ANALYTICAL EDITION
November, 1945
plete details were available as to the deviations from the recommended assay procedure, as practiced in some collaborating laboratories, it would be difficult to conclude from such data that any one deviation was solely responsible for the inconsistencies in assay values. It is realized that all vitamin assay laboratories are not similarly equipped, that they do not all have the same intensity of illumination, that different grades of reagents and solvents are frequently used in different laboratories, that different types and makes of instruments are used in measuring vitamin concentration, and that laboratories frequently differ in the facilities available for storing or holding vitamin-containing foods, extracts, solutions, etc. The authors believe that variations of this type may be reflected in the final results of the assay and that the inconsistencies in the data reported are not always the fault of the method. Investigators who do not have the facilities recommended by the author of the method frequently proceed to use available facilities and materials without fully realizing that they are not complying with the requirements of the method. There have been proposed certain changes or modifications of some of the assay methods employed in these studies which are supposed either to increase the accuracy of the methods or increase their application to a wider range of food products. I n some instances these modifications have tended to simplify the
Copper
Alkyl
713
method and a t the same time increase its usefulness. There are, however, investigators who insist that a t least some of the suggested revisions offer no significant advantage over the original method. The fact that the methods cited in this manuscript continue to be used rather extensively in their original form is proof in itself that the proposed rnodificatiow are not too genetally accepted. LITERATURE CITED
(1) Clifcorn, L.E., J . Nutrition, 28, 101-5 (1944). (2) Conner, R. T., and Straub, G . J., IND.ENQ.CHBM.,ASAL. ED., 13, 385-8 (1941). (3) Moore, L. A., and Ely, R., Ibid., 13, 600-1 (1941). (4) Morell, 9.A,, Zbid., 13, 793-4 (1941). (5) Pennington, D., Snell, E. E., and Williams, R. J., J . Biol. Chem., 135, 213-22 (1940). (6) Snell, E. E., and Strong, F. M., IND.ENO.CHEX.,ANALED., 11, 346-50 (1939). (7) Snell, E. E., and Wright, L. D., J . Biol. Chcm., 139, 675-86 (1941). SUPPORTBD in part by a grant from the National Canners Association-Can Manufacturers Institute Nutrition Fund. Authorised for publication Jan. 22, 1945, as Paper 1267 in the journal series of the Pennsylvania Agricultural Experiment Gtstion. This is the eleventh in a aeries on Nutritive Value of Canned Foods. Previous papers have appeared in the Journal of Nutritior, and the Journal of the American Dietetic Association.
Phthalates for the Estimation of Mercaptans ERNEST TURK AND E. EMMET REID
Hercules Experiment Station, Hercules Powder Company, Wilmington 99, Del. The preparation and use
of copper alkyl phthalates for the titration
of mercaptans are described.
These reagents, because of their definite composition, are an improvement over cupric oleate and naphthenate previously described. Copper butyl phthalate may be used as a standard substance after a given lot of the reagent has been analyzed. Results on'a variety of alkyl and terpene mercaptans are in good agreement with those obtained by the usual iodine titration, Thioglycolic acid and dithioethylene glycol are not amenable to titration with copper alkyl phthalates. Hydrogen cyanide, thiocyanates, organic sulfides, thiocyanoacetates, and terpenes do not interfere.
T
HIS paper describes the preparation of copper alkyl phthalates and their use for the titration of mercaptans. These reagents prepared from alcohols containing five carbon atoms or less have the advantages over cupric oleate and naphthenate described by Bond (1) that they are more easily prepared and have definite compositions. The compounds prepared from higher alcohols do not possess these advantages to such a marked degree. The method employing the copper alkyl phthalates is essentially that of Bond (1). A standard solution of the cupric salt in alcohol or naphtha is run into a solution of the mercaptan in the same solvent until the green color of the reagent is no longer discharged by reaction with the mercaptan to form the corresponding disulfide according to the equation 2Cu + +
+ 4RSH -+ 2CuSR + RS-S-R
+ 4H
per butyl phthalate, which is a dry crystalline solid, is preferred. After a given lot of this reagent has been analyzed, a standard solution may be made up by weighing the desired amount into R volumetric flask and making up to the mark with a solvent, such as Pentasol or butanol. If the titrations are to be made in hydrocarbon solvents, copper octyl phthalate, which is very soluble. in hydrocarbons, may be more convenient, although copper butyl phthalate can be used satisfactorily. The n-octyl salt is waxy, and solutions of it are standardized after being prepared. For economy 2-ethylhexanol may be used instead of n-octanol, but its copper salt is soft and not so convenient to handle. It was thought that ferric butyl phthalate might undergo reduction similarly with a change in color, but this wm not found to be the case. The intense red-brown color of the reagent did not change upon addition to a mercaptan solution. MATERIALS
The phthalic anhydride to be used in preparing the reagents must not be hydrated by exposure to moisture. A sample of it on being heated in a test tube should be clear at about 131' C. ; otherwise it will not combine proper1 Pentasol was used in this work as the solvent for copper gutyl phthalate, and HiFlash naphtha for copper octyl phthalate. Kerosene also may be used as a solvent for the latter. The mercaptans analyzed were commercial products, and experimental samples prepared in the authors' laboratories. In general they were used without careful purification.
