A N e w AI kalimetric Primary Standard, Sodium Hydrogen DigIycolate DONALD A. KEYWORTH and RICHARD B. HAHN Chemistry Deparfmenf, Wayne Sfate University, Detroit 2, Mich.
b Primary standard sodium hydrogen diglycolate may be prepared by the half neutralization of inexpensive, commercially available diglycolic acid. Two recrystallizations from water give a very pure salt which assays 100.00 =t0.05%. Sodium hydrogen diglycolate possesses a high equivalent weight, is soluble in water, and i s very stable in air. It is easily dried in 2 hours at 120" C. It contains negligible impurities and gives a readily detectable end point using phenolphthalein as indicator. Sodium hydrogen diglycolate is a stronger acid than potassium hydrogen phthalate and is easier to purify and dry.
substances have been proposed as alkalimetric primary standards. The more familiar substances include benzoic acid, constant boiling hydrochloric acid, sulfamic acid, and potassium hydrogen phthalate. Kolthoff reviewed these and many other alkalimetric references (4). Although potassium hydrogen phthalate is widely used, it has certain disadvantages. Small amounts of water are occluded in the crystals, and this water can be removed only with difficulty (crushing before drying, or drying for 120 hours or more a t 120' C.). In standardizing barium hydroxide n4th potassium hydrogen phthalate, insoluble barium phthalate is formed Iyhich occludes untitrated base and also makes the end point difficult to observe. The proposed standard, sotlium hydrogen diglycolate, has none of these faults, and the desired properties of a primary standard for alkalimetry are met. ARIOUJ
PREPARATION AND PROPERTIES
Socliuin hydrogen diglycolate can be prepared easily by the half neutralization of diglycolic acid which is commercially available a t low cost (E. I. du Pont de Nemours & Co., Inc., Kilrnington 98, Del., about 11 cents per pound in 50-pound bags). Diglycolic acid (HOOC-CHz-O-CH2COOH) ( 8 ) is a white, crystalline, diprotic acid. The monohydrated diglycolic acid is stable a t 25' C. and a t relative humidities above 72%. The
melting point is 148" C. The p H of a 10% aqueous solution is 1.4. The acid is stronger than phthalic acid, the ionization constants for the first and second stages of diglycolic acid being K l = 1.1 X 10-3 and KQ = 3.7 X 10-5. About $1.5 grains of diglycolic acid can be dissolved in 100 grams of nater a t 25' C. 81though diglycolic acid is easily purified by recrystallization from TT ater, the acid itself is not a useful primary standard in alkalimetry because of water of hydration. By half neutralization of the acid, however, sodium or potassium hydrogen diglycolate can readily be prepared. Because potassium hydrogen diglycolate is not very soluble (3.5 grams per 100 grams of water a t 25" C.), is more difficult to purify, and more expensive to prepare than sodium hydrogen diglycolate, sodium hydrogen diglycolate (available from the G. F. Smith Chemical Co., Columbus, Ohio) is a more suitable reference niaterial. Sodium hydrogen diglycolate (Sa00C-CH2-0-CH2-COOH) is a white crystalline nonhydrated, nonhygroscopic solid. About 5.1 grams will dissolve in 100 grams of water at 25" C. The p H of the solution is about 3. Sodium hydrogen diglycolate can be recrystallized readily from water, because about 20 grams will dissolve in 100 grams of water a t 90" C. and at 0" C. its solubility is about 2.6 grams in 100 grams of water. A very fortunate aspect of this purification by recrystallization from water is that both the free diglycolic acid and the disodium salt, which would be expected to be the major contaminants, are much more soluble in water than the acid salt. Disodium diglycolate has a solubility of 88.0 grams in 100 grams of water at 25" C. Purification of materials such as potassium hydrogen phthalate, whose parent acid, phthalic acid, is much less soluble than the desired potassium acid salt, requires much more care in recrystallization t o obtain comparable purity. Procedure. Dissolve 600 grams of D u Pont diglycolic acid in 1000 ml. of distilled mater. T h e m-ater may be warmed t o hasten solution. Filter this solution. Prepare a second solution of 180 grams of sodium hydroxide in
