Acetic Anhydride in Nonaqueous Titrimetry - American Chemical Society


ALCUIN F. GREMILLION1. Carbide and Carbon Chemicals Co., Division of Union Carbide and Carbon Corp., Texas City, Tex. Acetic anhydride has been used ...
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Acetic Anhydride in Nonaqueous Titrimetry Determination of Sulfuric Acid Catalyst in Ethyl Alcohol Esterification Mixtures ALCUIN F. GREMILLION' Carbide and Carbon Chemicals Co., Division o f Union Carbide and Carbon Corp., Texas City, r e x .

icetic anhydride has been used to determine sulfuric acid catalyst in industrial ethyl alcohol esterification rnixtures, and other solvents in the mixture have been studied. Acetic anhydride has been used as a solvent for the titration of a number of weak organic bases. Pyridine, some amines, and ureas have been titrated with considerable sharpness of end point, and in some cases large potential changes a t the end point. A technique for use of this solvent in titrating weak primary and secondary amine bases is applicable to the determination of some bases whose Kb values in water are 10-'6 or larger.

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concentration of sulfuric acid catalyst in continuous industrial ethyl alcohol esterification mixtures is important as iegards yield of ethyl acetate and the corrosion of some types of plant equipment. These esterification mixtures are composed in the main of varying amounts of ethyl alcohol, acetic acid, ethyl acetate, and water. A simple method of analysis for the sulfuric acid catalyst consists of the potentiometric titration of the sulfuric acid in a sample of esterification mixture with a standard acetic acid solution of sodium acetate after an equal volume of acetic anhydride has been added to the sample. The successful use of acetic anhydride in this instance has jJWmpted its use in the titration of a number of weak organir 1

1

Present address, Tulane University of Louisiana, New Orleans 18, La

2

I

I

I

6 8 MI. 0.1 N NoCZH30,

9

1

I

IO 12 in HOAC

I

I4

Figure 2. Titration Curves of Sulfuric Acid in Some Industrial Ethyl Alcohol Esterification Mixtures with Weight Per Cent of Water Shown Performed in presence of acetic anhydride

4-00-

bases including some with primary and secondary amine function. In addition, these results have been viewed as a part of the whole position of acetic anhydride in nonaqueous titrimetry (3-4, 6-8).

5 350v)

REAGENTS

1

Acetic acid, A.C.S. reagent grade. Acetic anhydride, A.C.S. reagent grade. Ethyl alcohol, Carbide and Carbon Chemicals Co. Ethyl acetate, Carbide and Carbon Chemicals Co. Sulfuric acid, Baker and Adamson, reagent grade. Sodium acetate, 0.1N, A.C.S. reagent grade in acetic acid. Allylthiourea, 0.02N, A.C.S. reagent grade in acetic acid. p-Aminodimethylaniline, 0.019N, technical grade in acetic acid. Dimethylaniline, 0.022N, technical grade in acetic acid. Diphenylamine, 0.02N, technical grade in acetic acid. Glycine, 0.02N, A.C.S. reagent grade in acetic acid. Pyridine, 0.018N, A.C.S. reagent grade in acetic acid. Urea, 0.02N, A.C.S. reagent grade in acetic acid. Perchloric acid, 0.1N, A.C.S. reagent grade in acetic acid.

I

5 i

8

300-

2

0

5

+

0

2

2 6 8 IO I MI. 0.1 N NaCZH302 in HOAc

PROCEDURE

Sulfuric Acid Determination. A 100-ml. sample of the esterification mixture is pipetted into a beaker containing a magnetic stirring bar. An equal volume of acetic anhydride is added. The sulfuric acid contained is titrated using a p H meter (Beckman, Model H2 glass electrode) fitted with a fibertip calomel and a glass elect,rode. A standard acetic acid solution of sodium acetate is used as titrant.

Figure 1. Titration Curves of Sulfuric Acid in a Low Water Content Industrial Ethyl Alcohol Esterification Mixture 9. Without acetic anhydride 5. With acetic anhydride C. With acetic anhydride plus 0.0286 gram of sulfuric acid

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ANALYTICAL CHEMISTRY

134

I n adding acetic anhydride to those esterification mixtures containing appreciable quantities of water or ethyl alcohol, a large amount of heat may be given off, so that it may be necessary to carry out the operation in a flask fitted with a reflux condenser.

Curve B represents the titration of the same material in the presence of 100 ml. of acetic anhydride and curve C is for the titration after some of the acetic anhydride had reacted with only 47.7% of the water present and undoubtedly some or all of the ethyl alcohol. Ethyl alcohol effects the titration in the same manner as does water. After the titration, 52.3% of the water was still present, as determined by the Karl Fischer method. The removal of some water from all of these systems tends to decrease the “leveling” power of the solvent. Total removal of water is not essential, but very large concentrations of water must be decreased somewhat before its adverse influence is sufficiently

650-

3001 I

I

I

I

I

I

600Figure 3. Titration Curves of Sulfuric Acid in a High Water Content Industrial Ethyl Alcohol Esterification Mixture Under various conditions of exposure in acetic anhydride

