lOy0 aluminum, as long as the amount of titanium in the aliquots does not exceed 20 mg.
(2) Ibid., 23, 580 (1951). (.3 .) Bastian. R.. Weherline. R.. Pallila. F., Ibid.,22, 160 (Ig50). ’ (4)Codell. bl.. Norwitz. G.. Ibid.. 2 5 . \
ACKNOWLEDGMENT
The author n-ishes t o thank Chase Brass & Copper Co. for permission to publish this lvork. LITERATURE CITED
(1) Bastian, R., (1949).
ASAL.CHEX 21, 972
,
143i (1953).
I
,
sorption Spectroscopy,” p. 194, Wiley, S e w York, 1950. Watertown Arsenal, Information Bull. KO.T8, Pt. 1, p. 43, Watertown Arsenal, Watertown, Mass., 1955. RECEIVED for revieff June 11, 1956. Accepted October 11, 1956. Division of Analytical Chemistry, 130th meeting, ACS, Atlantic City, 5 . J., Septem!xr 1956.
Photometric Determination of Phosphorus as Molybdovanadophosphoric Acid O D D B. MICHELSEN Norwegian Defence Research Esfablishmenf, Kjeller, Norway
b A modified molybdovanadophosphoric acid method for the determination of small amounts of phosphorus has been developed, using a spectrophotometer at 3 15 mp, and employing very dilute reagents. The method allows the determination of phosphorusi.e., phosphate-in concentrations as low as 0.1 y of phosphorus per ml.
I
considerable interest has been aroused concerning the molybdovanadophosphoric acid method, originally introduced by Misson ( 7 ) in 1908 for the determination of small amounts of phosphorus (phosphoric acid). Several papers (1-6, 8) have dealt with this subject, but there is still room for certain refinements, both with regard to the concentration of reagents and choice of wave lengths for the spectrophotometric determination. I n the course of other work, the analysis of very dilute phosphoric acid solutions became necessary. The search for sensitive and reliable methods for this purpose left no doubt that the procedures involving the molybdovanadophosphoric acid complex were by far the most satisfactory. Horrever, observations were made which gave rise to a modified procedure, considerably more sensitive than the previously known methods. A satisfactory absorption spectrum of a metavanadate-molybdate-phosphoric acid mixture has not yet been published, and an attempt to establish the absorption maximum of such a mixture proved unsuccessful, owing to N RECEKT YEARS,
60
ANALYTICAL CHEMISTRY
the strong absorption of the blank. The concentrations used \yere well within the limits recommended by Quinlan and De Sesa (S), except for the strong acid, which was 1.00N instead of 0.4iv. From previous data (3, 8 ) , a n absorption maximum at about 400 mp had been expected; the measurements, however, gave a n absorption curve more in accordance with that published by Gee and Deitz (4, Figure l ) , although the concentrations cited above did not allow reliable measurements below 355 mp. This observation at first led to a n analytical procedure operating at the lowest possible wave length, a procedure which has so far proved rather satisfactory. Hoxerer, after making an absorptionietric study of each reagent component separately, it \\-as decided to dilute the reagents radically. This made i t possible to extend the absorption spectrum of the mixture further into the ultraviolet, revealing a n absorption maximum a t 315 mp. This observation constitutes the basis of the present modified molybdovanadophosphoric acid method.
