(14) I. M. Kolthoff and J. J. Lingane, "Polarography", Interscience, New York, 1952. (15) M. Brezina and P. Zuman. "Polarography", Interscience, New 'fork, 1950. (16) P. N. Moorthy and E. Hayon, J. Am. Cbem. SOC.,97, 2048 (1975). (17) G.E. Adams and R. L. Wilson, J. Chem. SOC.,Faraday Trans. I , 69, 719 (1973). (18) J. R. Bolton, A. Carrington. and J. dos Santos-Veiga, Mol. Pbys., 5 , 465 (1962). (19) B. L. Barton and G. K . Fraenkel, J. Chem. Phys., 41, 1455 (1964). (20) H. Zeldes and R. Livingston, J. Phys. Cbem., 76, 3348 (1972).
(21) (22) (23) (24) (25) (26) (27)
H. Zeldes and R. Livingston, Mol. Phys., 27, 261 (1974). P. N. Moorthy and E. Hayon. J. Phys. Cbem., 7 6 , 2615 (1974). P. N. Moorthy and E. Hayon, J. Org. Chem., in press. U. Bruhlrnann and E. Hayon, J. Am. Cbem. Soc., 96,6169 (1974). M. Simic and M. Ebert, Int. J. Radlat. Phys. Chem., 3, 259 (1971). P. S. Rao and E. Hayon. J. Phys. Cbem., 79, 1083 (1975). M. Simic, P. Neta, and E. Hayon. J. Pbys. Chem., 73, 3794 (1969).
RECEIVEDfor review July 31, 1975. Accepted December 3, 1975.
Determination of Olefins by Direct Titration with Bromine in Propylene Carbonate Byron Kratochvil,* P. K. Chattopadhyay, and Richard D. Krause' Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Bromine In propylene carbonate was studied as a reagent for the determlnation of unsaturation by dlrect potentlometric titration. The bromlnation of cyclohexene, allyl alcohol, vinyl butyrate, and mixtures of 1- and 2-octenes was investigated at several temperatures; 25 "C was found to be optimum. Some substitution occurs with most aliphatic compounds. iodine values for oleic and linoleic acids and thelr methyl esters agree well wlth the iodine monochloride procedure, as do results for several vegetable oils. The stability of solutions of bromine in propylene carbonate, and the convenience of direct titration without need of a catalyst are advantages of the method.
Most classical chemical methods for the determination of olefinic unsaturation involve addition of a halogen across the double bond. Bromine is the most useful halogen for this purpose, chlorine being so reactive that extensive substitution is common. Iodine does not usually add readily, although the interhalogen compounds, iodine monochloride and iodine monobromide, have been utilized with some success ( I ). The bromination procedure commonly employed is addition of a known excess of bromine, followed after a suitable reaction time by back titration with standard thiosulfate of the iodine liberated on addition of excess iodide. For the determination of unsaturation in petroleum products, reagents employed include bromine in acetic acid ( 2 ) and in carbon tetrachloride ( 3 ) , aqueous bromate-bromide ( 4 ) ,tribromide in methanol (5), coulometrically generated bromine (6-9), and pyridinium bromide perbromide (10).In some cases, direct titrations have been investigated with the end point being determined either visually by the appearance of excess bromine ( Z ) , amperometrically (6, I ] ) , by the "dead stop" electrometric method (6, 12, 13), by constant-current potentiometry (7, 9 ) , and by spectrophotometry (8, 14). In the pharmaceutical field, the interhalogen compounds have been more popular. For example, in the determination of the iodine value (grams of iodine, or its equivalent, taken up by 100 g of sample) of fixed oils or fats, the official method of the U.S. Pharmacopeia employs iodine monobromide in glacial acetic acid (the Hanus method) (15), while that of the British Pharmacopoeia uses either iodine Present address, Calgary Medical Laboratories, 1638-10th Ave., S.W., Calgary, Alberta, Canada T 2 P 2M7. 568
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
monochloride (Wijs solution) or pyridinium sulfate perbromide in a mixture of glacial acetic acid and carbon tetrachloride ( 1 6 ) .In each case, excess reagent must be determined after a reaction time of 30 min or so, and the amount in excess must not be too large. A procedure using addition of excess bromine to determine unsaturation in fats and oils in a chloroform-dioxane solvent mixture has also been suggested (17). A direct titration of fats and oils with bromine in glacial acetic acid gave results that were low compared with Wijs method for most vegetable oils (2). None of the other direct titration studies investigated fats and oils, although several included oleic acid among the compounds studied. The use of propylene carbonate (PC) as a solvent for a number of analytical bromine substitution reactions has been described (18). Solutions of bromine in pure PC are relatively stable if stored in well-stoppered containers and protected from light; a decrease in titer of about 0.3% a day is observed. This work has been extended here to the investigation of PC as a medium for the direct titration of olefins, especially unsaturated fatty acids and oils, without the addition of a catalyst such as mercury(I1).
