Indirect Ultraviolet Spectrophotometric Determination of Phosphorus C.' H. LUECK and D. F. BOLTZ Wayne State University, Detroit, Mich.
b A new spectrophotometric method for traces of orthophosphate has been developed based on the ultraviolet absorptivity of the decomposition products of molybdophosphoric acid. After extraction of the acid with an etherisobutyl alcohol solution, a retrograde extraction with a basic buffer solution results in the decomposition of the molybdophosphoric acid and the transfer of the molybdate to the aqueous phase. Arsenate, germanate, and silicate ions seriously interfere. The optimum concentration range i s 0.1 to 0.5 p.p.m. of phosphorus when spectrophotometric measurements are made at 230 mp using 1 -cm. cells.
0
ions condense with molybdates in acidic solution forming molybdophosphoric acid. This heteropoly acid is stable in acidic or neutral solutions, However, in basic solutions heteropoly acids undergo hydrolysis (3). Berenblum and Chain (I) report that molybdophosphoric acid is almost completely dissociated in alkaline solutions. Wu ( 5 ) destroyed the yellow color due to the acid by making the solution basic with a saturated sodium carbonate solution. He applied this decolorization technique in detecting small amounts of the reduced molybdophosphoric acid (heteropoly blue) in the presence of large amounts of the yellow molybdophosphoric acid; the sodium carbonate solution transforms the unreduced acid into a colorless salt. This study was undertaken to develop an indirect method for determining phosphorus based on the spectrophotometric determination of the molybdenum in molybdophosphoric acid. The three essential steps in the resulting procedure are as follows: extraction of the yellow molybdophosphoric acid with an isobutyl alcohol-ether mixture, a retrograde extraction with a basic buffer solution which results in the decomposition products being transferred to the aqueous phase, and measurement of the absorbance of the basic molybdate solution a t a wave length of 230 mp. RTHOPHOSPHATE
EXPERIMENTAL ~PPARATUS.
The absorbance meas-
urements were made with a Beckman aqueous ammonium hydroxide-amModel DU spectrophotometer and a monium chloride buffered solution. Warren Spectracord using 1.000-cm. The decomposition products go into quartz cells. the aqueous buffer phase, and the Solutions. STANDARDPHOSPHATE absorbance measurements are made SOLUTION.Potassium dihydrogen phoson this aqueous phase. phate (2.198 grams) was dissolved in The extractant should effectively distilled water and diluted to 1 liter. extract the molybdophosphoric acid, A 20-ml. aliquot of this solution was extract none or only slight amounts of diluted to 1 liter with distilled water. This solution contained 0.01 mg. of the excess molybdate reagent, and phosphorus per milliliter. impart a small ultraviolet absorbance STANDARD MOLYBDATESOLUTION. to the buffer solution with which it Reagent grade sodium molybdate dicomes in contact. A mixture of diethyl hydrate, Na&IoO4.2HzO (0.252 gram) ether and isobutyl alcohol having a was dissolved in 1 liter of distilled water ratio of 5 to 1 was selected as a suitable containing 5 ml. of 720/, perchloric acid. extractant. Based on visual observaOne milliliter of this solution contained tion with concentrated molybdophos0.1 mg. of molybdenum. SODIUM MOLYBDATE. Sodium molybdate dihydrate, NazMo04.2H20 (25 grams) was dissolved in distilled water IO01 I and the solution was diluted to 250 ml. The solution had to be clear. BUFFERSOLUTION (1M in ammonium chloride and 1M in ammonium hydroxide). Ammonium chloride (53.5 grams) and concentrated ammonium hydroxide (70 ml.) were dissolved and diluted to 1 liter with distilled water. The isobutyl alcohol used for extraction was practical grade and the 95% diethyl ether was a purified grade. All other chemicals were reagent grade. The aqueous solutions were stored in polyethylene bottles to prevent contamination by silica. Ultraviolet Absorption Spectra for Molybdate. Molybdate solutions ex-
hibit general absorption in the ultraviolet region of the spectrum. Figure 1 shows the ultraviolet absorption spectra for sodium molybdate in an aqueous solution which is 0.23N in perchloric acid (curve 5 ) and in an aqueous ammonium hydroxide-ammonium chloride buffered solution having a pH of approximately 9 (curve 6). The absorption spectra differ in the tlvo media, because of the different polymeric forms of molybdate. However, in each case the solutions are stable, and conformity to Beer's law was observed at 230 mp for both solutions. Extractant. Extraction is a convenient means for isolating molybdophosphoric acid, separating it from excess molybdate in solution. After extraction of the acid into the organic phase, it could be decomposed readily by shaking the organic layer with an
220
230
240
250
W A V E LENGTH, rn p
Figure 1. Absorption spectra of molybdate solutions 1. Reagent blank solution for molybdophoEphoric acid buffer extraction method vs. water
