Determination of Perchlorate by Precipitation with Tetra-n-PentylammoniumBromide R. G . Dosch Sandia Laboratory, Albuquerque, N . Mex. RECENT TRENDS in the analytical chemistry of perchlorates have involved the precipitation of the perchlorate anion with large organic cations using various techniques t o provide quantitative results. The use of tetraphenylarsonium chloride in combination with potentiometric, conductometric, and gravimetric techniques to determine perchlorate has been described by Willard and Smith (I), Baczuk and Bolleter (2), and Glover and Rosen (3),respectively. The corresponding phosphonium and stibonium compounds, tetraphenylphosphonium chloride and tetraphenylstibonium sulfate, have been proposed as amperometric titrants for perchlorates by Nezu ( 4 ) and Morris (5), and as precipitating agents in titrimetric and gravimetric methods by Willard and Perkins (6). The use of these reagents has the disadvantage of requiring neutral or acidic solutions and a 2-3M chloride concentration for quantitative precipitation of perchlorates. The use of tetraphenylstibonium chloride is extremely limited because of the low solubility of the compound. I n the work reported here, tetrapentylammonium salts are proposed as analytical reagents for the determination of perchlorate by precipitation as tetra-n-pentylammonium perchlorate (TPAC104). The procedure is rapid and quantitative in neutral, acidic, and basic media, and is used in aqueous solutions containing n o extraneous salts. The data reported have been obtained by gravimetric techniques; however, potentiometric and conductxnetric techniques appear t o be directly applicable t o this system. The potentiometric titration of tetra-n-pentylammonium salts with a n iodine-iodide mixture has proven t o be a simple, quantitative procedure (7). Interfering anions have been determined qualitatively with the exception of the chlorate ion, which has been studied relative to its quantitative effect upon precipitation of the TPAClO4. A gravimetric method is described which allows quantitative determination of perchlorate in mixtures containing chlorate/perchlorate ratios of 100 :1 . EXPERIMENTAL Reagents. Tetra-n-pentylammonium bromide (TPABr) (Eastman Organic Chemical Co.) was purified by three successive recrystallizations from acetone using ethyl ether to precipitate TPABr. The recrystallized material was dried a t 40" C in a vacuum desiccator. However n o significant change in results was observed as a result of the purification procedure and the TPABr as supplied was used for this work. Sodium perchlorate (G. F. Smith Chemical Co.) was purified by recrystallization from acetone using ethyl ether to pre(1) H. H. Willard and G. M. Smith, IND.ENG.CHEM.,ANAL.ED., 11, 186 (1939). (2) R. J. Baczuk and W. T. Bolleter, ANAL.CHEM.,39, 93 (1967). ( 3 ) D. J. Glover and J. M. Rosen, Zbid.,37, 306 (1965). (4) H. Nezu, Bunseki Kagaku, 10, 561 (1961); Chem. Absrr., 56,
26f(1962). ( 5 ) M. D. Morris, ANAL.CHEM.,37, 977 (1965). (6) H. H. Willard and L. R. Perkins, Zbid.,25, 1634 (1953). (7) R. G. Dosch, Sandia Laboratory, Albuquerque, N. M., un-
published work, 1967.
cipitate NaC104. The product was dried 12 hours at 40" C in a vacuum desiccator t o avoid possible hazards resulting from perchlorate-organic addition products, followed by several hours at 110" C in a drying oven. All other chemicals were of analytical reagent grade, and were used without further purification. Stock solutions of 0.13M TPABr were prepared by dissolving 10 grams of TPABr in water, filtering through a medium porosity porcelain crucible, and diluting t o 200 ml. Solutions of 0.098M NaC104 and 0.0163M NaC104 were prepared by dissolving the required amount of NaC104 in water. The solutions used in washing the precipitated TPACIOl consisted of either a saturated TPAClOl solution or a solution containing 0.25 gram TPABr/100 ml water. Procedure. An aliquot of standard NaC104 containing between 2 and 50 mg of Clod- was pipetted into a 150 ml beaker. Sufficient amounts of distilled water were added t o provide the desired final solution volume. The pH was then adjusted t o the desired value using dilute solutions of HCl or NaOH, and then TPABr was added from a stock solution. Unless otherwise stated, the p H of all solutions used in this work was approximately 5.3-which was the nominal p H of the solutions without addition of acid or base. The precipitate was allowed to stand for a minimum of one hour