of (SH4)6110iOn4.4H20 in warm wn ter (50” C.). After cooling, make up to 1 liter. hlived reagent. Mix 1 volume of the metavanadate solution with 2 volumes of 2.5N hydrochloric acid. Finally add 2 volumes of the molybdate solution. Diluted mixed reagent. Dilute the mixed reagent described above 25 times with distilled mater. PROCEDURE
Mix an aliquot of the assay solution (phosphorus pentoxide dissolved in 0.1N hydrochloric acid) containing 0.2 to 3 y per ml., with an equal volume of the diluted mixed reagent. Wait for 2 minutes. Read the absorbance a t 315 mp, As reference a solution is employed containing diluted mixed reagent plus an equal volume of 0 . 1 s hydrochloric acid. The concentration of phosphorus pentoxide is found by means of a calibration curve prepared by the above procedure of known amounts of phosphorus pentoxide. All measurements to be compared should he made at the same temperature. EXPERIMENTAL A N D DISCUSSION
APPARATUS A N D REAGENTS
All measurements were made with a Model DU Beckman spectrophotometer, using 1-cm. quartz cells. Solutions: (a) 2.5N hydrochloric acid; (b) 0.234% ammonium metavanadate (w./v.), dissolve 2.34 grams of ammonium metavanadate in 500 ml. of hot water, add 28 ml. of concentrated hydrochloric acid, and after cooling dilute to 1 liter; ( c ) 3.53% ammomium molybdate (TV./V.), dissolve 35.3 grams
Absorption of the Molybdovanadophosphoric Acid Complex and Optimum Wave Length. The previous
modifications of t h e molybdovanadophosphoric acid method recommend t h a t t h e photometric measurements be carried out in t h e range 400 to 470 mp; the longer wave length is desirable possibly to reduce the influence of certain ions (particularly iron). This actually means operating on the slope
(4)suggests that the ratio of vanadiuni to phosphorus in the complex is 1 to 1, and that of molybdenum t o phosphorus, a t least 14 t o 1. If this is true, it means that a n appreciable proportion of the reagent vanadium and molybdenum is bound as components of the complex, and that the composition of the blank is no longer representative. The resultant absorption will still obey Beer’s law if the absorptions of all components involved do so, and if the complex is sufficiently strong to prevent any appreciable displacement of the equilibrium, The shape of the resultant absorption curve may, however, be somewhat different from that of the complex. As this does not affect the analytical applicability of the observations made, it has not been given further consideration. I n the present case Beer’s law is obeyed nithin the range 0 to 2.5 y of phosphorous per ml. (Figure 2). The limit 2.5 y of phosphorus per ml. corresponds to a proportion of phosphorus to vanadium of 1 to 1. The influence of further addition of phosphorus upon the absorbance was measured a t three different wave lengths; 400, 350, and 315 mp, B t 400 nip practically no change was noted, confirming the observation by Gee and Deitz (4), regarding the proportion of phospholus t o vanadium. At 360 and 315 mpq a certain increase was observed which was more pronounced a t the lower wave length. However, this may be simply explained by the fact that the still existing excess of molybdate will cause the formation of phosphomolybdate, the absorption of which adds to that of molybdoranadophosphoric acid complex Concentration of Reagents. The
h
Wovelength,
m/c
Figure 1. Absorption spectra of molybdovanadophosphoric acid complex and blank (diluted reagent) 1. 2. 3.
Molybdovanadophosphoric acid compared with water ( 1 yP/ml.) Blank compared with water Molybdovanadophosphoric acid compared with blank
of the absorption curve (Figure 1). The use of more dilute reagents makes it possible to vork on the absorption inaximuni, n-hich has the advantage of higher sensitivity, and usually a higher degree of accuracy. Whenever the disturbing effects of foreign substances are absent, or can easily be eliminated, the neiv procedure should therefore be preferable. Figure 1 shows the absorption spectra of a phosphoric acid-metavanadatemolyhdate mixture and the corresponding blank; in both cases distilled watei is used as a reference. The concentration of phosphorus is 1 y per nil. (calculated as phosphorus) ; the solutions are made according to the above procedure. The third curve represents the absorption of the phosphoric acidmetavanadate-molybdate mixture with the blank as a reference solution. I n order to test the effect of the dilution of the reagent on the absorbance of the molybdovanadophosphoric acid complex, comparative measurements n-ere made with diluted and undiluted mixed reagent on the same concentrations of phosphorus. Significant differences could not be observed in the wave length region above 355 nib, indicating that the dilution process does not seriously affect the formation of the molybdovanadophosphoric acid complex. Unfortunately, the concentrated mixed reagent cannot be used below 355 mp because of its strong
absorption; thus the evaluation of this point is considerably more complicated in this range. The important thing, however, a t least from an analytical point of view, is that Beer’s law is obeyed in the range where the resultant absorption has its maximum (315 mp). However, this maximum is not necessarily the real absorption maximum of the complex itself. The work of Gee and Deitz
/ /
/
b
10-
8P
0.0-
b 2
P
060.4 *
0.2-
/
0:2
0.4
0.6
0.8
1.0
12
l&
16
1.8
2‘0
2.2
2.L
2.6
PHOSPHORUS, y PER ML.
Figure 2.
