Separation and Determination of Lead with Salicylaldoxime I
J
W. B. LIGETT AND L. P. BIEFELD, Purdue University, Lafayette, Ind.
S
ALICYLALDOXIME has been used as a reagent for the gravimetric determination of copper (S), nickel (9), palladium (6),lead (e), bismuth (4), and zinc (4). Separations of copper from nickel (I, 6,9), copper from lead (Y),bismuth from zinc (Q), and bismuth from silver (4) have been carried out by control of pH or the use of ammonia complex formation. A careful study has been made of the effect of p H on the precipitation of copper salicylaldoximate and nickel salicylaldoximate ( I ) . It seemed desirable to study lead in a similar manner, inasmuch as the data obtained could be used as the basis for separating lead from other metals.
Determination of Optimum pH for Precipitation SOLUTIONS.Standard lead nitrate solutions were prepared from twice-recrystallized reagent grade lead nitrate,, dissolved in water containing approximately 0.02 mole of redistilled nitnc acid per liter. Standard lead acetate solutions were prepared from twice-recrystallized analytical reagent lead acetate dissolved in 0.02 molar acetic acid solution. The lead solutions were standardized by precipitation of the lead as lead sulfate. The salicylaldoxime was obtained from the Eastman Kodak Company. A 1 per cent solution was prepared according t o Ephraim (3) by slowly pouring a solution of 1 gram of salicylaldoxime in 5 ml. of 95 per cent ethyl alcohol into 95 ml. of water heated t o 80' C. The solution was cooled and fltered before using. It was freshly prepared the same day as used. PRECIPITATION TECHNIQUE.A 10-ml. portion of a 1 per cent salicylaldoxime solution was added to 25-ml. portions of the
FH FIGURE 1. PBECIPITATION O F LEADWlTH SALICYLALDOXIME
standard lead solutions. Ammonia solution was added in amounts estimated to give pH values over the desired range, and in each case water was added to give a total volume of 65 ml. The resulting precipitates and solutions were each stirred for 1 hour, and the precipitates were washed by decantation and fltered with suction through KO.4 Jena glass filtering crucibles. The precipitates were washed with water until free from salicylaldoxime as shown by the absence of coloration of the fltrate upon addition of ferric chloride solution, dried at 105' C. for 1 hour, and weighed as PbC7Hs02N. MEASUREMENT OF pH. The hydrogen-ion concentration was measured with a glass electrode pH meter (8) calibrated with Clark and Lubs buffer solutions. The measurement was taken upon the first filtrate decanted from the precipitate, and before any washings had been added. The pH values were reproducible t o ~ 0 . 0 pH 2 unit. p H RANGEFOR PRECIPITATION. The results of the precipitation of lead from lead acetate solutions are shown by curve 2, Figure 1. The amount of lead present in each determination was 0.1095 gram. The weight of precipitate should be 0.1809 gram if precipitation was complete and if the formula PbC',HbO& (Pb = 0.6053) was applicable to t h e compound precipitated. The results show that precipitation begins a t about p H 5.5, but that the p H must be equal to or greater than 9.4 before lead can be determined as lead salicylaldoximate using the theoretical factor 0.6053. Precipitation of the lead was actually complete a t a much lower p H than 9.4, as shown by the saturation of the fltrate with hydrogen sulfide. There was no evidence of formation of lead sulfide in any of the filtrates with p H above 7.3, indicating that the precipitates formed up to p H 9.4 are a mixture of lead salicylaldoximate and a basic lead salt. This is contrary to previously published results (6), which state that it is only necessary to have the p H above 6.5 to obtain complete precipitation of lead as PbC7H5O2N. Curve 1, Figure 1, is a plot of representative data obtained using lead nitrate solutions. The amount of lead present in each determination was 0.0971 gram, which should give a precipitate weighing 0.1604 gram on the basis of complete precipitation as PbC7HsOzN. As shown by the curve, precipitation of lead salicylaldoximate begins just above p H 4.8 and is complete as PbCyHS02N above p H 8.9. Precipitation of lead was actually complete a t p H 6.9 and above, as evidenced by no precipitation upon saturation of the filtrate with hydrogen sulfide, but the theoretical factor 0.6053 could not be applied to the precipitate obtained in the p H range of 6.9 to 8.9. The effect of the acetate ion, even though present in relatively small concentration, is to shift the curve above 0.5 p H unit to the right. Higher acetate-ion concentration would probably necessitate an even higher p H to obtain complete precipitation. It has been reported (4) that complete precipitation of lead can be prevented by use of ammonium acetate. The curves become nearly horizontal during precipitation of the last few percentages of lead. This flat portion of the curve, extending over about 2 p H units, represents the interval during which precipitation of the lead is complete, not solely as lead salicylaldoximate, but probably as a mixture of the salicylaldoxime complex and a basic acetate or nitrate. Thus, if it is desired only to separate the lead from solution, a pH of 6.9 for the nitrate solution and 7.3 for the acetate solution will suffice; but to weigh the precipitate as PbC,H50zN, the p H must be above 8.9 for the nitrate solution, and above 9.3 for the acetate solution. 813
INDUSTRIAL AND ENGINEERING CHEMISTRY
814
TABLEI. AMMONIA REQUIREDTO DISSOLVESALICYLALDOXIMATES
Metal Salicylaldoximate
Volume of Concentrated NHa Added
MI. Silver Zinc Cadmium Nickel Cobalt Copper