EXPERIMENTAL Propylene carbonate (Jefferson Chemical Co.) was purified as previously described (18). Chloroform (Fisher Chemical Co.), ACS grade, was distilled twice from calcium hydride. Tetraethylammonium perchlorate was prepared by neutralizing a solution of tetraethylammonium hydroxide with perchloric acid, recrystallizing the salt twice from freshly boiled distilled water, and drying 24 h under vacuum a t 60 "C. Linoleic acid and oleic acid (Fisher) and methyl oleate and methyl linoleate (Analab) were used as received. All other chemicals were reagent grade and were used as received. The cell, automatic titrator, and potentiometric end-point detection apparatus used for the titrations, and the procedure for the preparation and standardization of the bromine titrant, were as described before (18). For the determination of unsaturation of oils, a sample of from 0.1 to 0.5 g of commercial oil wa6 weighed into a dry titration cell, dissolved in a 2:1 PC-chloroform mixture, deaerated with nitrogen for about 10 min, then titrated with standard 0.5 M bromine in P C at a rate of about 2 mlimin; this rate automatically decreased in the region of the equivalence point. Titrations of other olefins were performed similarly, but in pure PC. T h e solutions in all cases were protected from direct exposure to light.
RESULTS AND DISCUSSION Results for the direct titration of several olefinic compounds a t 25 "C are shown in Table I. The change in poten-
Table I. Percentage Recoveries in Titration of Several Olefins with Bromine in Propylene Carbonate Compound
Percent recovery at 2 5 'C
Cyclohexene 99 Vinyl butyrate 95 100 1-and 2-Octene (mixture of isomers) Allyl alcohol 113 Allyl amine 118 Allyl ether 109a a Calculated on basis of 2 mol bromine consumed per mole of ether.
-0 .c c
0, c
0
a
Table 11. Effect of Sample Size on Percentage Recovery of Oleic and Linoleic Acids by Direct Br, in PC and Indirect IC1 Methods0 I
I
Compound
Sample size, g.
% Recovery b y Br, in PCb
9% Recovery by IClC
Oleic acid
0.1 107 104d 0.3 106 111 0.5-1.2 105.5 * 0.2 105 0.5-1.2 107.1 i 0.2 (0 " C ) ... 0.5-1.2 107.5, 107.8 ( 5 0 ° C ) ... 1.2-2.0 104 104 Linoleic acid 0.1 89.3 8 5d 0.2 88.7 90 0.35-0.4 84.2 r 0.2 86 0.65 81.5 ... a Temperatures controlled at 25 r 1 "C unless otherwise indicated. Individual values listed for one or two titrations, average and standard deviation for three or more. b Sample dissolved in 2 : l PC-CHC1, mixture. C Procedure of British Pharmacopoeia (16). Sample dissolved in 10 ml CCl,, 20 ml of 0.6 M IC1 added. Each value is the average of two titrations. d Sample dissolved in 1 0 ml CCl,, 20 ml of 0.2 M IC1 added.
