1599
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 work with titanium and iron. Before the samples were drained into the electrolysis cell from the Jones reductor, 90% of the theoretical quantity of ceric ion required for the titration of titanous ion was generated coulometrically, so that the reduced solution was caught in this and not in the pure cerous sulfate. This procedure necessitates knowing, within a reasonable range, the concentration of titanium in the sample. This means that either a preliminary titration must be performed, without the initial generation of ceric ion, t o obtain a very rough idea as to the amount of substance preqent, or the information must be obtained by some other means. The data in Table I1 show that in mixtures of iron and titanium, amounts of titanium ranging from 0.013 to 0.16 meq. can be determined with an arcuracy of within i.0.6% or better. I n these solutions 0.06 to 0.12 meq. of iron ran be determined with an accuracy of within &0.80% or better. The total amount of the sample present can b e establiqhed within a range of 0.66% or better. Traces of dissolved oxygen may account for the limits of pre(*isionand accuracy that are observed. Sulfuric acid is suspected of possible contamination bv traces of redox systems. Redistillation of the acid does not eliminate fluctuations in the reagent hlnnks that are observed in various procedures.
microcoulometric titrations are undertaken. Since the limiting factor with amounts of titanium larger than 0.1 meq. appears t o be the solubility of titanium sulfate, there is no reason why samples of this magnitude should have to be encountered. Through appropriate dilution of the sample solution, the amount of titanium to be determined can be brought within the range of concentrations found t o be satisfactory in this Tvork. Mixtures of titanium and iron can be determined coulometrically x i t h electrolytically generated ceric ion with reasonably good accuracy and precision. However, unless there is knowledge of the approximate amount of titanium in the sample, it is necessary to run a preliminary coulometric titration, and in a subsequent titration to generate about 90% of the required ceric ion before addition of the sample to he analyzed. LITERATURE CITED
Rricker, C. E., and Sweetser, P. B., A N ~ LCHEM., . 25, 764 (1953). Cooke, W. D., Reilley, C. S . ,and Furman, S . H., Ibid., 23, 1662 (1951). Ibid., 24, 205 (1952). Dilts, R. T., and Furman, Y. H., Ibid., 27, 1275 (1955). Furman, S . H., editor, "Scott's Standard Methods of Chemical Analysis," 5th ed., p. 984, Van Kostrand, Sew York, 1939. . Furman, iY.H., Cooke, K.D., and Reilley, C. N., A N ~ LCHEM., 23, 945 (1951). Shippy, B. A,, Ibid., 21, 698 (1949). and Peterson, H. E., IbLd., 24, 1175 (1952). Sill, C. W., Takeno, R. J., Chem. SOC..Japan, 5 5 , 196 (1934).
CONCLUSIOhS
Amounts of titanium alone can be determined coulometrically with ceric sulfate over fairly wide ranges of concentration with a n accuracy of within &0.6%. There appears t o be no reason why samples smaller than one microequivalent of titanium could not b e determined, provided that the precautions necessary for
R E C E I r E n for review March 24, 1955. Accepted July 20, 1955. BaPed upon a dissertation submitted by Robert V. Dilts in partial fulfillment of requirements for degree of doctor of philosophy a t Princeton Cniversity, 1954.
Direct Determination of Acetic Acid in Acetic Anhydride J. H. MCCLURE, T. M. RODER, and R. H. KINSEY' Polychemicals Department,
E. 1.
du Pont d e Nemours & Co., lnc.,Wilmington, D e l .
This investigation was undertaken to develop a rapid and accurate method for the determination of acetic acid in acetic anhydride. It has been extended to other acids in their anhydrides and other acids in acetic anhydride. Two methods are described w-hich depend on the reaction of a tertiary amine (triethylamine) with the free acid in the anhydride. The first method depends on the color change of methjl red at the end point, while the second measures the temperature rise when the reagent is added to the sample. A precision of & 0 . 0 7 7 ~absolute over the range 0.5 to 3.5'70 acid was obtained for the visual method and i 0 . 0 9 % absolute for the thermometric method over the range 0.8 to 5 . 5 7 ~acid. Potentiometric titration of acetic acid in anhydride using combinations of glass, calomel, gold, tungsten, silver, silversilver chloride, platinum, and antimony electrodes was unsuccessful.
T
HE literature lists only a few methods which can be used for the determination of acids in anhydrides. Sicolas and Burel (2) used aniline to produce the anilide from the anhydride, thus providing a method for calculating the acid and anhydride b y titrating the sample before and after anilide formation. Smith and Bryant ( 4 ) also performed two titrations, one with methanolic sodium methylate and one with aqueous caustic. Froin the-e data, the acid and anhydride can be calculated. As 1
Present address, Polychemicals Department, E. I. d u Pont de Nemours 6
Co.. Inc., Sabine River Works, Orange, Tex.