2. Buffer extract of molybdophosphoric . blank acid (0.1 p.p.m. of P) ~ J S reagent
fiolution
3. Buffer extract of molybdophosphoric acid (0.3 . p.p.m. of P) os. reagent blank .
solution
4. Buffer extract of molybdophosphoric acid (0.G p.p.m. of P) us. reagent solutinn
5 . Sodium molybdate solution (15 p.p.m. of 330) in 0.23N perchloric acid solu-
tion us. corresponding reagent blank solution 6. Sodium molybdate (15 p.p.m. of Mo) in buffer solution of pH 9 os. correfiponding reagent blank solution VOL. 30, NO. 2, FEBRUARY 1958
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phoric acid solutions, mixtures of isobutyl alcohol and ether are more efficient extractants than either solvent alone. The minimum amount of isobutyl alcohol was used because its ultraviolet cutoff is a t a longer wave length than 230 mp. This solvent mixture gives a very rapid and clean separation of layers. Procedure for Study of Solution Variables. A definite amount of standard phosphate solution was transferred to a conical flask and 5 ml. of perchloric acid were added. The volume !vas adjusted to approximately 40 ml. with distilled Tvater; in the case of diverse ion study, the ions were added before dilution. Five milliliters of the sodium molybdate reagent were added and the volume was adjusted to 50 ml. ivith distilled water. After the solution had been mixed and allowed to stand 5 to 10 minutes, it was transferred to a separatory funnel and extracted with an ether-isobutyl alcohol mixture. The nonaqueous phase was washed twice with perchloric acid. The molybdophosphoric acid complex was decomposed by shaking the organic phase with an aqueous ammonium hydroxide-ammonium chloride buffer solution. I n this retrograde extraction, the molybdate goes into the aqueous layer. The lower aqueous layer \vas drained off and diluted, and the absorbance was measured a t 230 m,u using water in the reference cell. All of the excess molybdate adhering to the separatory funnel had to be removed, as the slightest amount remaining would cause an error. EFFECT OF SOLUTION VARIABLES
Phosphorus Concentration. Figure 1 (curves 2, 3, and 4) shows the absorption spectrum for a solution containing the decomposition products of molybdophosphoric acid. Conformity to Beer's law was observed for 0.05 to 0.6 pap.m.of phosphorus when absorbance measurements were made a t 230 mp. Acid Concentration. The effect of acidity was studied using 0.2 p.p.m. of phosphorusin0.7,0.9, 1.2, and 1.6.V perchloric acid. A variation in acidity from 0.9 to 1.6N in perchloric acid does not affect the ultimate intensity as shown by the fact that absorbance values a t 230 mp of 0.479, 0.427, 0.429, and 0.424, respectively, were obtained. An intermediate acidity of 1.2N was used because the absorbance of the reagent blank increases a t loner acidities. Molybdate Concentration. A relatively high concentration of molybdate was used t o ensure the formation of molybdophosphoric acid. Using 0.2 p.p.m. of phosphorus and 4, 5, and 6 ml. of the 10% sodium molybdate reagent per 50 ml. of solution, absorbance values of 0.420, 0.423, and 0.435 lvere obtained. (Distilled water was 184 *
ANALYTICAL CHEMISTRY
used as reference.) The absorbance of the blank increases slightly when larger amounts of molybdate reagent are used. This accounts for the increased absorbance n-hen 6 ml. of molybdate solution per 50 mi. were used. Five milliliters of 10% sodium molybdate in a final volume of 50 ml. were chosen for the procedure. Temperature. The effect of the initial temperature upon the ultimate absorbance was determined using 0.2 p.p.m. of phosphorus. The solutions were brought to the desired temperature and extracted 5 minutes after the addition of the molybdate reagent. Absorbance values of 0.419, 0.426, and 0.431 were obtained, corresponding to temperatures of lo", 24', and 40' C. Thus, a slight increase in absorbance, +3%, mas observed vhen the temperature was increased from 10' to 40" C. Extractant. B considerable quantity of the extractant dissolves in the aqueous phase during the extraction process. Forty milliliters of the extractant was a convenient volume, and it was sufficient t o permit this solubility loss and still quantitatively extract the molybdophosphoric acid. In checking the efficiency of the extraction it was found that with 0.2 p.p.m. of phosphorus two extractions with 25-ml. portions of extractant gave an absorbance of 0.428 as compared with 0.427 when one extraction with a 50-ml. portion