Borohydride-Flame Determination of Arsenic in Phosphoric Irving May* and
L. Paul Greenland
U.S. Geological Survey, Reston, Virginia 22092
T h e requirement for t h e arsenic content of ACS grade of phosphoric acid is limited to not more than 1ppm As ( I ) . The ACS test method is a photometric silver diethyldithiocarbamate procedure. Difficulties with the test have been reported (2) because of interference by larger concentrations of antimony frequently present in phosphoric acid. We undertook t o test a n alternative procedure for determining arsenic in phosphoric acid which would avoid the antimony interference. This was accomplished with a procedure for evolving arsenic using sodium borohydride and determining the arsenic by atomic absorption spectrophotometry using the nitrogen-hydrogen flame. This procedure is a variation of procedures used by others (3, 4 ) and also in this laboratory for geochemical samples. T h e work presented here is the first application of a borchydride-atomic absorption method to the determination of arsenic in phosphoric acid. This procedure is more rapid than the silver diethyldithiocarbamate procedure currently used in t h e ACS test method ( I ) .
EXPERIMENTAL Atomic Absorption Instrument. A Varian Model 1200 was used for this study. The peak-reading mode was employed without background correction. The optimum instrument settings are: Wavelength, 197.2 nm; spectral band pass, 1 nm; current, 7 mA; H2-7(approximately 11 L/min); N2-6 (approximately 10 L/min); burner height, set for maximum sensitivity. Hydride Generating Apparatus. The reaction flask (Figure 1)consists of a two-necked 100-mL flask connected to a distillation trap and then to a gas inlet adapter. The outlet tube of the adapter is connected to the burner inlet with a short length of Tygon tubing. Reagents. Sodium borohydride, freshly prepared 5% aqueous solution. Dissolve 2 pellets of sodium hydroxide per 100 mL. Potassium iodide, 10% aqueous solution. Hydrochloric acid, 6N. Standard solution: Dissolve sufficient As203 in a NaOH solution to give a 1000 ppm As solution. Just before use, dilute 2.0 mL of the standard to 100 mL with 6 N HC1 to prepare a standard solution. Procedure. Pipet 1.0 mL (1.67 g) of a phosphoric acid sample or 0.01-0.06 mL of the standard solution into the reaction flask. Add 20 mL of 6 N hydrochloric acid and swirl the contents of the flask to ensure complete mixing. Add 0.5 mL of the iodide solution and mix. Insert the glass distillation trap and join the assembly with the gas inlet unit. Fill a plastic-tripped plunger-type pipet such as an Oxford pipet with 5 mL of the sodium borohydride solution. Insert the pipet into the 10/30 joint of the reaction flask so as to make a gas-tight seal. Observe absorbance readings using the "A" damping control until the readings stabilize at a minimum value. Set the zero, depress the "peak" and "read'' buttons. Deliver the borohydride solution by rapidly depressing the plunger (vigorous evolution of hydrogen produces thorough mixing). Record the peak readings. Repeat for each of the standard solutions. Plot the absorbances vs. the weights of As taken for the standards. Determine the As contents of the sample taken from the curve.
RESULTS AND DISCUSSION T h e 197.2-nm line was selected because of its adequate sensitivity but greater freedom from noise compared to the more sensitive 193.7-nm line. The hydride evolution apparatus used here was assembled from standard glassware. An advantage of this generator is t h e reduction of spray entering t h e burner compared with other generators. Results comparable t o ours should be obtainable with other hydride generating apparatus and with other instruments. There is a significant difference in sensitivity with most hydride methods between As(1II) and As(V), 40% greater for 2376
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
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b Figure 1. Hydride Generating Apparatus. (1) Nitrogen inlet. (2) Spray trap. (3) 100-mL generating flask. (4) Side-arm inlet for pipet
Table I. Standard Addition D a t a
As, PPm 0 0.2 0.4 0.8
1.2
Absorbance 0.012 0.065 0.117 0.191 0.236
0.010
0.010a
0.055 0.103 0.197 0.269
0.059' 0.083a 0.193a 0.254'
' Added as As(V). Table 11. Test of ACS Reagent Grade Phosphoric Acids As found, Recovery of 1 ppm spike, Sample PPm % Mfr. 1 lot 1