tial in the region of the end point for the titrations is on the order of 150 t o 200 mV. Rates of reaction were rapid; titrations typically required only 5 to 6 min to complete without use of a catalyst. The high recoveries obtained in several instances may be attributed to the occurrence of substitution reactions as well as addition; substitution tends to take place to a greater extent in polar solvents than in nonpolar ones (19,ZO). In PC, substitution of a bromine atom releases a molecule of HBr. Measurement, therefore, of the bromide ion concentration after reaction to estimate the extent of substitution was explored, since potentiometric titration of bromide salts with silver perchlorate in PC had been shown t o be feasible (18). In all cases, high results were accompanied by the formation of HBr, but the amount of bromide found after correction for a titrant blank was always higher than predicted and could not be used as any more than a rough measure of the extent of substitution. A study of the effect of temperature on the reaction stoichiometry of the compounds in Table I showed that bromine consumption generally increases a t temperatures above or below 25 "C. Below 25 "C, the rate of substitution becomes faster relative to the rate of addition, but the temperature at which this increase appears varies with the compound. For allyl alcohol and a mixture of 1- and 2-octenes, the percentage recoveries were little changed between 0 and 25 "C but increased about 20% a t -20 "C; for vinyl butyrate, recoveries were unchanged down to -20 "C but increased about 60% a t -45 "C; and for cyclohexene, recoveries increased about 7% a t 0 "C, and about 30% a t -20 "C. Above 25 "C, results were less precise, but no over-
2
4
Volume 0.5
8
6 Br, in
I
PC , m l
Figure 1. Direct titration of linoleic and oleic acids esters with bromine in 70 % PC-30 % CHC13
and their methyl
all trend was seen; a t 50 "C, for example, bromine consumption by vinyl butyrate was about 10% below, and by allyl alcohol about 40% above, that predicted for addition only. Determination of Unsaturation of Oils and Fats. Most oils and fats are not sufficiently soluble in neat PC to allow titration. Addition of 30% by volume chloroform provided sufficient solubility for all the systems studied without affecting potentiometric detection of the endpoint. Examples of titration curves of oleic and linoleic acids and their methyl esters are given in Figure 1. Linoleic acid, which has two double bonds per molecule, shows two potential breaks; the second break was used for all analytical calculations. Titrations of ten mixtures of oleic and linoleic acids in varying ratios gave overall recoveries within 1%of the sum of the individual acids. The percentage recovery is independent of the rate of titrant delivery up to a t least 2 ml/min, but does depend on sample size, smaller samples giving higher recoveries. A dependence on sample size also is observed in the analysis of samples of oleic and linoleic acids by the iodine monochloride procedure outlined in the British Pharmacopoeia ( 1 6 ) ,which involves adding excess iodine monochloride, waiting 30 min, then back-titrating the excess. In this method, the amount present in excess must not exceed 50% of the amount consumed, so that with samples of unknown composition, a trial run is necessary to determine the approximate quantity of IC1 to be used. Table I1 compares percentage recovery with sample size by the direct Bra in PC and indirect IC1 methods. Recoveries are similar for the two methods, and both give lower recoveries as the amount of sample taken is increased, although the direct Brz in the PC method is affected less. The IC1 method is also dependent on the amount of excess IC1 added; recoveries drop about 5% when the excess is reduced (Table 11). The percentage recovery for the titration of a 0.9-g sample of methyl oleate was 105.4, the same result as for oleic acid; while for a 0.9-g sample of methyl linoleate, it was 91.7; and for a 0.4-g sample, 94.8. These values for methyl linoleate are appreciably higher than for linoleic acid, perhaps because of higher purity of the ester. Commercial chloroform contains about 0.75% ethanol as preservative. Because ethanol undergoes bromine substitution readily, it must be removed from the chloroform solvent before use. The effect of oxygen on the titration was also examined. With both oleic and linoleic acids, deaeraANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
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Table 111. Comparison of Iodine Values of Vegetable Oils Determined by Br, in PC and IC1 Methods Iodine valueb Oila
"
I
I
I
I
Volume
I
I
I
I
I
L
0.5 M Br, in PC
Flgure 2. Titration of unsaturation in vegetable oils with bromine in 70% PC-3OYo CHCls
tion with nitrogen prior to titration gave slightly higher results, but the effect was small, and although oxygen was removed in all the titrations reported here, this step does not appear necessary in routine work. Vegetable oils contain a variety of fatty acids. Of the unsaturated acids, oleic and linoleic are the most common. Iodine values for a number of oils by direct bromination and by indirect iodine monochloride determination are compared in Table 111. The general shape of the bromine titration curves is shown in Figure 2; those oils with higher percentages of molecules containing two or more double bonds show evidence for two potential breaks. The results for the bromine method tend to be higher than for the IC1 method by about 5 to 10%. However, iodine numbers are useful primarily for relative comparisons among various oils and, for this purpose, a direct titration is more rapid and convenient.