both of these methods are indirect, they are not very satisfactory for determining small amounts of acid in anhydride. Siggia and Floramo (3) used potentiometric titration of the acid with a tertiary amine in a solvent such as acetone. Their method is limited to acids with a pK, 3 or greater. Thus, acetic acid cannot be determined b y their procedure. The visual titrimetric and thermometric methods presented here permit the determination of acids with dissociation constants a t least as small as that of benzoic acid (6 X 10-6); they also reduce the analytical time and tend to eliminate the lack of precision arising from the difference in large numbers in the Sicolas-Burel or Smith-Bryant methods. I n the visual method triethylamine in benzene is used as titrating agent. Acetic anhydride, as the solvent, appears to enhance the acidity of the acetic arid t o the point nhere it will yield a color change with methyl red. The molarity of the triethylamine-benzene reagent is determined by titration against standard hydrochloric acid, and consequently, the reagent must be free of primary and secondary amines. The purification step is relatively. simple and involves treating the triethylamine with excess acetic anhydride to remove primary and secondary amines, neutralizing the excess anhydride with sodium hydroxide, and extracting the amine into benzene. The benzene-amine mixture is dried b y distillation, which removes the benzene-Lyater gzeotrope. The amine-benzene azeotrope is then collected, diluted with dry benzene, and standardized. The titration of a sample is made t o a n end point which matches the color of methyl red in benzene. The thermometric method IS entirely empirical and involves
1600
ANALYTICAL CHEMISTRY
Table I. Sample so. 1
Precision of \-isual llethod for Determination of Acetic 4cid in -4cetic Anhydride %
.kcon
Found
3 4
.4v.
0
0.606
0.044
2.2s4
0.102
3.364
0.075
n ti22
I)
0.632
5 6
7 8 0
li, 11 1.7 13 I4 1._ .i
16
2QR - _-I)
2.120 2,346 2.346
3.277 3.419 3 323
3 434
Standard deriation for all samples 0 . 0 7 0 % absolute.
Table 11. Recoveries of Added ..icetic Acid by Visual &lethod .4cOH .4dded, Grams 1 .65 2 36 3.42 Arerage of four results.
.4cOH Found, Gramsa 1.53 2.3s 3.51
A
+0.10 -0.02 -0.09
the measurement of the change in temperature when reagent grade triethylamine is added t o arctic anhydride in a Dewar flask. Since the method is empirical, any water and/or primary and secondary amines in the reagent will have the same effect on the standards as on the samples and consequently no purification of the reagent is needed. -4potentiometric study of the reaction was attempted using a Beckinan Model H-2 p H meter and all possible combinations of glass, calomel, gold, tungsten, silver, silver-silver chloride, platinum, and antimony electrode.. The only combination M hirh n a s found to show any promise was glasp-silver. Horvever, the breaks obtained were not reproducible. S o potentionieti ic ptudy of any si-stem except acetic acid-anhydride was made.
of acid into another 250-ml. Erlenmeyer flask. Add 5 drops of methyl red indicator solution per 100 ml. of sample and titrate immediately with triethylamine-benzene reagent unt,il the solut,ion color matches t,hat,of the indicator in benzene. The color is st,able in benzene, but it will change rapidly in acetic anhydride after the end point has been reached.
Discussion of Visual Method. Initial studies concerned the stiochiometry of the reaction. By titrating known amounts of wetic acid in anhydride it x a s determined that 1 ml. of hnse was equivalent to 0.4184 gram of acetic acid. Standardization of the base against aqueous hydrochloric acid showed it to be 2.2294.11. From these data it can he shown that the ratio of acetic arid t,o trieth?.lamine is 3.13 to 1. Thus, within experimental error, the equation for the reaction appears to be 3AcOH
+ S(CyHj)a
+
N(C~H~)~.~ACOH
S o attempt was made to characterize the product further. Iiaufman and Singleterry ( 1 ) gave an excellent bibliography on this type of problem and indicated a precedent for this stoichiometry. The indicator, methyl red, was chosen after consideratiori indicated that the indicator must contain no group3 which are readily acetylated such as hydroxy1 or primary and secondary amino groups. Other azo indicators and the indicator quinoline Iihe (1,l '-diisoamyl-4,4'-quinoc>-anine iodide) Il-ere a1r.o t lied I L P I V ~ Pa mixed indicator ineth3-1 red-quinoline blue: none \vas ,mitable. With the exception of methyl red all the indicators tried !yere already in the basic form x-hen added to the anhydride mixture or would give no color change in the medium. Table I shows the precision of the visual method over the range 0.6 to 3.4% acetic acid. The standard deviation for the sixteen result? is O . O i % absolute. Table I1 indicates the arcuracy of the method in t e r m of acid recovered from known sainple~. .4n extension of the method was made t o the determination of acetic arid in acetic anhydride-hydrocarbon sample.. In this
VISUAL JIETHOD
Reagents. Triethylamine in benzene, 2-11. Dry the contents of a 500-gram bottle of C.P. triethylamine with potassium hydroxide pellets overnight. Decant into a 1-liter flask and cautiously add 25 ml. of acetic anhydride. (This will generally provide a sufficient excess of anhydride.) Reflux the mixture for 30 minutes. Allow to cool. Add 125 ml. of 10M sodium hydroxide with constant stirring. (The small temperature rise which accompanies this addition is an indication that an excess of acetic anhydride was used.) -4llow to stir for 10 minutes. Pour the mixture into a separatory funnel and add 500 ml. of benzene. Shake thoroughly, and d r a n off and discard the xater layer. Wash the benzene layer u i t h another 125 ml. of sodium hydroxide. Transfer the triethylamine in benzene solution to a distilling flask fitted with a simple condenser set for distilbtion. Distill at least 200 ml. and discard (temperature >81 (3.). Replace the receiver and condenser n ith dry apparatus and distill until 100 ml. remain in the pot. Protect the receiver cvith an Ascarite-Drierite tube a t all times. Dilute the distillate n i t h an equal volume of dry (
