Continuous Conductometric Determination of Acetal de hyde ITALO A. CAPUANO Special Instrumentation Division, Engineering Department, Union Carbide Olefins Co., South Charleston 3, W. Va.
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method to measure concentrations
(0 to 1.2% by volume) of acetaldehyde in an aqueous liquid stream i s described. The measurement i s based on Schiff’s test for aldehydes and employs the conductometric determination of the sulfurous acid which i s formed as one of the products of such chemical reaction. Continuous, indirect determination of acetaldehyde i s made with a conductometric analyzer employing a differential type of measurement.
T
HIS W O R K was undertaken to develop a rapid and reliable analytical method for the continuous determination of 0 t.0 1.27, by volume concentrations or‘ acetaldehyde in an aqueous liquid stream containing inipurities of nonionizing compounds such as acetone, 2-butanone, ethyl acetate, etc. Sei-era1 anall-tical methods for acetaldehyde are available in the lit’erature; however, the presence of ket,one$ in the sample stream and the range of acetaldehyde concentration to be measured limit the application of such methods. The similar chemical and physical properties of bot,li aldehydes and ketones render impossible the use of some physical methods. eliminate the use of ultraviolet and infrared spectroscopy, and exclude the application of most of the chemical ieaction:: n-hkh both carbonyl compound; unclergo. The applica-
tion of several reactions which are usually employed to determine aldehydes in the presence of ketones also presents some difficulties. The colorimetric methods are not satisfactory in this case, as Beer’s law is not obeyed in the measurement of acetaldehyde in the per cent concentration range. Some other reactions form precipitates, which would interfere with the functioning of a continuous analyzer. The reaction of the Schiff’s test for aldehydes was satisfactory for the solution of this analytical problem. When a basic fuchsin solution is made to react with sulfurous acid. its intense red color disappears. This decolorization appears to involve the formation of the leucosulfonic acid, and also the addition of sulfur dioxide to two amino groups. Subsequent reaction \%-ith a n aldehyde gives a n addition product which loses sulfurous acid to form a nelv colored compound ( 2 ) .
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Schiff’s reagent (colorless) NHSOZCHOHR
Aldehyde addition product (colored) Although this reaction is generally employed to determine aldehydes colorimetrically. in the work reported here acetaldehyde was determined indirectly by measuring the change in electrical conductance due to the sulfurous acid formed in the reaction.
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APPARATUS
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The differential conductometric determination performed by measurement of
Basic fuchsin (colored)
RECORDER1
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1
-
:0.8VAC MIXING CHAMBER
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-
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1AIRPAC I CHOPPER I I I
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CONVERTER
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Figure 1.
Diagram of acetaldehyde analyzer
Figure 2.
Alternating current conductivity bridge VOL. 32, NO. 8, JULY 1960
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108-ACETALDEHYDE
9684--
W'
\DRAIN
Figure 3.
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CHART DIVISIONS
_ -
-
-
- -
CONCN ---
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-
~
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- ACETALDEHYDE
.
vs
80-
-
3 4
5
Acetaldehyde, % by Volume A4dded Found Difference 0.190
0.380 0.535
0.780 0.995
0.180
0.385
-0.010
CO.005 0.545 +O.OlO $0.010 0.790 -0,015 0.985 Uean dev. 3 t O . 010
Table II. Reproducibility of Acetaldehyde Determination by Differential Conductivity Method
yo.of Detns. 1 2 3
4 5
1026
Acetaldehyde, yo by Volume Added Found Difference 0.378 -0.002 0.380 0.380 0.377 -0.003 0.380 0.382 +0.002 0.380 0.384 +0.004 0.380 0.378 -0,002 hlean dev. 1 0 . 0 0 3
ANALYTICAL CHEMISTRY
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CHART DIVISIONS
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Conductivity cell
Table 1. Determination of Acetaldahyde by Differential Conductometric Method
1 2 -
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PLUG
the conductance of a sample solution before and after the addition of a reagent provides a high degree of selectivity (8). This type of determination was used to measure acetaldehyde. A schematic flow diagram of the analyzer used is illustrated in Figure 1. The most important parts of this instrument are the differential conductivity bridge and a flowing type conductivity cell. Figure 2 outlines the circuit of the alternating current conductivity bridge used. The bridge consists of a regulated power supply voltage, a bridge circuit with zero and range controls, and external reference and measuring cells, and a low level alternating to direct current converter circuit. For the particular application in which the apparatus was used, 0.8 volt, 60 cycles n-as applied to the bridge. T o eliminate transients and fields on the detecting circuit, a magnetic mu metal shielding n-as used. The output of the bridge was designed to operate into a high impedance recorder. A drawing of the conductivity cell is shown in Figure 3. The cell block consists of a transparent plastic (Lucite),
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0 7 0'8'08 09 ACETALDEHYDE CONCN, %
Figure 4.