I n some cases the reaction between the acetic anhydride and the water and/or ethyl alcohol caused the generation of heat so that the mixutres came to a boil before the titrations were completed. However, in many cases duplicates of the titration mixtures remained as much as 15 to 30 minutes longer than would be necessary for titration before they came to a spontaneous boil. This boiling was preceded by a rapid change in temperature,

Titration of Amine Bases. I n the titration of a base containing primary or secondary amine function, an aliquot portion of an acetic acid solution of the base is added to 100 ml. of acetic anhydride a t 0 ” C. The base is then rapidly titrated with a standard solution of perchloric acid in acetic acid or acetic anhydride. For bases that do not readily react with acetic anhydride, the titration may be carried out a t room temperature. ROLE OF ANHYDRIDE I N SULFURIC ACID TITRATIOh

I n all cases the presence of acetic anhydride in the titration mixture is of value. Without the acetic anhydride the titration proceeds sufficiently well in some rases (Figure l ) , but in other cases (Figures 2 , 3 ) acetic anhydride must be added if the presence of the sulfuric acid is to be known. This has prompted an investigation of the influence of the constituents of the esterification mixture. Sulfuric acid titrates as a monobasic acid in the solvent systems used. The advantage of using acetic anhydride as a solvent in the titration of sulfuric acid is well illustrated in Figure 4. The influence of ethyl acetate is exemplified by the data of Figure 5 . These data were more readily obtained than the data in the ethyl acetate curve of Figure 4. An adverse effect is attendant with the presence of water as illustrated by the data of Figure 2 collected for several industrial esterification mixtures. The weight per cent of water for each case was determined by the Karl Fischer method. For the cases of the larger water concentrations this effect can be prevented by partial removal of the water through reaction with acetic anhydride. Three titrations performed on one esterification mixture containing 19.7y0water are shown in Figure 3. Curve A represents the titration performed in the absence of acetic anhydride.

InEtOAc \

2 - 5500

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3 500-

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Acz(

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0

2

+ I

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MI. 0.1 N Figure 4.

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8 10 12 IY NaCZH302 in HOAc

Titration Curves of Sulfuric Acid in Several Solvents

n

2-600 550

\ 2 Y 6 8 1 0 1 2 1 ’

0

MI. 0.1 N NaCLH302 in HOAc Figure 5 .

Titration Curves of Sulfuric .4cid in .-icetic 4cid-Ethyl Acetate Mixtures ITith per cent ethyl acetate shown

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V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5 removed. In some cases not all of the water originally present had been removed. Therefore, it may be that the results are not attributable to complete removal of water only. Russell and Cameron ( 7 )have concluded that "sulfuric acid, perchloric acid, and certain sulfonic acids show increased acidities in the presence of acetic anhydride which cannot be accounted for by dehydration of the solution."

Table I.

Base Dissociation Constants in Water Reference

Kb

Base Diphenylamine Glycine Pyridine Thiourea Urea

TITRATION OF BASES IN ACETIC ANHYDRIDE

The use of acetic anhydride as a solvent has permitted very sharp end points to be obtained in the titrations of some weak bases (Figures 6, 7 , and 8). In the titration of a tertiary amine there has been no difficulty due to reaction with acetic anhydride. However in the case of a primary or secondary amine the reaction with acetic anhydride has been prevented only by titrating in systems a t 0" C. This has been accomplished in four cases (Figures 6 and 7 ) . ACETIC ANHYDRIDE IN TITRIiMETRY

Acetic anhydride has been used as a dehydrating agent for either titrant ( 2 , 3 , 6) or some organic solvent ( 4 )or as a reactant for the removal of some interfering substance. Wagner, Brown, and Peters (8) have used it in this latter way in the determination of tertiary nitrogen. I n titrating some weak organic bases, Fritz and Fulda ( 4 )used acetic anhydride-nitromethane mixtures as the solvent. They concluded that completely anhydrous con-

Glycine

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2

+ I

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6 8 IO 12 MI. 0.02 N HClOq. in HOAc

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Figure 6. Titration Curves of Weak Bases in ..icetic Anhydride at 0" C.

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0

2

9

6

8

10

12

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MI. 0.02 N HCILOq in HOAc Figure 8. Titration Curves of Weak Bases at Room Temperature in Acetic Anhydride

MI. 0.02 N HCLQy in HOAc Figure 7 . Titration Curves of Weak Bases at 0" C.

A.

10 ml. of 0.019N p-aminodiniethylaniline in acetic acid added t o acetic anhydride 10 ml. of 0.018.%' pyridine in acetic acid added t o acetic anhydride C. 10 ml. of 0.022.X- dimethylaniline in acetic acid added t o acetic anhydride

B.

136

ANALYTICAL CHEMISTRY

Table 11. Titrations of Sulfuric Acid in Several Solvent Mixturesa Solvent Mixture, 100 MI. Acetic acid

Sulfuric Acid Content Used, Found, gram gram 0.057 0.053

Acetic acid 50% Acetic anhidride, 50%}

0.057

O.Ob7

0.057 0.171

0.058 0. I72

0.057 0.171

0.056 0.172

Ethyl alcohol 30% Acetic anhydride, 70%}

b

E t h y l acetate, 50% 1 Acetic anhydride, 50%j

Water 30% 0,057 0.057 Acetic'anhydride, 70%} 0.171 0.170 Mixtures were brought t o a boil, then cooled t o room temperature before titrating. b Because of spontaneous reaction of water and ethyl alcohol with acetic anhydride, concentration of each component of solvent was uncertain a t moment of titration.