Calibration curve for determining phosphorus at 3 15 mp VOL. 2 9 , NO. 1, JANUARY 1957
61
concentration of t h e reagents chosen for t h e final procedure mas in this case determined by t h e dilution necessary t o reduce t h e background absorption t o a suitable level and is not claimed to be optimal. Comparatively small variations of the concentration of molybdate (in the range 0.001 to 0.002 M) do not cause any significant changes in the resultant absorbance. At 315 niu the absorbance of a blank measured with water as reference solution amounts to about 0.25 (1-cni. cells). It is likely that further dilution of the reagents is possible without affecting the resultant absorption of the mixture, and thus the background absorption may be even further reduced. Acid and Acid Concentration. Originally nitric acid as used in t h e procedure, but in order to minimize the background absorption, it was later rejected because of t h e absorption band a t 300 mp. As a substitute, sulfuric, perchloric, and hypochloric acids all have been tried without noticeable difference. The choice of hydrochloric acid is arbitrary. The concentration of strong acid does, however, play an important role. and affects both the coniplex formation and the reaction velocity. Complete exclusion of acid immediately causes the mixed solution of metavanadate and molybdate to turn intensely yellow. The shape and position of the absorption curve, after addition of phosphoric acid, show a certain resemblance to the ‘‘normal” curve encountered in the presence of strong acid (and to the absorption spectrum of phosphomolybdate), but is considerably weaker when referred to the same concentration of phosphorus. It is also not constant with time. Subsequent experiments were performed with a mixed reagent of the composition previously given, the contribution of this reagent to final acidity being 0.021N. Variations in the concentration of acid were effected by adjusting the acidity of the assay solu-
0.04
0.06 0,08 0,‘G G.12 0.14 Nonnoiity of strong ac,d
0.16
I
Figure 3. Effect of strong acid on absorption of blank at 315 mp
tion. A final acidity in the range 0.021 to 0.071-V produces maximum absorbance m-ithin 2 minutes. The spectrophotometer reading remains constant for a t least 2 hours, and no dependency of the absorbance upon the acidity could be observed within this range. Higher concentrations of acid interfeie niarkedly with the formation of the complex, and with the reaction velocity. The latter steadily decreases, but the formation of the complex is greatly impaired as acidity increases. Acidities in excess of about 0.3N give rise t o scarcely any complex formation. The choice of final acidity within the favorahle range mentioned may be based on the fact that the absorption of the blank also depends on the acid concentration (Figure 3). The loryest absorption (at 315 nip) is found using about O . O i N acid. For the purpose of the present work however, the concentration 0.07N was selected, because, for other reasons, the assay solution had to be made 0.1N in strong acid. For other purposes one may more conven-
iently incorporate all acid in the mixed reagent. Precision and Accuracy. Some indication of t h e precision of t h e method is given by t h e calibration curve in Figure 2 . The precision and accuracy of t h e method mere further tested on solutions of known phosphorus concentrations, Solutions calculated t o give final concentrations of 0.1 and 1.0 y of phosphorus per nil., respectively, were made by dissolving potassium dihydrogen phosphate in O.1N hydrochloric acid and a series of determinations was carried out according to the described procedure. The results recorded in Table I indicate an accuracy within +370 when working at the ultimate lower limit of the method, and better than = t l %a t a concentration of 1 y of phosphorus per ml. ACKNOWLEDGMENT
Thanks are expressed to the Norwegian Defence Research Establishment for the permission to publish this r o r k , and to Unn Sollid for valuable assistance in obtaining the data. LITERATURE CITED
Table I.
Estimation of Precision and Accuracy
Concn. Sample 1
2
3
4
5 6
7
8 9
10 11
12
62
of P,
y
per 1111. 0.100 0.100 0.100 0.100 0.100 0.100 1.000
1.000 1.000 1,000 1.000
1,000
ANALYTICAL CHEMISTRY
Spectrophotometer Reading 0.065 0.064 0.065
0,067 0.068
0.065 0.662 0.667 0.663 0,670 0.667 0.667
(1) Anderson, B., Wright, W. B., Jr.,
Concn. of P Found, y per M1.
Error,
0,099
-1.0
0.097 0,099 0.102 0.103
% -3.0
-1.0
+2.0 $3.0
0.099
-1.0
1.007
-0.4 +0.3 -0.3 f0.7 f0.3 $0.3
0.996 1.003 0.997 1,003
1.003
U. S. Atomic Energy Commission Y-900 (August 1952). ( 2 ) Baghurst, H, C., Norman;-V, I., ANAL.CHEM.27, 1070 (19as). (3) Barton, C. J., Zbid., 20, 1068 (1948). ( 4 ) Gee, A., Deitz, IT. R., Ibid., 2 5 , 1320’ 11953). --, \ -
( 5 ) Gericke, S., Kurmies, B., 2. anal. Chem. 137, 15 (1952). (6) Kitson, R. E., Mellon, M. G., IXD. EKG.CHEM.,ANAL. ED. 16, 379,
(1944).
(T) Misson, G., Chem.-Ztg. 32, 633 (1908).
(8) Quinlan, K. P., De Sesa, A. M., A 1 . 4 ~ . CHEM. 27, 1626 (1955).
RECEIVED for review April 6, 1956. Accepted -4ugust 30, 1956.
