Spectrophotometric Determination of Hypophosphite in the Presence of Phosphates
r-l-7
-’-I
0.7
2 z 1.0-
=1 m
d
H PO’. 2.5 N HCL 0.0 L
c \ 2
500
600 WAVELENGTH, mp
300
Figure 1 .
,
700
800
Molybdenum blue complex of hypophosphite
Conc., 2 mg./25 ml.; 1-cm. cell; Cary Model 1 1
SIR: According to the colorimetric method based on the development of the molybdenum blue complex in the presence of sulfurous acid, hypophosphite cannot be determined directly in the presence of phosphates (1). However, by modifying the initial method and adjusting the total acid normality, in this case HCl, hypophosphite can be determined without interference from phosphates. The shift in absorbance maximum of the molybdenum blue complex with respect to acid normality is illustrated in Figure 1. EXPERIMENTAL
A Beckman Model DU spectrophotometer equipped with 1cm. Corex absorption cells was used for all photometric measurements. Wavelength scans were obtained with a Cary Model 11 MS spectrophotometer. Reagents. T h e ammonium molybdate (NH4)6L101024~4H20 was prepared by dissolving 10 grams in 200 ml. of distilled water, to which 60 ml. of concentrated HC1 were added and the Apparatus.
Table I. Effect of Oxyphosphorus Compounds on Hypophosphite Determination Determination of hypophosphite in presence of 5 mg. of interfering oxyphosphorus
Interfering compound
anion Hypophosphite analysis hlilligrams Rel. Added Found error, 70 2.00 2.00 2.00 2.00 2.00 2.00
1422
2.08 1.98 1.95 2.00 2.10 2.05
ANALYTICAL CHEMISTRY
+4.0 -1.0 -2.5 0.0 +5.0 +2.5
solution was diluted to 1 liter with distilled water. A sodium sulfite solution containing 20 grams in 100 ml. of distilled water was also prepared. Hydrochloric acid ( 6 N ) was used to adjust the acidity of the color complex to the proper normality. The various oxyphosphorous compounds used in this study were purchased as sodium salts from Fisher Scientific Co. Procedure. Transfer a n aqueous aliquot of t h e hypophosphite-containing sample (not exceeding 5 ml. total volume and 5 mg. of hypophosphite) into a 25-ml. volumetric flask. Add 10 ml. of 6 N HCl, 5 ml. of the molybdate reagent, and 5 ml. of the sodium sulfite solution. Record t h e absorbance after 30 minutes a t 400 mp in a 1-cm. cell against a reagent blank. The phosphate content in a second aliquot of sample can be determined by excluding the 10 ml. of 6N HC1 and by measuring the absorbance a t 670 mp after 30 minutes. A calibration curve for the hypophosphite is constructed by plotting the absorbance at 400 mp within a concentration range of 1 to 5 mg. per 25 ml. of total volume. A similar curve can be constructed for the phosphate analysis (Figure 2). Effect of p H . According t o d a t a plotted in Figure 3, t h e hypophosphite will absorb at 400 mp a t all normalities, with an absorption maximum occurring at 670 mp at 0.36N. Phosphates, as typified by the response of H2P0?-, will not form any complex a t normalities greater than 1.6. The normality calculated is based on the total acid present, including the contributions from the reagent in the final analytical volume (25 ml.). Interference. No interference was encountered with phosphates, at a concentration 2.5 times t h a t of t h e hypophosphite (Table I). Phosphates in excess of 5 times the hypophosphite level will contribute interfering background absorbance at 400 mp.
0
1
2
3
4
5
m 9.125 ml.
Figure 2. Concentration curve for H2P02- and HzP04Cell, 1 cm.; references, reagent blank; Beckman Model DU HzPOz- absorbance recorded after 30 minuter, 2.5N HCI HzPOa- absorbance recorded after 30 minutes, 0.1 4N HCI DISCUSSION
This technique is applicable to the analysis of hypophosphite without any interference from phosphate and pyrophosphate. The method B is +0.06 at 2.4 mg./25 ml. hypophosphite in the presence of 8 mg./25 ml. of phosphate The hypophosphite(Table 11). molybdate complex develops only upon the addition of sodium sulfite, which in contact with acid produces the required sulfurous acid (1). Phosphates at the lower normalities developed a yellow color prior to the addition of sodium sulfite. -
Table II. Precision of Hypophosphite Method in Presence of Hypophosphate
Hypophosphate conc. added, 8 mg./25 ml. Hypophosphite conc. added, 2.4 mg./25 ml.
Sample
H2POZ- found, mg.
1 2 3 4 5 6 7 8 9 10
2.36 2.36 2.29 2.39 2.41 2.41 2.45 2.46 2.47 2.29
Av. ==9 Std. dev. = f 0 . 0 6
absorbance and the total absorbance can be related to the phosphate content graphically or by the use of the absorptivity of the known phosphate. For the concentration of phosphate studied, the procedure described does not require adjustment to 0.36N normality. However, solutions containing
