Rapid Colorimetric Determination of Phosphorus in High Explosive Compositions Following a Wet-Ashing Procedure SIR: For approximately seven years, this laboratory has determined phosphorus in organic compounds and mixtures by a combination of liquid-fire combustion, as described by Smith ( 1 , 4 ) , and molybdovanadophosphoric acid procedures. The method is rapid and accurate and has proved safe for use with high explosive mixtures. The reagents and conditions have been found suitable for the decomposition of pentaerythritol tetranitrate (PETN), hexahydro - 1,3,5 - trinitro - s - triazine (RDX), octahydro - 1,3,5,7 - tetranitro-s-tetrazine (HMX), cellulose nitrate, 2,4,6-trinitrotoluene (TNT), and other explosives, and it is believed that a controlled solution technique is considerably safer than dry-ashing methods. Because many materials will explode violently if dropped into hot perchloric acid, it is unacceptable for use in decomposing samples directly; hoivever, with mixed acids safe quantitative oxidations are possible through the tcmperature range 100' to 203' C. The oxidation potential increases smoothly with increased temperature as the effective concentration of the perchloric acid increases. The molybdovanadophosphoric acid colorimetric method for phosphorus, developed by Kitson and Mellon ( 2 ) , iq recommended for the determination of phosphorus after the decomposition. The color, once formed, is stable, an? the reagents may be stored for long periods. Perchloric acid does not interfere with the colorimetric method, an I is the best medium for the color dcvelopment. This has been confirmrd by Quinlan and DeSesa (3). EXPERIMENTAL
Reagents. Ammonium lllolybdate, 0.2M. Dissolve 35.3 grams of animonium molybdate tetrahydrate in deionized water. Dilute t o 1 liter and filter if necessary. Vanadate, 0.2M in 0.4.U' Perchloric I c i d . Dissolve 1.17 grams of ammo-
1050
0
ANALYTICAL CHEMISTRY
nium metavanadate in 400 ml. of deionized water containing 25 ml. of 8 M perchloric acid. Dilute t o 500 ml. 8 M Perchloric Acid. Dilute 345 ml. of 70% acid t o 500 ml. Liquid-Fire Reagent. Add 40 ml. of 70% perchloric acid t o 360 ml. of concentrated nitric acid. Standard Phosphate Solution for Calibration. Dissolve 0.2236 gram of Kational Bureau of Standards potassium dihydrogen phosphate in demineralized water and dilute t o 1 liter. This solution will contain 50 pg. of phosphorus per ml. Procedure. Weigh sufficient sample t o contain 20 t o 400 pg. of phosphorus. Place t h e sample in a 500-ml. Erlenmeyer flask (or Kjeldahl flask if Kjeldahl apparatus is used) and add 20 ml. of t h e liquid-fire reagent. Place a spray t r a p in the neck of t h e flask, place t h e flask on a hot plate, and lower a vapor collector over t h e flask. T h e vapor collector should be connected through a flask filled n i t h water t o a water aspirator. Heat the suspension t o a boil. For most samples i t is perfectly safe t o keep the hot plate a t full temperature all of t h e time. Continue boiling the suspension until white fumes appear. Cool t h e solution and add approximately 10 ml. of water. Boil t o white fumes a second time. This second step is probably unnecessary; however, t h e presence of traces of nitrogen dioxide in the solution could cause high readings. Cool t h e solution conipletely and transfer i t t o a 50-ml. volumetric flask. T h e amount of perchloric acid added in t h e liquid-fire reagent is just sufficient t o make t h e final solution 0.4M in acid; therefore, if t h e sample size or amount of reagent is changed, i t will be necessary t o adjust the amount of acid in t h e final solution. T h e acid concentration is not critical, b u t best results are obtained at 0.4M. Add sufficient water t o give a total volume of approximately 25 ml. D o not exceed a total volume of 30 ml. Add 5 ml. of the vanadate reagent, mix. and add 10 nil. of t h e molybdate
reagent. Mix and make u p t o volume with water. Allow the sample t o stand for a t least 15 minutes and read absorbance us. a fresh reagent blank at 400 mp. Calculate t h e percentage of phosphorus from a previously determined calibration curve. Few esplosive samples have a tendency t o be refractory; horn-erer, certain samples need a catalyst during t h e decomposition. I n these cases it has been found advantageous t o add the 5-ml. portion of t h e vanadate reagent prior t o t h e combustion step. RESULTS A N D DISCUSSION
Several hundred samples of an esplosire composition containing 3% tris(2 - chloroethy1)phosphate (CEF) hare been analyzed by this method, and sanip,ling was found the most critical factor to analytical accuracy. The accuracy of the procedure was =k0.017, CEF nith a standard deviation of O . O l i % CEF (urn = 0.008% CEF). The average sample size taken was i 5 mg. The total required m-orking time per sample was approximately 7 minutes. P E T S samples have been checked for plasticizer contamination by the above procedure and combustioiis proved smooth and uneventful. Samples of phosphate plasticizers have been awayed and the plasticizer content of pladics has been determined. LITERATURE CITED
(1) Diehl, H. C., 2 909 (1959).
Smith, G. F., Talanta
(2)'1iitson, R. E., hlellon, M. G.,
IND.
AXAL.ED. 16, 379 (1944). ( 3 ) Quinlan, K. P., DeSesa, M. A., AKAL. CHEM.27, 1626 (1955). ( 4 ) Smith, G. F., Anal. Chim. Acta 8, 397 (1953). E S G . CHEM.,
R. N. ROGERS
University of California Loe ..liarnos Scientific Laboratory Los .4lamos,
?;.
11.
