1734
Anal. Chem. 1984, 56, 1734-1737
Registry No. Morphine, 57-27-2; heroin, 561-27-3. LITERATURE C I T E D (1) Clarke, E. G. C. “Isolatlon and Identiflcatlon of Drugs”; Pharmaceutical Press: London, 1969; p 292. (2) Fishbein, L. “Chromatography of Environmental Hazards“; Elsevier: New York, 1982; Vol. IV, p 209. (3) Manura, J. J.; Chao, J. M.; Safertein, R. J. forensic Sci. 1978, 23, 44-56. (4) Bertulll, G.; Mosca, L.; Pedroni, G. Boll. Chim. farm. 1978, 777,
..-
1 7 n ..-. -i~
(5) Taisky, G.; Mayring, L.; Kreuzer, H. Angew. Chem., Int. Ed. €rig/. 1978 77, 785-799.
Feii, A. F. Trends Anel. Cbem. 1983, 2 (No. 3), 63-66. Kltamura, K.; Mallma. R. And. Chem. 1983, 55, 54-56. Lawrence, A. H.; MacNeii, J. D. Anal. Cbem. 1982, 54, 2365-2387. Gill, R.; Bal, T. S.; Moffat, A. C. J . forenslc Sci. SOC. 1982, 22, 165-1 7 1. (10) Grasselli, J. G. “Atlas of Spectral Data and Physical Constants for Organic Compounds”; CRC Press: Cleveland, OH, 1973; pp B-660-B661. (11) Lawrence, A. H. Trends Anal. Cbem. 1983, 2 (No. 12), V-IX, (6) (7) (8) (9)
RECEIVED for review January 27, 1984. Accepted March 26, 1984.
Extraction-Spectrophotometric Determination of Trace Phosphorus in Chromium-Bearing Materials Which May Contain Silica, Nloblum, Tantalum, Zirconium, Titanium, and Hafnium Vladimir J. Zatka* a n d Nelson Zelding
J.Roy Gordon Research Laboratory, INCO, Ltd., Sheridan Park, Mississauga, Ontario, Canada L 5 K 129 Methods for the determination of low phosphorus by solvent extraction and spectrophotometry are well publicized (1-4) with potential pitfalls and problems extensively discussed. What is rarely mentioned is the interference by chromium(II1) which is responsible for low phosphorus recoveries. Pakalns (5) noted a slight chromium effect when he analyzed nickel alloys. Shelton (6) reported on a serious chromium interference in the determination of phosphorus in chromites by the vanadomolybdophosphoric acid method (7). He separated P from Cr by collection in ferric hydroxide, a technique also recommended for umpire analysis of chromium ores (8). Neither approach was found satisfactory. Particularly in Shelton’s procedure, large variable quantities of Cr(II1) were left tied up in the ferric hydroxide precipitate. This residual chromium had to be carried through an oxidation/reduction step (9) and required a special “low phosphorus” chromium ore to be used for calibration. Nothing demonstrates the problems associated with trace phosphorus determination in chromium-bearing materials more clearly than a wide spread of results reported in a certification program for analytical standard reference materials (SRM). Participating laboratories usually give preference to spectrophotometric techniques often combined with solvent extraction preconcentration steps. The surprisingly poor performance of the long established and widely used methods, such as the V-Mo-phosphoric acid method, reflects in our view the underestimated effects of chromium and, frequently, of silica as well. In addition, the methods are not suitable for measurement of phosphorus in materials containing niobium, tantalum, etc., as in high-temperature nickel alloys. In this paper, a general extraction-spectrophotometric molybdenum blue method is described for the determination of phosphorus in chromium-bearing materials, e.g., chromite ore, complex nickel alloys, stainless and specialty steels, magnetic alloys, etc. Refractory elements and silica do not interfere. T o avoid low recoveries, phosphorus must be extracted from chromium-free solutions. The separation is achieved by precipitating ferric or zirconium phosphate after all chromium has been oxidized to the hexavalent form. Selective fluoride complexation controlled by boric acid is used to eliminate interference by Nb, Ta, Zr, Hf, or Ti. The presence of silica can be tolerated since it is screened as the very stable fluorosilicic acid. The accuracy of the method was tested by analyzing a wide variety of chromium-bearing SRMs, most of which were 0003-2700/84/0356-1734$01.50/0
certified for phosphorus. Duplicate analyses on two different days were performed whenever possible. While an excellent agreement was obtained in most instances (Table I), distinct differences were found in some SRMs, the majority of them issued before 1970. In such cases, results obtained by the proposed method invariably were lower than the certified phosphorus values. Independent evidence (10) makes it appear that the high bias of at least some of the recommended phosphorus values is silica related, i.e., due to incomplete silica removal. The tolerance of the proposed method for silica and refractory elemenb simplifies its application and widens the scope beyond that of chromium-bearing materials. Excellent results have been obtained on phosphorus determination in silicates, refined nickel and cobalt metals, ferronickel, etc. EXPERIMENTAL SECTION Reagents. All reagents used throughout the study were analytical grade, and so were the organic solvents, isobutyl alcohol (3-methyl-1-propanol) and chloroform. The molybdate reagent solution (5% NazMo04.2Hz0in 1M NaOH) was stored in a plastic bottle. Chloride wash solution (0.4 M NaC1,0.6 M HCl) contained 