0.05
0.4 1.2 6.6 6.7 50.0
NH: Concentratipn of Resulting Solution Molar 0.01
0.1 0.3 1.5 1.5 7.0
Separation of Lead These data show that lead can be quantitatively precipitated from strongly ammoniacal solution. It seemed likely that the separation of lead from other metals which form insoluble complexes with salicylaldoxime might be effected if precipitations were carried out in solutions containing a high concentration of ammonia. SOLUBILITY OF COMPLEXES IN AMMONIA.Solutions of Bi+++ Cd++, Cot+, CU++, Fe++, Mg++, Mn++, Hg++, Ni++, AgC: and Zn++ ions were prepared from reagent grade salts to contain 4.0 grams of the metal per liter of solution. To 10 ml. of each of these solutions were added 20 ml. of a 1 per cent salicylaldoxime solution. The solutions were diluted to 65 ml. with water. One drop of 4 molar ammonia solution was added to each to produce precipitation of the metal salicylaldoximate, except that the addition of ammonia was not necessary to produce precipitation with copper or cobalt. Concentrated ammonia solution was then added dropwise, with stirring, to determine which of the complexes were soluble in ammonia solution, and the relative ease with which they dissolved.
It was found that bismuth, ferrous, magnesium, manganous, and mercuric salicylaldoximates did not dissolve in ammonia solution in concentrations u p t o 8 molar. The salicylaldoximates of the metals which form ammonia complexes dissolved upon addition of varying amounts of ammonia, as shown in Table I. These results do not represent equilibrium conditions; the precipitates undoubtedly would have dissolved with less ammonia if a longer time had been allowed between addition of successive increments. However, from an analytical standpoint, the data as given are more valuable than would be the data for equilibrium conditions. Solutions of copper, cobalt, zinc, SEPARATION TECHNIQUE. and cadmium were prepared by dissolving reagent grade acetate salts in 0.02 M acetic acid. Nickel and silver solutions were preared from reagent grade nitrates dissolved in 0.02 M nitric acid. Each solution contained 4.0 grams of metal per liter. Fifteen-millilite; portions of 1 per cent salicylaldoxime solution were added to mixtures of lead acetate solutions, prepared and standardized as described above, and solutions of the various metals from which precipitation was to be attempted. Concentrated ammonia solution was then added in amount sufficient to precipitate lead salicylaldoximate and to prevent precipitation, if possible of the salicylaldoximate of the other metal. Water was added to make 65 ml. The precipitate and solution were stirred for 1hour, and the precipitate was then permitted to settle. The supernatant liquid was decanted through a No. 4 Jena glass filtering crucible. The precipitate mas washed, by decantation, with a solution of salicylaldoxime and ammonia of the same concentration as that in which precipitation had occurred. Finally, the precipitate was washed with 20 per cent ethyl alcohol solution, by decantation and also after transfer to the filtering crucible, until it was free from salicylaldoxime. It was dried for 1 hour a t 105" C., cooled, and weighed as PbC,HsO?N. Copper. Attempts to separate lead from various mixtures of copper solution and lead acetate solution were unsuccessful. Copper salicylaldoximate precipitated with lead salicylaldoximate from solutions with ammonia concentrations as high as 8 M . Cobalt. Portions of the cobalt solution were mixed with lead acetate solution and separations were attempted employing the technique outlined above. The data in Table I indicate that c?balt salicylaldoximate would not precipitate in 1.5molar ammoma
Vol. 13, No. 11
solution, but coprecipitation of the cobalt complex with the lead actually occurred in ammonia concentrations u to 8 M . The precipitate obtained in all cases was a mixture o!cobalt and lead salicylaldoximates. Nickel. The results obtained were very similar to those with cobalt. Coprecipitation of nickel salicylaldoximate was not so pronounced as was the coprecipitation of cobalt salicylaldoximate, but was sufficient to make separations impossible. Silver. s Separations of lead from mixtures containing portions of the silver solution and of the lead acetate solution were accomplished, followin the procedure given above. The results are listed in.Table I f A volume of 2.5 ml. of concentrated ammonia solution was used in a total volume of 65 ml. of mixture. The data in Table I suggest that a much lower concentration of ammonia would prevent precipitation of silver salicylaldoximate. Zinc. It was possible to precipitate lead salicylaldoximate uncontaminated by the zinc complex from mixtures of the zinc solution and lead acetate solution (Table 11). Although zinc salicylaldoximate does not precipitate from a zinc solution containing less than 0.5 ml. of concentrated ammonia solution in a total volume of 65 ml., it was necessary to have 12.5 ml. of concentrated ammonia solution in a total volume of 65 ml. to revent coprecipitation of zinc salicylaldoximate with lead salicyfaldoximate. Cadmium. The experiments on the separation of lead from cadmium gave results similar to those with zinc (Table 11). A volume of 12.5 ml. of concentrated ammonia solution was used in a total volume of 65 ml. Metals Not Forming Ammonia Complexes. Since the solubility experiments showed that bismuth, ferrous, manganous, magnesium, and mercuric salicylaldoximates do not appreciably dissolve in ammonia solutions, separations of lead from these ions by salicylaldoxime and high ammonia concentrations are impossible.