500 ml. of water. Filter this solution using a sintered-glass funnel. Remove a 10-nil. aliquot from t h e diglycolic acid solution and titrate n i t h t h e sodium hydrouide solution t o t h e pink end point of phenolphthalein. Calculate the nuniber oi niilliliters of sodiuni hydroxide solution required t o half neutralize 1 nil. of the diglycolic acid solution. 11ultiply this value by the volume of diglycolic acid solution remaining. Slonly add the calculated number of milliliters of sodium hydroxide solution to the diglycolic acid solution with constant stirring, Continue to stir; then cool the solution to room temperature. Filter the solution through a sintered-glass funnel. Continue suction to damp dryness. Remove the sodium hydrogen diglycolate from the funnel, and dissolve it in 1 5 liters of water. Heat to near boiling to dissolve. The solution -hould be clear and colorlcss. Filter hot to remove any metal hydro-des. Cool to room temperature, stirring cwnstantly. Filter the recrystallized sodium hydrogen diglycolate using a sintered-glass funnel. Kedissolve in 2.8 liters of distilled m t e r , and recrystallize a second time. Spread the tnice recrystallized sodium hydrogen diglycolate on a flat dish of borosilicate glass and dry overnight a t 110" C. Store in ordinary screw-cap bottles. This procedure yields about 250 grams of sodium hydrogen diglycolate. The yield may be increased by further cooling the mother liquor, which contains about 350 grams of sodium hydrogen diglycolate. 24b0Ut half of this can be recovered by cooling to 0" C. EXPERIMENTAL
The number of recrystallizations from ryater required to purify sodium hydrogen diglycolate was determined : -4 sodium hydroxide solution, about 0 . l N and free of carbon dioxide, was prepared. Dry samples of the unrecrystallized, once, twice, and thrice recrystallized sodium hydrogen diglycolate were weighed, and a normality for the sodium hydroxide was calculated from the titration of each sample. This calculated normality was compared t o the normality obtained with Sational Bureau of Standards sample 84a, potassium hydrogen phthalate. The titrations were performed using a weight buret and matched pink phenolphthalein end points. The data summarized VOL. 30,
NO. 8, AUGUST 1958
1343
Table I.
Purification of Sodium Hydrogen Diglycolate, Lot 1
(Sormality") Once Twice Thrice Unrecrystallized Recrystallized Recrystallized Recrystallized 0 11675 0.11671 0.11834 0.11673 0.11820 0.11675 0.11669 0.11677 0.11827 0.11670 0.11672 0.11670 Av. 0.11827 0 11673 0.11672 0.11672 Calculated assay 101.34 100.02 100.01 100 01 a Xormality of TaOH calculated from XaHD titration; NBS Standard KHP 84a gives .V = 0.11670.
Table II. Analysis of NBS Standard Potassium Hydrogen Phthalate 84a' and Sodium Hydrogen Diglycolate Using Gravimetric Hydrochloric Acid Reference, Lot 2
yo after Drying Potassium HydroSodium Hydrogen Diglycolate _ gen _ Phthalate ~ _ _ _ 120 2 hours 2 120 hours (crushed) hours hours 100.03 100.05 99.98 100.02 99.98 98.98 100.03 100.00 99.98 100.01 100 02 100.02 Av. 100.033 99 987 lOO.OO3 100.006 S B S Standard KHP 84a hy differential potentiometric titration assayed 99.94470 ( 1 ) . 0
Table 111.
Weight Loss on Drying at
120"
Uncrushed KHP
NaHD
c.