SAMPLE FLOW RATE 5 5 m l l r n i n i REAGENT FLOW RATE IlOmUmin I BRIDGE RA NGE POT 9 8 6 K BRIDGE ZERO POT 2 5 8 K RECORDER RANGE 5mv ~
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04-05
1,
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BRIDGE Z E R O P O T 2 8 9 K RECORDER RANGE 20mv
sample
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CONCN
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12
Calibration curves
and contains an inlet and outlet for the flowing liquids, a vent to expel any trapped gas bubbles, and a sliding plug drain for cleaning. All these elements are equipped with plastic poly-flow adapters suitable for connection to l/4inch polyethylene tubing. The platinized electrodes were made rigid to withstand vibrations, and were placed flush with the inside cell block walls inch apart from each other. A Foxboro Dynalog electronic recorder] modified to have both a 5- and 20-mv. range, was employed to record the conductivity bridge output. To agitate the sample and the reagent in a mixing chamber, a magnetic stirrer was used. Two Lapp Pulsafeeder metering pumps metered constant flows of sample and reagent through the analyzer's sampling system. Because the mobility of most increases approximately 247, per 1 C. increase in temperature ( I ) , the conductivity cells, the mixing chamber, and part of the sampling system mere temperature-controlled at 35' C.
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REAGENTS AND PROCEDURE
The reagent was made by dissolving 0.0789 gram of basic fuchsin 99% dye (Hartman-Leddon Co., Philadelphia, Pa.) per liter of distilled water containing 52.2 ml. of sulfurous acid (Baker reagent grade containing 6.4% SOZ). The reagent stability was good for several days. Some acetaldehyde samples were redetermined after 2 weeks using the same reagent and the results were reproducible. The reagent was metered a t a flow rate of 11.0 ml. per minute through a reference conductivity cell, and then into the mixing chamber where i t reacted with the acetaldehyde sample introduced at a flow rate of 5.5 ml. per minute. The reacted solution was passed through a measuring cell and discharged to a drain. Under these
conditions a determination was completed in 3 minutes. The instrument was calibrated for the determination of concentration of acetaldehyde ranging from 0 to 1.2% by volume. The calibration is shown in Figure 4. To keep the measurement within a sufficient degree of linearity, two determination ranges were used. One calibration curve which includes the acetaldehyde concentration of 0 to 0.8% was obtained using the 20-mv. range of the recorder, n-hile the 0.8 to 1.2% concentration range was i ecorded on the 5-mv. recorder span. Some actual determinations of acetaldehyde n-eie made using these calibration data and the results are shown in Table I. Tests for the reproducibility of a determination were also made and are indicated in Table 11. CONCLUSIONS
Although the work reported here deals strictly with the determination of acetaldehyde, the chemical reaction applied for such determination is a general and specific test for aldehydes; thus, other Ivater-soluble aldehydes, such as formaldehyde, etc., should offer the same possibility of being determined with this instrumental method. With this type of measurement, the presence of traces of any materials that do not ionize and react with the Schiff's reagent to form ionizable products can be tolerated; however, impurities of ionizing substances such as organic acids, etc., if present in the sample will definitely interfere with the measurement. The method of analysis described here provides a simple and accurate means for continuously determining acetaldehyde in a process stream, as well as for making routine determinations in
the laboratory. This type of nieasurement appears to be very satisfactory as it has a good reproducibility, is insensitive to variation of flow rate, and possesses an excellent long-term stability.
