or nitrogen tetroxide on dry heating. This finding led directly to a rather sensitive preliminary test (detection limit 0.2 to
20 y) for nitro and nitroso Compounds, oximes, hydroxamic acids, and amine oxides (6). Nitrous acid and the two oxides result likewise from other classes of nitrogenous organic compounds, if they are heated along with an excess of manganese dioxide. It cannot be stated with certainty n-hich of the partial processes are responsible for the formation of nitrous acid during the pyrolysis of organic compounds in the presence of manganese dioxide. Undoubtedly oxidation is involved, because the manganese dioxide may be replaced by lead dioxide, red lead, cobaltic oxide, nickel sesquioxide, or manganic oxide. Because the cupric oxide is ineffective, it seems likely that the atomic oxygen released by the higher oxides on heating promotes the pyrolytic opening of the carbon frameworks by the formation of carbon dioxide and acidic nitrogen oxides. The latter can be fixed by basic heavy metal oxides through formation of nitrates or nitrites, which in turn decompose in known fashion to give metal oxide and nitrogen tetroxide. The assuniption of the intermediate formation of nitrate or nitrite is buttressed by the finding that heating of these organic nitrogenous compounds along nith acidic reducible metal oxides-e.g., chromium trioxide, tungsten trioxide, and vanadium pentoxideyields no or only a little nitrous acid. Furthermore, the compounds cited above yield nitrous acid in greatly in-
