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the difference in the rate of removal of the bulk soil, out also preferential removal of individual components of the soil, where chemical reaction resulting in a water-soluble product is not a factor. ACKNOW LEDGMEh T
The investigation described herein R as supported by Solvent01 Chemical Products Co. of Detroit, Mich. LlTERATURE CITED
Vol. 45, No. 12
( 3 ) Hensley, J. W., Skinner, H. -4.,and Sutter, H. R., Am. SOC. Testing Materials, S p e c i d Tech. Pub. N o . 115, 18-32 (1952). (4)Osipow, Lloyd, and Snell, Foster Dee, paper presented a t XIIIth International Congress of Pure and Applied Chemistry, Stockholm, Sweden, July 29 to -4ug. 3 , 1953. ( 5 ) Overbeek, J. Th. G., “Colloid Science,” Vol. 1, ed. by H. R.
Kruyt, Amsterdam, The Setherlands, Elsevier Publishing Co., 1952. (6)
Reich, Irving, and Snell, Foster Dee, IND. ENG.CHEM.,40, 1233-7
(1948). (7) Zhid., pp. 2333-7.
(1) Campbell, A. M., and Brown, E. ii., J . Am. Chem. Soc., 60, 3055-60 (1938). (2)
Campbell, C. A., U. S. Patent 2,399,205 (1940); 2,583,165 (1952).
RECEIVEDfor review May 25, 1953. ACCEPTED August 17, 1933. Presented before t h e Division of Colloid Chemistry a t t h e 124th Meeting, AMERICAXCHEMICALSOCIETY,Chicago, Ill., 1953.
Sequestration by Sugar Acids C. L. MEHLTRETTER, B. H. ALEXANDER, -4ND C. E. RIST Northern Regional Research Laboratory, Bureau of Agricultural a n d Industrial C h e m i s t r y , U . S . D e p a r t m e n t of Agriculture, Peoria, Ill.
T
HE recent interest in organic sequestering agents ( 2 , 13)
has stimulated the investigation of many substances for utilization in this rapidly expanding field. Sugar acids are a class of compounds that show promise for this purpose. Included in this category are the fermentation acids, lactic, citric, gluconic, and 2-ketogluconic as well as the aldonic and dibasic acids produced from sugars by chemical oxidation. The ability of sugar acids, in general, to form water-soluble complexes with metal ions stems from the capacity of their carboxyl and hydroxyl groups t o bind cations in ring form by means of coordinate and covalent bonds. A knowledge of the specific conditions under which such complexing is most effective should lead to new outlets for these products. I n this investigation data were obtained on the sequestering ability of a number of readily accessible sugar acids toward calcium, ferric, and cupric ions in acid to strongly alkaline solutions. The effect of such variables as“concentration of sequestrant and of alkali on sequestering power also was observed. Methods (4,14, 16) are known for determining the degree of chelation of calcium by organic sequestrants but those for iron and copper have not been made generally available. Procedures, therefore, were devised for evaluating the sequestering action of sugar acids toward the latter metal ions under acid, neutral, and alkaline conditions. METHODS FWR DETERMINING SEQUESTERING CAPACITY O F SUGAR ACIDS
CALCIUM. The sequestering power of the sugar acids toward calcium ions was measured by Zussman’s method (16). Stock solutions containing 2% (2 grams per 100 ml. of solution) of the sodium or potassium-sodium salts of the sugar acids were prepared, For studies on sequestration in acid solution, 10 ml. of the stock solution (0.20 gram of sequestrant) were diluted to 20 ml. with distilled water in a 50-ml. beaker. Two milliliters of 2% sodium oxalate solution were then added, and the acidity was adjusted t o pH 4 with acetic acid. The solution was mechanically stirred, and standard 1% calcium acetate solution introduced dropwise from a buret until the first appearance of permanent turbidity. If considerable precipitation occurred on standing for 24 hours, further titrations were made with a lesser amount of standard solution until only a slight precipitate was observed after 24 hours. Although the slight permanent precipitate obtained immediately was used by Zussman for determining the calcium-