and was filtered through a previously weighed medium porosity porcelain filter crucible. The precipitate was transferred from the beaker to the filter using either the TPAC104 or TPABr wash solutions. Any precipitate adhering to the glass was removed by standard techniques. The precipitate was then dried at 110" C for one hour and weighed as TPAC104. RESULTS AND DISCUSSION Tetrapentylammonium halides (Cl-, Br-, I-) are all commercially available; however, TPABr was the most satisfactory in this application. TPACl, as received, appeared t o be very wet and when dissolved in water, produced a cloudy solution which was difficult t o filter. The solubility of TPAI is too low to allow its practical use. TPABr has a solubility between that of the corresponding chloride and iodide. A solution containing 10 grams in 100 ml is easy to prepare, and the resulting solution contains very little insoluble material-2-3 mg insolublejl0 grams TPABr. The solubility of TPAC10, in water is approximately 2.5 x 10-4M, which necessitates the presence of excess reagent during the precipitation t o achieve a quantitative recovery. A 2.5 fold excess of TPABr is sufficient for quantitative work. High results obtained with an excess of TPABr greater than 2.5 times the weight of perchlorate probably result from adsorption of the reagent on the precipitate. During the filtration step, it is advisable to keep liquid in the filter at all times. If the filter goes dry, TPAClO4 is pulled into the fritted disc, and is exposed t o the air. After exposure t o air, TPAC104 is difficult t o wet, and, consequently, the filter becomes essentially sealed, making subsequent filtration very slow. After use, the filters are easily cleaned, as TPAC10, is very soluble in acetone. Precipitation of TPACIO, is complete about one hour after addition of TPABr. For practical purposes, filtration imVOL. 40, NO. 4, APRIL 1968
829
Table I. Quantitative Determination of the Effect of Chlorate on the Precipitation of TPACIOa MI of 0.13M TPABr 5 5
5 5 5 5 5
Conc. HCI (ml)
Total vol
NaC103 (mg)
... ... ... ... ... ... ...
... ... ... ... ... ...
100
...
100 100 100 100
... ...
...
...
10
100 100 100 100
60 60 60 60 12 24 36 60 60 60 60 60 60 60 84 120 170 170 170 170 170
1
...
5 5 5 5 5 15 15 15
5
...
5 5 5 ... 10
... ...
... ...
...
20
...
15 15 15 15
15
...
15 15
... ... ... ...
15 15 5 5 5 5
15 15 15
...
1
...
1
...
1 1 1
10 a
NaHS03 (8)
... ...
100 100 100
200 200 200 200 200 200 200 200 200 100 100
100 100 100
...
Taken 48.73 19.49 8.13 3.25 1.68 48.73 48.13 48.73 48.73 48.73 48.73 48.73 48.73 48.73 48.73 9.75 11.39 29.25 48.73 48.73 48.73 9.75 29.25 1.63 4.89 97.50
Clod- (mg) Founda 48.82 19.62 8.28 3.53 1.63 54.27 52.76 51.98 51.93 49.06 49.71 50.39 50.74 49,60 50.08 9.46 11.29 29.52 48.92 49.61 50.17 9.88 29.47 1.71 5.07 97.42
Each number represents the average of at least two determinations. Table 11. Qualitative Determination of Possible Interfering Anions Hz0 0.1N HCl 10% .' NHiOH Orange ppt No PPt No ppt White ppt No PPt No ppt White ppt No ppt No PPt No PPt No PPt No ppt No ppt No PPt No ppt Orange ppt No ppt No PPt Orange ppt Orange ppt No ppt White ppt White ppt No ppt No ppt No PPt No PPt No PPt No PPt No PPt No ppt No ppt No PPt No PPt No PPt No PPt White ppt No ppt No ppt No ppt No ppt No PPt No PPt No ppt No PPt No ppt No PPt No PPt No ppt No PPt No ppt White ppt White ppt White ppt Purple ppt Purple ppt Purple ppt White ppt White ppt White ppt White ppt White ppt White ppt
NHdVOI Na?WOa NazMoOa NazHAsOa NazHP04 NazCr04 K~Cr207 KIOi NaC1O3 NaB03.4H20 Na?B407 Na2SOa KzSz08 Na3C6H507 Na2SZO3 NazC4H40s K&07 KI
KMn04 NaSCN KRe04
mediately after precipitation is feasible as aging for an hour reduces the error from about 0.5% to about 0.1 %. Control of the pH in this procedure is not critical as quantitative recoveries are possible with hydrogen ion concentration between0 _< p H 13. I n this work, all data were obtained with a precipitation formation time of one hour. The TPABr wash solution was used in the procedure. This introduces a slight error into the results as a precipitate of TPAClO4 weighing 200 mg retains approximately 0.3 ml of the wash solution. This represents a weight of 0.23 mg of perchlorate (0.5% relative error) in the final calculation. This error decreases as the amount of precipitate decreases, because less of the wash solution is retained. It was found that if the TPAClO4 precipitate was washed into the bottom of the filter, the TPABr