.
PREPARATION OF COPPER ALKYL PHTHALATES +
There are two important advantages of this method over the usual iodine titration (a); the reaction takes place in a homogeneous solution, and there is no interference by unsaturates. Thus, it is suited for following the addition of a mercaptan to an unsaturate. The use of copper butyl phthalate and copper octyl phthalate, which are representative of the copper alkyl phthalates, is described. When titrations can be made in alcoholic solvents, cop-
COPPERBUTYL PHTHALATE. A mixture of 74 grams (0.5
mole) of finely round phthalic anhydride and 50 ml. (46.2 ml. = 0.5 mole) of n-[dutanol is heated in a 500-ml. flask over a free flame with shaking until the temperature is about 105' C. The agitation is continued as the flask is removed from the flame. The temperature continues to rise to about 120" C. and the mixture becomes clear within a few minutes. As sa011 as the product is cool, it is poured into a solution of 20 grams of sodium hydroxide in 1500 ml. of water. If the resulting solution is not acid to litmus, it should be made so with acetic acid. The acid solution is filtered through a folded filter into a 4-liter beaker. A clesr
INDUSTRIAL A N D ENGINEERING CHEMISTRY
714 Table
I.
Analyses of Mercaptans by Copper Alkyl Phthalate and Iodine Methods
-
Per Cent -SH
Substance
Calcd.
%
Iodine, found
%
..
Co per butyl pgthalate, found
copperoctyl phthalate, found
%
%
%
Pentadecanethiol-8O 13.5 .. 13.6,13.4 13.2,13.7 22.3 Butyl thioglycolatea 22.0,22.0 1 9 . 4 18:8,18:6 19.2,19.2 Terpene mercaptans 19:5,16:5 3 6 . 7 34.0,34.O, 3 4 . 8 3 3 . 9 , 3 3 . 8 n-Butyl mercaptan 34.6,34.7 3 6 . 7 3 4 . 5 , 3 5 .O 34.8,34.8 Isobutyl mercaptan tat-Butyl mercaptan 3 6 . 7 35 : 7 , 3 4 :7 14:9,14:7 1 6 . 3 14:6,14:6 L a w 1 mercaptan 12.8 1 2 . 3 , 1 2 . 1 ,1 2 . 2 i i : g , i i : i i Cet yi'mercaptan 42.3 4 i :3,4i:4 41.3,41.2 0-Mercaptoethanol . . .. 19.4 18.5,18.7 Terpene mercaptan .. 19.4 Terpene mercaptanb 17:9,17:9 .. Methyl thioglycolate 3 1 . 1 . . .. 30:4,30:2 0 These compounds were laboratory preparations which were carefully purified. All others were either commercial products or compounds prepared in the author's laboratory and used without careful purification. Data on impure compounds are presented to show reproducibility of the method, and not for comparison between calculated % -5H and that actually found. b Tertiary mercaptan.
.. ..
.. . .
solution of 65 grams of copper sulfate (CuSO4.5Hz0, calculated for 0.5 mole, 62.5 grams) in 500 ml. of water is added slowly with vigorous stirring. The precipitated cupric butyl phthalate is collected on a Biichner funnel, washed thoroughly with water, and air-dried. It is pulverized and dried in a vacuum desiccator. The yield is 95%. The purity of the product is determined by dissolving a weighed sample, 0.3 to 0.5 gram in 5 ml. of glacial acetic acid, diluting with 50 ml. of water and titrating iodometrically. The purity is 95% or higher. The dry reagent has been kept for more than a half year in a stoppered bottle without deterioration. COPPEROCTYLPHTHALATE. This reagent is prepared in exactly the same way as cop r butyl phthalate, except that 68 grams (calculated 65 gramsyof octanol are used in place of the butanol. Copper octyl phthalate is not so easy to filter and dry as the butyl compound. If desired, the mother liquor may be poured off, and the product washed by decantation and dissolved in a high-boiling hydrocarbon. A suitable portion of the clear solution may be diluted to prepare a standard solution. STANDARDIZATION OF THE SOLUTIONS
For a 0.1 N solution of copper butyl phthalate, 25.29 grams of the salt are required. If P is the percentage purity of the salt 25.29 x 1OO/P grams should be taken. This amount is weighed into a volumetric flask, dissolved in 50 ml. of acetic acid, and diluted to 1liter with Pentasol or some similar solvent. Standardization of, and titrations with, the solution should be made at nearly the same temperature; however, a difference of 10' C. between the temperature at standardization and that during the titration causes an error of only 0.1 ml. in a 10-ml. titration. For a 0.1 N solution of copper octyl phthalate, the calculated amount is 30.9 grams. Somewhat more than this amount is dissolved in a hydrocarbon and made up to 1 liter. The solution is standardized by hydrolyzing a suitable volume with dilute hydrochloric acid and determining the cupric ion iodometrically using the procedure of Weatherburn, Weatherburn, and Bayley (3). I n this case the normality is: M1. of Na&Oa X N X 2 = ml. of Cu Solution
cu
The factor 2 arises from the fact that only one half of the merca tan is converted to the disulfide by the cupric ion. !'he copper alkyl hthalate solutions should be stored in a dark bottle or cupboarcf to retard peroxide formation. Solutions which contain considerable precipitate after having stood for some time should be discarded. METHOD OF ANALYSIS
The analytical procedure is essentially that described by Bond (1). A sample assumed to contain 0.1 to 0.3 gram of mercaptan according to its molecular weight, is added to 50 ml. of Pentasoi or hydrocarbon solvent in a 125-ml. Erlenmeyer flask and titrated by the addition of copper alkyl phthalate in increments of about 0.5 ml. to near the end point u-ith shaking. The solution may darken somewhat during the titration, but in all cases it brightens and becomes clear just before the end point, which occurs when the blue-green color of the reagent persists. The end point can be observed most readily when the flask is illuminated and viewed against a white background.
