are listed in Table 111. All four dyes are suitable indicators for the photometric titration of strong and intermediate bases, as the relative error values fall within the probable error range of k4.0 p.p.t. for these determinations. Urea behaves as a base that is slightly weaker than the indicators used in the titration. Because the dyes included in this study extend from the strongest to the weakest bases of the triphenylmethane dyes, it is doubtful that any dye Of fsmily malachite green) is suitable for indicat-
ing the exact equivalence point in the titration of urea with perchloric acid. ACKNOWLEDGMENT
m e authors express their indebtedne88 to Research Corporation for financia1 support of this investigation. LITERATURE CITED (1) @'ddU, R., Hume, D., A N A L -
26, 1681 (1954). (2) Higuchi, T., Rehm, C., Barnskin, C., M . ,28, 1509 (1956).
(3) Kolling, 0. W., J . Am. Chem. Sor. 79, 2717 (1955). (4) Kolling, 0. W., J . Chem. Educ. 35,452 (19%): (5) Kolling, 0. W., Trans. Kansas h a d . Sa'.59,422 (1956). (6) Kolthoff, I., Bmckenskirl, s., J . A m . Chem. Soc. 7 8 , 7 (1956). (7) Rsmeth, R., sandell, E., ibid,, 78, 4872 (1956). (8) Rehm, C., Higuchi, T., ANAL.CHEM. 29,367 (1957). (9) Seward, R., Hamblet, C., J . A m . Chent. Soc. 54,557 (1932).
RECEIVEDfor review January 12, 1959. Accepted .4ugust 5, 1959.
Titrimetric Determination of 2-Mercaptoacetic (or Thioglycolic) Acid by Copper(l1) SUSEELA B. SANT' Carr Chemical laboratories, Mount Holyoke College, South Hadley, Mass. BHARAT R. SANTl Codes Chemical laboratories, Louisiana State University, Baton Rouge, l a .
b A mettrod for the determination of 2-mercaptoacetic (or thioglycolic) acid in aqueous solution is based on the titration of a standard cupric salt solution with thioglycolic acid. At the end point a permanent yellow precipitate is formed. The method is accurate within 0.3%.
D
of mercaptans by reaction with cupric alkyl phthalate is rapid and accurate although somewhat less accurate than the iodine method (2, 7, 9, 10). This procedure is effective in the presence of many substances which would otherwise interfere in the iodine method. The reaction between copper(I1) and mercaptan takes place as follows. 2Cu++ 4RSH + 2CuSR RSSR 4H+ (1) ETERMINATION
+
+ +
The method is of general applicability. Titrations are carried out in a nonaqueous m e d i p , usuaily Pentawl or a hydrocarbon solvent. The appearance of the green color of cupric ion marks the end point. However, thioglycolic acid cannot be determined by this method, because the deep blue cupriccuprous mercaptide produced obscures the end point (2). In the c o m of these investigations, thioglycolic acid was found to undergo quantitative reaction with cupric ion in aqueous solutions. Experimental con'Present addresa, Department of Chemiatry, University of Toronto, Toronto, Ontario, Canada.
ditions for a direct titration method using a standard cupric salt solution for the estimation of thioglycolic acid are described. REAGENTS
Copper sulfate (J. T. Baker analyzed) was used to prepare a 0.1N solution in water. An aqueous solution of thioglycolic acid, HS-CHxCOOH, was prepared by dissolving a 98 to 99% pure sample. Its mercaptan content was determined by oxidation with a known e x c w of iodine solution, followed by titration of the excess with standard sodium thiosulfate (6):
+
~HS-CHICOOH Is -C (S--CHzCOOH)r
+ 2HI
PROCEDURE
A measured portion of the copper sulfate solution was transferred to a 150-
Erlenmeyer flask or, preferably, a porcelain dish. The white background of the latter is convenient for discerning the correct end point. The thioglycolic acid solution was added from a buret until the deep violet precipitate first formed changed to a permanent yellow. Titrations near the end are performed slowly with vigorous shaking or stirring of the solution. ml.