LITERATURE CITED (1) A. Polgar and J. L. Jungnickel, in "Organic Analysis", Vol. 111, J. Mitchell, Ed., Interscience, New York. 1956, pp 206-255. (2) K. Uhrig and H. Levin, lnd. Eng. Chem., Anal. Ed., 13, 90 (1941). (3) P. C.Mcllhiney, J. Am. Chem. SOC., 16, 275 (1894). (4) A. W. Francis, lnd. Eng. Chem., 16, 821 (1926). (5) H. P.Kaufmann and E. Hansen-Schmidt,Arch. Pharm. (Weinheim), 263, 32 (1925).
Br, in PC method
IC1 method
Corn 145 131 Cottonseed 124 119 Linseed 168 168 Olive 91 87 Peanut 103 97 Rapeseed 125 118 (low erucic acid variety) Soybean 123 112 Safflower 143 139 a Commercial oils used as received. b Calculated as equivalent number of grams of iodine that would have been consumed by 100 g of oil. All values are the average of two titrations.
(6) F. A. Leiseyand J. F. Grutch, Anal. Chem., 28, 1553(1956). (7) F. Baumann and D. D. Gilbert, Anal. Chem., 35, 1133 (1963). (8) J. W. Miller and D. D. DeFord, Anal. Chem.. 29, 475 (1957). (9) W. Walisch and M. R . F. Ashworth. Mikrochim. Acta, 1959, 497. (10) T. Williams, J. Krudener, and J. McFarland, Anal. Chim. Acta, 30, 155 (1964). (11) I. M. Kolthoff and F. A. Bovey. Anal. Chem., 19,498 (1947). (12) B. Braae, Anal. Chem., 21, 1461 (1949). (13) H. D. DuBois and D. A. Skoog, Anal. Chem., 20, 624 (1948). (14) P. 8. Sweetser and C. E. Bricker. Anal. Chem., 24, 1107 (1952). (15) "United States Pharmacopeia", 18th Revision, Mack Publishing Co., Easton, Pa., 1970, p 905. (16) "The British Pharmacopoeia", The Pharmaceutical Press, London, 1968, pp 1266-1267. (17) F. Said, M. M. Amer, A. K. S. Ahmed, and A. A. Said, J. Pharm. Pharmacob, 16, 210 (1964). (18) R. D. Krause and B. Kratochvil, Anal. Chem., 45, 844 (1973). (19) J. Bloeseken and E. T. Gelber, Rec. Trav. Chim., 46, 158 (1927); Chem. Abstr., 21, 1628 (1927). (20) C. F. Van Duin, Rec. Trav. Chim., 45, 345 (1926); Chem. Abstr., 20, 2441 (1926).
RECEIVEDfor review August 19, 1975. Accepted December 8, 1975. Financial support by the National Research Council of Canada and by the University of Alberta is gratefully acknowledged.
Removal of Copper and Iron Prior to Water Hardness Titration James S. Fritz' and Jeffrey N. King
Ames Laboratory, ERDA, and Department of Chemistry, lowa State University, Ames, lowa 500 7 I
Silica gel was reacted with 3-aminopropyitriethoxysiiane or with the N-methyl derivative of the same reagent to produce a material with an amino silyl functional group. If a water sample in the pH range of 5.0 to 7.5 is passed through a short column of this material, iron(l1) and copper(11) are completely retained, while calcium(l1) and magnesium(il) pass through. This permits an accurate water hardness titration without adding any cyanide.
In the determination of water hardness by the EDTA titration method, several metal ions interfere by blocking the indicator ( I ) . In natural water supplies, the chief interfering ions are iron(I1) and copper(I1). Interference from these ions can be avoided by masking with cyanide, although re570
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 3,
MARCH 1976
action of cyanide with iron is sometimes incomplete and the end point of the titration is not as sharp as when iron is absent. Cyanide is extremely poisonous and thus its use may be hazardous, especially when large numbers of samples are being analyzed. The purpose of this study was to develop a quick, simple method for removing iron and copper from water so that water hardness titrations could be performed without the use of cyanide. Various workers have incorporated amine functional groups in silica material by reaction with different alkoxy silanes which contain an amine group. Thus, Leyden and co-workers (2-4) have shown that silica gel containing a chemically bonded organic amine or diamine will quantitatively retain metal ions such as mercury(II), copper(II), zinc(II), and manganese(I1). Dingman et al. ( 5 ) prepared