Vol. 17, No. 11
In some cases, principally with low-molecular-weight mercaptans, the yellow cuprous mercaptide precipitates out, but in others it remains in solution, In either case, the green color is easily detectable. I t takes an appreciable amount of the copper solution to impart a definite green color to 50 ml. of solution. This blank should be determined by each analyst under his particular operating conditions and subtracted from the buret reading to get the volume of solution required by the mercaptan. In the analyses given below, this blank m'as 0.3 ml. According to the reaction above, 1 ml. of 1 N co per alkyl phthalate solution corresponds to 0.0331 gram of -kH. If V ml. of a solution of N normality is required for W gram of sample then: (V-blank) X N X 0.0331 X 100 = % -sH W RESULTS
A wide variety of mercaptans, including primary, secondary, and tertiary compounds, were titrated with the copper alkyl phthalates. In all cases the reaction was rapid and complete. Two mercaptans, thioglycolic acid and dithioethylene glycol, were not amenable to titration by this method. Thioglycolic acid reacted normally at first to give the cuprous mercaptide, which then reacted with excess cupric ion to give the dark blue insoluble cupric cuprous mercaptide. A sharp change of color from yellow to blue did not occur. Methyl and butyl thioglycolates, however, behaved normally. Dithioethylene glycol formed a stable, dark blue cupric salt upon addition of copper alkyl phthalates, and reduction to the cuprous state did not occur. Table I gives a comparison of the results obtained using copper alkyl phthalate solutions with those found by the iodine method of Kimball, Kramer, and Reid ( 2 ) . The copper reagents were standardized by shaking known volumes of their solutions with dilute hydrochloric acid and titrating the liberated cupric ion iodometrically. There is substantial agreement between the results with copper alkyl phthalates and those with iodine. The variations of individual determinations from the mean are about the same. A solution calculated to be 0.1151 N was prepared by dissolving 3.036 grams of copper butyl phthalate (95.9% pure) in sufficient Pentasol to give 100 ml. With this solution, n-butyl mercaptan and cetyl mercaptan were found to contain 34.1 and 33.8, and 12.3 and 12.0% S H , respectively. A second solution, 0.1185 N , similarly prepared from the same lot of copper salt 3 months later, gave 13.6% -SH for pentadecanethiol-8. These results are in good agreement with the values recorded in Table I and show that copper butyl phthalate can be used as a standard substance once its purity is known. Hydrogcn cyanide, organic thiocyanates, organic sulfides, thiocyanoacetates, and terpenes were added to solutions containing known amounts of mercaptans which were then titrated. No interference was observed when any of these substances was present in amount five times that of the mercaptan present; this demonstrates that mercaptans can be determined in the presence of these compounds. The method is not applicable when hydrogen sulfide is present in the mercaptan solution. SUMMARY
Copper butyl phthalate and copper octyl phthalate have been used as reagents in the titration of a variety of mercaptans. The preparation of these new analytical reagents is described. Copper octyl phthalate is a waxy material while copper butyl phthalate is a crystalline solid which may be used &s a standard substance and is, therefore, superior to cupric oleate previously described. Ferric butyl phthalate was investigated and found to be unsatisfactory as a reagent for the titration of mercaptans. LITERATURE CITED (1) Bond, G. R.,IND.ENQ.CHEM.,ANAL.ED., 5, 257 (1933). (2) Kimball, J. W., Kramer, R. L., and Reid, E. Emmet, J. Am. Chem. SOC.,43, 1199 (1921). (3) Weatherburn, A. S., Weatherburn, M. W., and Bayley, C. H., I N D . ENG.CHEM., -4N.4L. ED., 16, 703 (1944).