TABLE 11. DETERMINATION OF LEAD Lead Present Gram
Metal Present Gram
Precipitate Found Gram
Lead Found Gram
Error
Mg.
I n the Presence of Silver
I n the Presence of Zino 0.100 0.080
0.0438 0.0876 0.0876 0.1095
o.os0
0.0219 0.0438 0.0657 0.1095
0.100 0.100 0.100 0.060
0.060
0.0725 0.1447 0.1450 0.1811
0.0439 0,0876 0.0878 0.1096
+o.
1 0.0
f0.2 +0.1
In the Presence of Cadmium 0.0362 0.0725 0.1082 0.1810
0.0219 0.0439 0.0655 0.1096
0.0 +0.1 -0.2 +o. 1
Summary
Lead can be determined by precipitation as lead salicylaldoximate, weighing the resulting precipitate after drying at 105' C., and calculating the amount of lead on the basis of the formula PbCTH602N. From nitrate solutions, precipitation begins just above p H 4.8 and is complete at 6.9. However, the precipitate obtained at p H 6.9 does not have the composition represented by the formula PbGH602N. Precipitation must be carried out at p H 8.9 or above in order to obtain a compound t o which the theoretical factor of 0.6053 for lead in lead salicylaldoximate can be applied. The effect of acetate concentrations as low as 0.05 molar is to increase the p H necessary for the beginning of precipitation, for complete precipitation, and for complete precipitation as lead salicylaldoximate. This increase is about 0.5 p H unit. Lead can be separated as the salicylaldoximate from silver, cadmium, and zinc in strongly ammoniacal solutions. Salicylaldoxime reagent cannot be used in conjunction with high ammonia concentrations to separate lead from one or more of the following metals in solution: copper, nickel, cobalt, bismuth, iron, magnesium, manganese, and mercury.
A N A L Y T I C A L EDITION
November 15, 1941
Literature Cited (1)
Biefeld, L. P., and Howe, D. E., IND.ENG.CHEW ANAL.ED., 11,
251 (1939). (2) Chambers, M., Chemist-Analyst, 26,52 (1937). (3) Ephraim, F., BeT., 63,1928 (1930). ANAL.ED., (4) Flagg, J. F., and Furman, N. H., IND.ENQ.CHEM., 12,663 (1940). ( 5 ) Holzer, H., Z.anal. Chem., 95,392 (1933).
815
(6) Ishibashi, hl., and Kishi, H., Bull. Chem. Sac. Japan, 10, 362 (1935). (7) Ishibashi, M.,and Kishi, H., J. Chem. Sac. Japan, 55,1067 (1934). (8) MeUon, M. G., “Methods of Quantitative Chemical Analysis”, p. 413,New York, Macmillan Co., 1937. (9) Riley, H. L.,J. Chem. Sac., 1933,895. PRESENTED before the Division of Analytical and Micro Chemistry a t the lOlst Meeting of the American Chemical Society, 8t. Louis, Mo.
Determination of Starch by the A. 0. A. C. Malt-Diastase Method J
Effect of Pretreatment of Samples R. T. BALCH AND J. K. PHILLIPS’ Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Washington, D. C.