of the extractant was performed. Stability. The final aqueous extracts were stable for a t least 6 weeks when stored in glass-stoppered flasks. A solution containing 0.2 p.p.m. of phosphorus gave absorbance values of 0.420 and 0.418 a t elapsed times of 5 minutes and 2 n-eeks. ilnother solution containing 0.2 p.p.m. of phosphorus gave absorbance values of 0.426 and 0.428 a t elapsed times of 5 minutes and 6 weeks. Diverse Ions. The effect of various diverse ions was studied using 0.2 p.p.m. of phosphorus. Errors of less than 2.5% of the phosphorus present were considered negligible. The anions tested were added as sodium, potassium, or ammonium salts, and the cations as perchlorates, chlorides, or sulfates. No interference was observed \Then 500 p.p.m. of the following ions mere present: acetate, aluminum, ammonium, bromide, calcium, chloride, chromium(III), cobalt, copper(II), dichromate, fluoride, iron(III), lead, manganese(II), molybdate, nickel, oxalate, perchlorate, permanganate, silver, sulfate, vanadate, and zinc. The permissible amounts of nitrate and tungstate ions mere 200 and 20 p.p.m., respectively. Arsenate, arsenite, germanate, nitrite, and silicate ions must
be absent within the ordinary limits of purified reagents. RECOMMENDED PROCEDURE
Treat a suitable quantity of the sample containing up to 0.065 mg. of phosphorus in such a manner that the phosphorus is present finally as orthophosphate. (The concentration of other ions should be within tolerance limits.) Add 5 ml. of 72% perchloric acid to this sample solution. The final acidity on dilution to 50 ml. should be between 0.9 to 1.6N; 5 ml. of perchloric acid per 50 ml. gives an acidity of approximately 1.2N. Dilute the solution to approximately 45 ml. and add 5 ml. of 10% sodium molybdate reagent. The final volume should be 50 ml. f 5 ml. Mix and allow to stand 5 to 10 minutes. Transfer the solution to a 125-ml. separatory funnel. Extract with 40 ml. of an ether-isobutyl alcohol mixture (5 l), shaking the funnel 30 to 60 seconds. Allow the layers to separate and discard the lower aqueous layer. Swirl the funnel to collect the water droplets into one globule and discard. Wash twice with 25-ml. portions of dilute perchloric acid solution (1 to lo), shaking the funnel 15 to 30 seconds in each case. Swirl the funnel after each washing and discard the remaining aqueous phase. Wash the funnel tip with a jet of distilled water to remove any remaining traces of excess molybdate. Add 30 ml. of the ammonium hydroxide-ammonium chloride buffer solution. Shake the funnel 15 to 30 seconds and drain the lower buffer phase into a 100-ml. volumetric flask. Add another 15-ml. portion of buffer solution to the funnel and again shake 15 t o 30 seconds. Drain 6he buffer phase into the 100-ml. volumetric flask containing the previous portions of buffer. Swirl the funnel to collect the remaining buffer phase. Dilute the solution to the mark with distilled water and measure the absorbance a t 230 mp in a 1-cm. cell using a reagent blank in the matched reference cell. Refer the reading to a standard calibration curve obtained using standard phosphate solutions.
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SUMMARY
This method is more sensitive than any of the previous heteropoly methods for orthophosphate. The molar absorptivity for phosphorus is 57,400 liters per mole-em. which is more than twice as sensitive as the heteropoly blue procedure of Boltz and Mellon (9) which has a molar absorptivity of 26,800 liters per mole-em. a t 830 mu. The ultraviolet modified molybdophosphoric acid of ST'adelin and Mellon (4) has a molar absorptivity of 24,400 liters per mole-cm. An indication of the precision of this indirect ultraviolet spectrophotometric method was ascertained from the results of seven samples, each con-
taining 0.2 p.p.m. of phosphorus. These samples give a mean absorbancy value of 0.420 .ivith a standard deviation of 0.006, or 1.4%. The solutions are stable for weeks and adjustment Of the is not Exact duplication of technique in ex-
traction and washing of extract is important. LITERATURE CITED
(1) Berenblum, I., Chain, E., Biochem*J . 32, 287 (1938).
(2) Boltz, D. F., Mellon, M. G., ANAL. CHEM.19, 873 (1947).
(3) Huckel, JV., “Structural Chemistry of Inorganic Compounds,” p. 183, Elsevier, New York, 1950. (4) Radelin, C., blellon, M. G., ANAL. CHEM.2 5 , 1668 (1953). (5) WU,H., J . Bd.Chem. 43, 189 (1920). RECEIVED for review May 3, 1956. Accepted October 11, 1957.