The data of Table I1 have been collected on a number of mixtures into which a known amount of sulfuric acid had been added. Table I11 gives the quantitative data on the titration of sewmi weak bases in acetic anhydride.

Table 111. Titrations of Weak Organic Bases in .tcetic An hpdride" Purity Found,

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ditions so obtained give rise to the large potential changes observed a t the end point. Although the conditions under which the sulfuric acid has been titrated seem favorable for dehydration, the titrations have been carried out in the presence of water. Acetic anhydride makes possible the simple and rapid determination of some weak organic bases (Table I ) whose base ionization constants in water are less than 10-12. I n the absence of Kb for allylthiourea the value for thiourea has been included in Table I. It is suspected thst the Kb values for these two substances are about the same. Markunas and Riddick (6) have recommended the use of acetic acid in titrating weak organic bases whose Kb values in water are or larger. Urea, which does not give an insoluble perchlorate in acetic acid, has been listed by Fritz ( 2 )as giving an unsatisfactory end point in acetic acid. The use of acetic anhydride in this instance removes this difficulty (Figure 7).

h

%

Base .kllylthiourea Dimethylaniline p - Aminodimethylaniline Diplienylamine Glycine Pyridine Urea All bases were used as received. Reagent grade materials. Technical or practical grade.

100; 99 98;5C 100

lOOb,

1006 l00.5h

LITERATURE CITED

(1) Hell, K. P., "Acids and Bases. Their Quantitative p. 66, London, Methuen and Co., Ltd., 1952.

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(2) Fritz, J. S., ANAL.CHEM.,22, 1028-9 (1950). (3) Ibid., 25, 407-11 (1953). (4) Fritz, J. S., and Fulda, M. O., Ibid., 25, 1837-9 (1953). (5) Lange, N. A., "Handbook of Chemistry," 8th ed., p . 1234,

Sandusky, Ohio, Handbook Publishers, Ino., 1952. (6) lfarkunas, P. C., and Riddick, J. A., ANAL.CHEM.,23, 3 3 i - 9 (1951).

(7) Russell, J., and Cameron, 8 . E., J . Am. Chem. Soc.. 60, 1345-8 (8)

(1938). Wagner, C. D., Brown, R. H., and Peters, E. D., I b i d . , 69, 2609-10 (1947).

RECEIVEDfor review February 26, 1954. Accepted September 1, 1054. Presented before the Division of Analytical Chemistry a t t h e Regional SOCIETY, New Orleans, La.. December Conclave of the AMERICASCHEMICAL 10, 1 9 3 .

Semimicromethod for Determination of Cyanate Ion in Presence of Interfering Substances WILLIAM

H. R. SHAW

and JOHN 1. BORDEAUX

The University of Texas, Austin, r e x .

The method described employs ion exchange'removalof interfering cations, conversion of cyanate ion to ammonium ion by dilute acid, separation of ammonium ion by ion exchange, and photoelectric colorimetric analysis of ammonium ion with Nessler's reagent.

COLORIMETRIC method for the determination of cyanate ion in the presence of interfering substances has been developed. The mixture containing cyanate ion is passed through a cation exchanger; the effluent is collected; the column is eluted with a sodium hydroxide solution and rinsed, thus freeing it from all cations except the sodium ion. The original effluent containing only anions, sodium ions, and neutral molecules is acidified, and cyanate ion is rapidly converted to ammonium ion. The resultant solution is again passed through the cation cxchanger. Ammonium ion is quantitatively retained, and the anions, exchanged sodium ions, and neutral molecules are rinsed from the column and discarded. Ammonium ion is eluted, the elutriate is treated with Sessler's reagent, and the absorbance is determined. A calibration curve relates absorbance to the cyanate ion concentration. Interference may be expected from a rather limited number of substances that react with dilute acid t o

produce cations which, in turn, interfere with the Sewler's reaction. -kt elevated temperatures in aqueous solutions, urea tiwomposes according to the following reaction

+ CNO-

CO(NHz)z - NH:

(1)

In the course of a kinetic study of this reaction (Is),an e l tremely rapid conversion of cyanate ion to ammonium ion h!dilute sulfuric acid was observed.

CNO-

+ 2H+ + 2H20 -

'4"

+ H&Os

(2)

This same reaction had been employed by Hertig (4, 5, 1 4 ) qand others a t elevated temperatures for the quantitative determination of cyanate. Previous work (6-8, 1 2 ) had established a convenient method ior the determination of the ammonium ion based on an ion exchange separation and subsequent colorimetric analysis with Sessler's reagent. Consequently, if Reaction 2 were quantitative a t room temperature, cyanate ion could be easily determined by a technique similar to that employed for ammonium ion. Since this method had proved useful in the presence of interfering substances, and treatment of complex mixtures containing