6% v/v of isobutyl alcohol. Reducing solution was made fresh daily by mixing 5 mL of stannous chloride stock solution (20% SnClZ-2H20in 6 M HCl) with 10 mL of 9 M sulfuric acid (50% v/v) and diluting with distilled water to 250 mL. Standard phosphorus solution, 100 mg L-l P, was prepared from KH2P04 in 0.04 M HN03 Eppendorf digital pipets were used to aliquot the standard solution for calibration or when spiking samples. Apparatus. A Beckman Model DU-2 spectrophotometer was used for absorbance measurements. Procedures. Chromite Ore. Fuse 1.00 g of finely ground sample with 5 g of sodium peroxide in a zirconium crucible. Leach the cooled melt with 100 mL of water, acidify with 15 mL of 16 M nitric acid, and heat to clarify the solution. Cool and transfer to a 250-mL volumetric flask and dilute to volume. Take an aliquot containing less than 100 bg P, dilute to 150 mL, add 3 mL of 7% Fe(N03)3.9Hz0solution, and heat to boiling. Add 10 mL of 1.5% AgNOa and 30 mL of 10% (NH&S& and boil for 10-15 min to oxidize all chromium to Cr(V1) and to decompose excess of peroxydisulfate. Dilute to 250 mL and add concentrated ammonia solution until the appearance of a permanent ferric hydroxide precipitate. Add 15 mL of ammonia in excess and digest for 15 min. Filter the precipitate through a fast paper containing some filter pulp, and wash chromate free with 1% NH4NOa solution. Transfer the filter with the precipitate to a plastic beaker, and add 10 mL of 8 M nitric acid and 2 mL of concentrated (48%) hydrofluoric acid. Heat to dissolve the precipitate and to pulp 0 I984 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Table I. Phosphorus in ChromiumBearing SRM’sby the Proposed Method % phosphorus found year of certified day 1 issue SRM NBS 103a, chrome refractory 121c, stainless steel
121d, stainless steel 123c, stainless steel 160a, stainless steel 346, valve steel 348, high-temp alloy 349, waspaloy 897, “tracealloy” A 898, “tracealloy” B 899, “tracealloy” C
1962 1958 1971 1971 1959 1965 1981 1959 1980 1980 1980
0.004 0.028 0.019 0.024 0.027 0.018 0.015 0.002
BCS 31011. nimonic 90 345, IN 100 alloy 365, alcomax 111 371, commercial Ni 384, hycomax I11 387, nimonic 901 474, stainless steel 475, stainless steel
SARM 8, chromite ore 9, chromite ore
1969 1982 1974 1973 1975 1974 1982 1982
0.007 0.008 0.037
1978 1978
0.0039 0.0029
the paper, With use of plastic ware, filter and wash with 30 mL of 4% H3B03solution and finally with water. Make sure the total volume of the filtrate does not exceed 60-70 mL. Transfer the filtrate to a separatory funnel, add 2 mL of 10% tartaric acid, 1mL of 5% sulfamic acid, and 10 mL of molybdate reagent. Mix and extract with 20 mL of isobutyl alcohol by shaking for 2 min. Allow phases to separate and transfer the lower aqueous layer into another separatory funnel. To this phase add an additional 5 mL of molybdate reagent and 10 mL of isobutyl alcohol and repeat the extraction. Discard the aqueous phase and combine the extracts in the first separatory funnel. Wash the combined extracts three times, each time by shaking for 30 s with 20 mL of chloride wash solution. Discard the aqueous washings. To the washed organic extract add, in order, 40 mL of chloroform, 30 mL of distilled water, 10 mL of freshly prepared working reducing solution, and 2 mL of 1%oxalic acid. Without delay, shake for 1min. Discard the colorless heavier organic layer. Transfer the blue aqueous solution to a 100-mL volumetric flask containing 5 mL of 6 M hydrochloric acid. Dilute to volume with water. Let the solution stand for at least 20 min before measuring at 700 nm in 10- or 20-mm cells against water as reference. The blue solution is stable in excess of 20 h. Carry a reagent blank through the entire procedure. Convert the absorbance readings of the sample and the reagent blank solutions into micrograms of P by using the calibration graph. Calculate the P content in the sample as follows: % P = ( A - B)C-l, where A is micrograms of P in sample, E is micrograms of P in reagent blank, and C is grams of sample present in the sample aliquot taken. To calibrate, carry blank solutions apiked with 0, and up to 100 Mg of P, each containing 5 mL of 16 M nitric acid and 3 mL through the oxidation, precipitation, and of 7% Fe(N03)3-9H20, extraction steps. Measure absorbances of the blue solutions against the “zero” solution used as reference to obtain net absorbance for each phosphorus spike. Plot the data to obtain the calibration graph which is linear for the suggested concentration range. Nickel Alloy or Stainless Steel. Dissolve in a small beaker 0.200-0.300 g of sample in 10 mL of a mixture of 5 parts of 12 M hydrochloric and 1part of 16 M nitric acids. Add 5 mL of 9 M sulfuric acid and evaporate to dense fumes but not to dryness. Cool to room temperature and dissolve the salts in 5 mL of 16 M nitric acid and 30 mL of water. The solution may remain turbid in the case of high-temperature alloys. Dilute to 150 mL, add 3 mL of 7% ferric nitrate solution, and proceed with the oxidation,
0.001 0.025 0.018 0.024 0.024, 0.025 0.0124, 0.0121 0.015 0.002 0.0010 0.0011 0.0009 0.0019 0.0009 0.0097 0.001 0.0065 0.0065, 0.0065 0.007 0.040 0.0023