Determination of 1,l ’-Ferrocene Dicarboxylic Acid in Presence of Ferrocene Monocarboxylic Acid by Infrared Spectroscopy EUGENE F. WOLFARTH’ Materials laboratory, Wright Air Developmenf Center, Air Research and Developmenf Command, Wright-Patterson Air Force Base, Ohio
A method is described for obtaining quantitative infrared absorption data for highly insoluble materials using potassium bromide disks. The preparation of the disks and treatment of the data is given, along with an example using ferrocene carboxylic acids.
F
E~;~;c,
(dicyclopentadienyl because of the resonance between the iron and the conjugated ring, shows a high degree of aromaticity and consequently, a high degree of stability. I n the course of studying the chemistry of ferrocene compounds, it was of interest to determine the amount of ferrocene dicarboxylic acid in a mixture of both the mono- and dicarboxylic acids. A spectrometric method of analysis \vas desired because of the inherent speed and accuracies of absorption methods, and the small amounts of sample available (usually 3 to 7 mg.). I n a scan of the entire spectral region from 0.2 t o 35 microns, only two bands were found which would be suitable for quantitative measurements. Both were infrared bands associated with the unsubstituted cyclopentadiene ring. The first band, at 9.02 microns, was assigned (1-5) to an antisymmetrical ring breathing vibration and the second band, at 9.98 microns, to a C-H bending vibration. Both bands are very strong in the spectrum of the unsubstituted ferrocene molecule. I n monosubstituted ferrocenes, the two bands are still present but of diminished intensity ( 5 ) . Both bands are completely absent in 1,l’-disubstituted E NC
Present address, Ohio State University, Columbus, Ohio.
ferrocenes ( 4 ) . As can be seen in Figure 1, the ferrocene carboxylic acids follow these generalizations. Because ferrocene dicarboxylic acid is insoluble in the solvents useful in infrared absorption nwasurements, the potassium bromide disk technique was investigated. A method was devised to obtain semiquantitative data. MATERIALS A N D APPARATUS
Both the ferrocene mono- and dicarboxylic acids were prepared in the laboratory. Elemental analysis indicates they are of high purity. The potassium bromide was an infrared spectral grade reagent supplied by Harshaw Chemical Co.; it was dried a t 120’ C. for 2 days prior to USP. The potassium bromide disks, 0.5 inch in diameter, were prepared in a PerkinElmer wmium die. The die \vas pressed in a 20-to11 Carver laborntory press t o approximately 23,000 pounds per square inch. Weighings ryere made on a Sartorius Dial-0-matic analytical balance to an accuracy of 50.05 mg. Absorbance measurements were made on a Perkin-Elmer Model 112 doublepass infrared spectrometer equipped with sodium chloride optics. The instrument was calibrated to AO.01 micron using atmospheric water and carbon dioxide bands and ammonia vapor. The absorbance values were measured directly using the Model 112 densitometer attachment, applying the “sample in, sample out” technique. The values are accurate t o +0.02 absorbance unit. PROCEDURE A N D DISCUSSION O F RESULTS
For each pure acid, a mixture of potassium bromide and the acid was prepared consisting of 0.0111 gram of ferrocene carboxylic acid per gram of pure potassium bromide. The resulting
mixtures were thoroughly ground, mixed in a carbide mortar, and placed in a desiccator. They contained exactly 4.830 X loes mole per gram of ferrocene monocarboxylic acid and 4.054 X mole per gram of ferrocene dicarboxylic acid, respectively. A weight of exactly 0.3000 gram !vas selected as the standard disk size. Varying amounts of the two mistures were combined for a total weight of approximately 0.3 gram to give a series of niixturcs ranging from 0 to 100% ferrocene dicarboxylic acid. These final mixtures n-ere again ground and mixed, the disk was prepared by the usual procedures, and the finished disk weighed. To determine the absorbance value. the Perkin-Elmer Mode1 112 was set at the desirfd m v e length, the slits set a t 0.9 mm., and the gain set at 0.02 pv. full scale. The absorbance value for each disk was then read directly from the densitometer scale. T o refer these absorbance values to a common point, the standard 0.3000gram disk. a weighted absorbance value was calculated as follows: W.A. =
0.3000
disk weight in grams X measured absorbance This procedure can be justified by considering Beer’s law. A = kml where A is the absorbance, k is the extinction coefficient, m is the number of moles of sample, and 1 is the path length. Because the diameter of the disk and the ratio of organic material to potassium bromide (the density) remain constant, the only significant variable is 1. The path length is directly proportional to the amount of material present and, therefore, to the disk weight. I n Figures 2 and 3, these weighted absorbances were plotted against the VOL. 30, NO. 2, FEBRUARY 1958
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