sequestering ability of polyamine carboxylic acid salt, he realized that further precipitation on standing introduced an element of uncertwinty in the results. The sequestering values of the sugar acids calculated on the basis of immediate precipitation of calcium oxalate are considerably higher in most cases than those determined from the slight permanent precipitate obtained in 24 hours. The authors considered the latter to be the more reliable values. For similar studies in alkaline solutions, 10 ml. of 2% stock solution of sequestrant were mixed n i t h 10 ml. of sodium hydroxide solution containing twice the coilcentration of alkali desired before introduction of the sodium oxalate indicator and standard calcium acetate. IRON.The sequestering pon-er of the sugar acids toward ferric ions in acid and neutral solution was carried out as follows: Ten milliliters of 2% stock solution of the sequestrant were added to 180 ml. of distilled water containing 5 ml. of 2% potassiuni ferrocyanide solution. The resulting mixture was adjusted t o p H 4 or 7 with dilute hydrochloric acid. A standard solution containing 90.0 grams of ferric sulfate per liter was then introduced dropwise from a buret with continuous stirring of the sequestrant solution until a permanent precipitate due to the formation of insoluble ferric ferrocyanide appeared. Potassium ferrocyanide reagent is sensitive to small amounts of ferric ion over the p H range 4 to 7 . Control experiments carried out without the addition of sequestrant utilized 0.3 ml. of ferric sulfate solution t o obtain a permanent precipitate of ferric ferrocyanide. I n alkaline solution, ferric ions precipitated as the more insoluble ferric hydroxide and ferrocyanide reagent was not required. HOT< ever, the mode of addition of ferric sulfate was important for satisfactory evaluation of sequestration. The direct introduction of standard ferric sulfate solution to alkaline solutions of the sequestrants produced an immediate precipitate of ferric hydroxide which was solubilized t o o slowly for test purposes. If, however, the alkali solution was added to a mixture of t h e sequestrant solution and an amount of ferric sulfate solution below the chelation limit of the sequestrant, which was determined by trial, the ferric hydroxide precipitate initially formed was rapidly solubilized. By successively increasing the amount of ferric sulfate in separate trials, the quantity required to obtain a permanent slight precipitate of ferric hydroxide was determined. Specifically, 10 ml. of 2% potassium sodium saccharate were mixed with 20 ml. of standard ferric sulfate solution. To this solution were
December 1953
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g[20-/T y
15-
53
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3a
woa 3a 10a
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Figure 1. Effectof Alkali Concentration on Sequestration of Calcium by Potassium Sodium Saccharate at 25 O C.
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concentrations of alkali higher than 4% markedly reduced the sequestering capacity of saccharic acid (Figure 1). Optimum sequestration of saccharic acid was achieved in 2 to 4% sodium hydroxide solution where 20 grams of calcium were complexed by 100 grams of potassium sodium saccharate after 24 hours. When the concentrations of saccharic and gluconic acids were increased in 3 % sodium hydroxide solution their sequestering power toward calcium increased in direct proportion (Figure 2). The considerable difference in complexing capacity for calcium that was noted immediately and after 24 hours is shown in Table I. Of the sugar acids tested the best sequestrants for calcium
contrast, the higher sugar acids sequestered large amounts of iron and copper in the presence of alkali (Tables I1 and 111).
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STRUCTURAL COXSIDERATIOXS
In general, the sequestering action of sugar acids is more pronounced in alkaline solutions than in acid or neutral media. Presumably, this effect is due to the increased ionization of the hgdrogen of the hydroxyl groups in sugar acids in aqueous sodium hydroxide. Relatively high concentrations of alkali, however, reduce the sequestering ability of sugar acids, possiblj- through the formation of stable sodium salts with t,he hydrosyl groups.
CAPACITY O F SVGAR ACIDSALTS FOR TABLE 11. SEQUESTERING IROSAT 25” C. (Grams of iron per 100 grams of sequesrrant under acid, neutial, and alkaline conditions)
Sodium Hydroxide in Solution, % pH4pH7 1 2 3 4 5