RESULTS AND DISCUSSION
The reactions taking place during the titration are: complexstion of copper with thioglycolic acid, reduction of copper(I1) to copper(1) with simultaneous oxidation of thioglycolic to dithioglycolic acid and, at the end point,
Table 1. Titrimetric Determination of Thioglycolic Acid by Copper(l1)
Thioglycolic Acid, Mg. Iodine Copper(I1) method method 15.91 15.89 31.78 31.82 45.82 45.67 91.87 91.65 137.7 137.5 183.2 183.3 228.8 229.1
Difference 0.02 0.04 0.15 0.22 * 0.2 0.1 0.3
formation of yellow cuprous mercaptide. The over-all process represented by the following equation
+
~ C U + + ~HSCHICOOH --c 2CuSCH&OOH (S--CHzCOOH)* 4H'
+ +
(2)
is the .same as that of Equation 1. I t follows from Equation 2 that 1 ml. of a 1M cupric sulfate solution is equivalent to 92.12 mg. of thioglycolic acid. Experimental results are given in Table I. Cupric nitrate, chloride, or acetate may be used instead of cupric sulfate. A pH of 4.0 to 4.4, which is the case with aqueous solutions of cupric salts except the acetate, is best suited for the titration. At higher pH valuea the reaction is faster, but the end point is not stable because of the oxidation of cuprous mercaptide. At too low a pH the reaction is slow. When cupric acetste is used, the desired pH m y be obtained readily by the addition of a reqVOL 31, NO. 11, NOVEMBER 1959
0
1879
uisite quantity of dilute acid. The end pomts are stable and reproducible under the prescribed conditions. Organic sulfides, disulfides, and thiocyanates do not interfere. Interference is caused by ions such as thiosulfate, thiocyanate, sulfide, cyanide, and others which react with copper ions. Sulfite ion does not interfere. Thioglycolic acid, like other mercaptan compounds, can be estimated by direct titration with iodine (6, 8, 1 2 ) or silver nitrate (1, 3, 4, 6). The present cupric method appears to be relatively more simple and rapid. No indicator is needed and a single titration requires only 2 to 5 minutes. Besides, cupric sulfate can serve as a primary standard. Experiments a t different time intervals revealed that 0.05 to 0.2N aqueous
solutions of thioglycolic acid are stable under ordinary conditions of temperature and day light for 2 weeks. ACKNOWLEDGMENT
The authon are indebted to Lucy W. Pickett and Philip W. West for research facilities. One of them (S.B.S.) acknowledges the award of a Special Skinner Fellowship. Appreciation is also due to Evans Chemetics Inc., New York, for a gift sample of thioglycolic aeid. LITERATURE CITED
J. Mitchell, et al., eds., Vol. I, p. 343, Interscience, New York, 1953. (3) . . Karchmer, J. H., ANAL. CHEM.29, 425 (1957). . (4) Kolthoff, I. M., Harris, W. E., IND. ENQ.CHEM.,ANAL.ED. 18, 161 (1946). (5) Gamer, .H., H., J. Asso; Assoc. Oflc. Agr. Chemists 35, 285 (1952). (6) . . Leisv, F. A., ANAL. CHEM.26. 1607 (1954j.’ (7) Siggia, S. “Quantitative Organic Analysis Via Functional Groups,” p. 88, Wiley, New York, 1949. (8) Stephen, W. I., Mfg. C h k t 26, 450 (1955). (9) Stone, K.
G., “Determination of Organic Compounds,” p. 188, McGrawHill, New York, 1956. (10) Turk, E., Reid, E. E., IND.ENQ. CHEM.,ANAL.ED. 17, 713 (1945). (11) Walker, G. T., Mfg. C h i s f 24, 376 429 (1953).
(1) Cecil, R., McPhee, J. R., B i o c h a . J. 59,234 (1955). (2) Dal Nogare, S., in “Organic Analysis,”
RECEIVEDfor review April 14, 1959. Accepted July 28, 1959.