IK
THE production of starch from sweet potatoes at the Laurel Starch Plant, Laurel, Miss., i t was found impossible to obtain a starch balance based on t h e starch content of t h e raw materials entering the factory and on that contained in the final products-commercial starch, pulp, and waste waters-as determined by the official A. 0. A. C. maltdiastase method (1). The unknown loss was much greater than could be ascribed to faulty sampling or to any losses which were not being estimated. In order t o demonstrate whether this loss was actual or fictitious, starch was mechanically extracted from potatoes of known weight and determined starch content, with the aid of water and other solutions used at the Laurel plant: sulfurous acid (approximately 0.015 N ) , alkaline sulfite (approximately 0.04 N sodium hydroxide and 0.02 N sulfur dioxide), and lime water (approximately 0.02 N calcium hydroxide). As lime water had been used exclusively during the previous two operating seasons at t h e plant, particular attention was given in this study t o this solution and its effect on the analysis of sweet potatoes and the products derived therefrom. The mechanical extraction of starch from sweet potatoes in the factory (6) is accomplished by.grinding the potatoes very finely, mixing the ground mass with water or solution, and separating the starch suspended in the liquid by screening. The washing and screening operations are repeated several times on a countercurrent principle. I n these laboratory tests 500 grams of potatoes, ground with a sugar-beet sampling rasp, were thoroughly agitated with 2500 cc. of water or one of the solutions mentioned above, and the starch milk was separated from the pulp with a 200-mesh screen. This operation was repeated twice, employing a fresh portion of liquid in each cycle. The starch water obtained from the three extractions was combined and allowed to settle in a rather shallow layer for about 4 hours, after which the supernatant li uid was siphoned off and allowed to settle further overnight. %he fist crop of starch was collected in a Buchner funnel and washed, then transferred to a drying tray, where it was dried a t about 45’ C. The dried starch was weighed and analyzed. The second settlin s were simply collected and the entire amount was subjectefi to analysis The residual pulp was dewatered by filtering in a Bdchner iunnel, after which it was weighed and analyzed. These products were analyzed by the A. 0. A. C. malt-diastase (1) method, with very slight modifications. As it was thought that the potatoes and pulp were sufficiently disintegrated for the determination of starch, the samples were simply weighed out: transferred to Gooch crucibles, and washed with water until 1
Present address, Yoder Brothers, Bsrberton, Ohio.
sugar-free. Ordinarily from 100 to 150 cc. of water were used in this step. The contents of the crucibles were trapsferred to beakers, the starch was gelatinized by boiling for 15 minutes on a hot plate, and 25 cc. of malt diastase, prepared by digesting 10 grams of freshly ground barley malt grain with 150 cc. of water for at least 1.5 hours were added and allowed to act for 1 hour a t 55” C. The boiling and malt treatment were repeated as specified. After the malt conversion, the solutions were acidified with 5 to 8 drops of glacial acetic acid to aid clarification, cooled, made up to 250 cc., and filtered; 200 cc. of the filtrate were digested with hydrochloric acid for 2.5 hours on a steam bath, cooled, neutralized, and made to a final volume of 500 cc. Reducing sugars were determined in the resulting solutions by the LaneEynon titration method (S), employing 10 cc. of Fehling’s solution. For determination of the blank, a double portion of the malt extract-100 cc.-was digested with hydrochloric acid and made up to a final volume of 500 cc. It was later found that this procedure for determining the blank, although specified by the A. 0. A. C. method, introduced a slight error, which, however, did not alter the conclusions based upon these data. The error in the blank arises from the fact that heating the malt extract, acidifying, and filtering apparently remove from solution something which possesses a slight reducing action after acid digestion. Applying these extraction and analytical procedures to several lots of sweet potatoes, an average unaccountable loss of 4.2 per cent of the starch in the potato was obtained with water extraction, 7.3 per cent with lime water, 7.1 per cent with alkaline sulfite, and 3.1 per cent with sulfurous acid. With the exercise of every possible precaution to prevent appreciable loss of starch, the only logical conclusion was t h a t the methods of analysis were faulty; otherwise a rather constant unknown loss would be expected, regardless of the solution used in aiding the extraction of starch from potatoes. It mould not seem logical to obtain a progressively larger unknown loss of starch in changing the reaction of the liquid phase from acid to alkaline. It was certain t h a t the alkaline solutions did not dissolve a n y starch and there was no evidence that t h e actual loss of starch in the alkaline waste waters was a n y greater than in the acid waters. In view of such consistent results, i t was decided to pretreat duplicate samples of potatoes taken for analysis, in the last two extraction experiments, with the reagents used for t h e extractions-water, 0.02 N calcium hydroxide, and alkaline sulfite (0.04 N sodium hydroxide-0.02 N sulfur dioxide). This treatment was made after samples were weighed out and prior t o washing them in the Gooch crucible. The starch values obtained were rather surprising. The samples given