70 Loss 2 24 hours hours 0,020 0.032 0.002 0.007
-
120 hours 0.081 0 009
in Table I show that there is no change in purity of the sodium hydrogen diglycolate after the second recrystallization. T o determine the exact purity of twice recrystallized sodium hydrogen diglycolate and to obtain data on more than one preparation of sodium hydrogen diglycolate, a new lot of twice recrystallized salt mas prepared. d sodium hydroxide solution free of carbon diovide \vas prepared. The normality of the sodium hydroxide was determined with a standard hydrochloric acid solution, the concentration of which had been carefully determined using the silver chloride gravimetric method ( 5 ) . A weight buret was used in all titrations, and the final end point vias approached using a microburet containing dilute sodium hydroxide ( 3 ) . Samples from 1.0 to 1.2 grams of sodium hydrogen diglycolate were titrated. The end point of the titration was determined using a p H meter and titrating to the theoretical end point of 8.2 pH, for conversion of sodium hydrogen diglycolate to disodium diglycolate, and of 8.6 pH, 1344
ANALYTICAL CHEMISTRY
for conversion of potassium hydrogen phthalate to sodium potassium phthalate. A titration curve of sodium hydrogen diglycolate under the prescribed experimental conditions was made t o verify the pH of the theoretical end point. A sharp break in the curve was noted a t the end point. All water used was freed of carbon dioxide by boiling and was protected from the air thereafter. Keights were corrected to in vacuo. The purity of sodium hydrodiglycolate is compared with the purity of the Kational Bureau of Standards potassium hydrogen phthalate sample 84a in Table 11. The Bureau of Standards report3 an assay of 99.99% for standard 84a if the material is dried for 120 hours at 120" C. and 99.93% if the material is dried for 2 hours without crushing. T o obtain the higher assay with an ordinary drying period, the Bureau of Standards recommends light crushing of the sample to break open the crystals prior to drying. Without extended drying or crushing, sodium hydrogen diglycolate which has been twice recrystallized shows theoretical assay within the precision limits of the analytical method employed. T o determine the loss on drying a stored ?ample, a twice recrystallized sample of sodium hydrogen diglycolate dried overnight a t 100" C. n-as stored in a common screw-cap bottle for several months. This stored sample and a sample of reagent grade potassium acid phthalate were dried a t 120" C. for various lengths of time. The data (Table 111) showed that sodium hydrogen diglycolate is easier to dry than potassium hydrogen phthalate and that the loss of weight on drying a stored sample of sodium hydrogen diglycolate is of a very lorn order of magnitude. T o test the stability of sodium hydrogen diglycolate in air, a 10-gram sample which had been dried overnight a t 100" C. was placed in an open weighing bottle. The sample was weighed periodically on dal-s of widely different humidity and temperature. During exposure t o air for 2 months, the sample changed less than 2 parts in 10,000 in weight. Such remarkable stability in air accounts for the negligible loss
of weight when stored samples of sodium hydrogen diglycolate are redried in an oven prior to use. Various qualitative chemical tests were used to detect limits of impurities in the twice recrystallized sodium hydrogen diglycolate. Sulfate and chloride were absent, and the amount of iron present was below 0.05 p.p.m. Infrared absorption spectra detected no free diglycolic acid and no free disodium diglycolate. Loss of weight on drying a t 120" C. for 2 hours n-as only 0.02%. Spectroscopic examination showed that the total amount of heavy metals was less than 20 p.p.m. The second and third recrystallizations showed no spectral differences in either the ultraviolet or infrared region. The low order of magnitude of impurities is expected from the excellent assay of the twice recrystallized sodium hydrogen diglycolate. CONCLUSIONS
The equivalent weight of sodium hydrogen diglycolate is 156.075. A 0.6-gram sample requires about 40 ml. of 0.1S sodium hydroxide for neutralization. Phenolphthalein may be used to detect the end point with excellent results. The stoichiometry is simple, the titration curve being very similar to that for potassium hydrogen phthalate. The solubility of sodium hydrogen diglycolate, about 5 grams in 100 grams of water a t room temperature, compares favorably with the solubility of potassium hydrogen phthalate in water. The metal diglycolates of all the common cations are soluble, and therefore no insoluble products are produced during titrations. ACKNOWLEDGMENT
The authors wish to thank E. I. du Pont de Kemours h Co., Inc., for supplying information and samples of diglycolic acid and H. A. Bright of the Sational Bureau of Standards for his helpful advice. LITERATURE CITED
(1I Bates, R. G., Wichers, E., J . Research .Yatl. Bur. Standards 59, 9 (1957).
(2,Bruner, W. M.,Sherwood, L. T., Jr., Ind. Enq. Chein. 41, 1653 (1949). (3) Kolthoff, I. AI., "Volumetric *$nalysis," L'ol. 11, p: 19, 2nd ed., Interscience. New l o r k . 1947. ( 4 ) Ibzd., p. 7 3 . ( 5 ) \li!lard, H. H., Furman, S H., . Elementary Quantitative dnalpsis," 3rd ed., p. 320, \-an Tostrand, Kern Tork, 1940.
RECEIVED for review October 21, 1957. Accepted March 17, 1958. Division of Analytical Chemistry, 132nd LIeeting, ACS. Sen- York, N. Y.,September 1957.