Determination of Fluorine as Lithium Fluoride EARLE R. CALEY and GERALD R. KAHLE’ McPherson Chemical laboratory, The Ohio State University, Columbus Fluorine present as fluoride in aqueous solution is quantitatively precipitated by the addition of an equal volume of a 3% solution of lithium chloride in 95% ethyl alcohol. The precipitated lithium fluoride is easily filtered off and washed in a filter crucible. After drying for an hour at 110” C., it is weighed. No special apparatus is needed and accurate results are obtained.
V
objections have been raised against the gravimetric methods advocated for the exact determination of fluorine in large proportion as fluoride ion. One is the difficulty of filtering and washing such gelatinous precipitates as calcium fluoride or thorium fluoride (2). Another is the serious interference of many commonly associated anions-i.e., sulfate, when barium, calcium, lead, or thorium ions are used to precipitate the fluoride (1). By using lithium ion as the precipitant, a precipitate is obtained that is easy to filter and wash, and the interference from associated anions is less. ARIOUS
was found to be very suitable. Unless the concentration of ethyl alcohol is at least 50% by volume, precipitation is not quantitative in the presence of lithium ion in the low concentration needed to produce precipitates with desirable physical properties. Higher concentrations of ethyl alcohol are also undesirable both from the standpoint of producing precipitates with desirable physical properties and the probability of increased interference from the precipitation of various salts. Because of the appreciable solubility of lithium fluoride in 50% ethyl alcohol solutions containing lithium ion in low concentration, the volume of the solution must be restricted in order to obtain quantitative precipitation. This is evident from the results of some preliminary experiments shown in Table I, using a similar procedure to the one in the text. Ethyl alcohol is added slowly in the form of ‘a dilute solution of lithium chloride in 95% ethyl alcohol to the warm aqueous solution containing the fluoride. No other way .of Table 1.
CONDlTlONS FOR PRECIPITATION
Precipitation is far from complete in aqueous solution regardless of the concentration of the added lithium ion. The addition of some miscible organic solvent is necessary, and ethyl alcohol Present address, Phillips Petroleum Co., Bartlesville, Okla. 1
1880
ANALYTICAL CHEMISTRY
IO, Ohio
Need for Restricting Volume of Solution
Final Volume, M1.
F Taken, Mg.
F Found, Mg.
Error, Mg.
200 100 50 50 30 30
100.0 100.0 100.0 50.0
95.2 98.5 100.2 49.7 50 :1 24.9
-4.8 -1.5 +0.2 -0.3 +0.1 -0.1
50.0
25.0
combining the components produces a precipitate that may be filtered and washed without difliculty. RECOMMENDED PROCEDURE
Adjust the neutral fluoride solution containing from 20 to 200 mg. of fluorine to a volume of from 15 to 40 ml., depending on the probable amount of fluorine present. Heat to 70” C. and add slowly with stirring an equal volume of a solution of lithium chloride, 30 grams per liter, in 95% ethyl alcohol. Allow the mixture to cool to room temperature and stand until the supernatant liquid is clear. Filter the mixture through a weighed porcelain filter crucible of medium porosity using a rubber-tipped stirring rod as a transfer aid. After all the liquid has passed through the crucible, wash the precipitate with 50% ethyl alcohol saturated with lithium fluoride until the washings give a negative test for chloride with silver nitrate test solution. After the final portion of wash solution has p a d through the crucible, wash the precipitate with 5 ml. of 95% alcohol. Dry the crucible and precipitate a t 110” C. for 1 hour. Multiply the weight of the precipitate by 0.7325 to obtain the weight of fluorine. Use of Glassware. I n the preliminary experiments precipitations were first made in polyethylene beakers because glass or porcelain vessels were attacked by fluoride, preventing accurate determinations. However, polyethylene beakers, were found t o be unsatisfactory for accurate determinations by this method because the precipitate hati a tendency to adhere to the walls of auch beakers, and ita close similarity to polyethylene in color and