of the Special Instrumentation Division who made the conductivity bridge used in this work, and particularly TV. R. Thompson for improving it. Appreciation is also expressed to other personnel of this division whose cooperation made this work possible,
ACKNOWLEDGMENT
The author thanks the members of the Electronic and Nucleonic Group
LITERATURE CITED
(I) Harley, J. H., Wiberley, S. E.,
“Instrumental Analysis,” p. 265, Wiley, New York, Chapman & Hall, London, 1954. (2) Xoller, C. R., “Chemistry of Organic Compounds,” p. 633, W. B. Saunders Co., Philadelphia, Pa., 1955. (3) JTherry, T. C., DeFord, D. D., J . Control Eng. 5 , 119 (March 1958). RECEIVED for review September 15, 1959. Accepted March 28, 1960. Division of Industrial and Engineering Chemistry, 136th Meeting, ACS, Atlantic City, K.J., September 1959.
Conductometric Titration of Very Weak Bases in Aqueous Medium FRANC0 GASLlNl and LUClO ZION NAHUM Research Division, Cartiera Vita Mayer &
b Twenty-five weak bases with ionization constants between 1 O-s and 10-l’ have been conductometrically titrated. The determinations are carried out b y dissolving the samples in an excess of aqueous acetic acid and titrating with trichloroacetic acid. Sharp angles are obtained a t the equivalence point and results are accurate. The method has been applied to the differential titration of diacid bases and mixtures of monoacid bases which have been resolved in the presence of ethyl alcohol.
V
TEAK bases are not satisfactorily titrated in aqueous niediuni by the usual conductometric methods (I), because hydrolysis causes very obtuse angles a t the equivalence point and large roundings in its vicinity. Therefore, many authors have reported nonaqueous titrations of very iveak bases using high-frequency conductometric apparatus. These methods have been recently summarized by AIcCurdy and Galt (4), who investigated solvents or solvent mixtures nhich increased the sharpness of the equivalence point angles, thus improving the accuracy. Very \T-eak acids can be conductonietrically titrated with lithium hydroxide, using an aqueous solution of ammonia as a solvent ( 2 ) . By this procedure, which considerably increases the ionization of the acids, the intersection angles were as satisfactory as those given by strong acids using the usual conductometric method, and very weak acidic groups, which ordinarily were not revealed, could be determined. In this work, this same principle was applied to the conductometric titration of very weak bases, titrating these compounds a i t h a strong acid and using an aqueous solution of a weak acid as a solvent.
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Co.,Milan, Italy The presence of an excess of the weak acid, AH, strongly increases the dissociation of the base, B, shifting the following equilibrium B
+ AH e B H + + A -
toward the right. The portion of the titration curve before the equivalence point represents the change in conductance due to the replacement of the weak acid anions by the strong acid anions, the former combining with the hydrogen ions of the titrant. After the equivalence point, the conductance increases sharply as a consequence of the rapid increment of the free H + ions of the excess titrant. Therefore, the difference beh e e n the mobility of the disappearing anion and that of the anion of the
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VOLUME OF T I T R A N T Figure 1. Conductometric titration of weak bases in aqueous acetic acid with 2N trichloroacetic acid I. Pyridine II. Quinoline 111. Aniline
titrant, as n-ell as the ionization constant of the latter, is important in determining the sharpness of the intersection angle. Hence it appears advisable to use as titrant an acid having a 1017 anionic mobility and a large ionization constant and, as solvent, a weak acid with a high anionic mobility. The ionization constant of the weak acid must be much smaller than that of the displacing acid, but large enough to allow a satisfactorily high dissociation of the higher base being titrated. The aqueous titration of several water-insoluble bases is made possible by this procedure. Aniong the various couples of strong and weak acids tried in thePe experiments, results were best with trichloroacetic acid as titrant and aqueous acetic acid as solvent. Some bases were also titrated with benzenesulfonic acid; the intersection angles were of the same order of magnitude as those obtained with trichloroacetic acid, the higher anionic mobility of the benzenesulfonic acid being counterbalanced by the higher ionization constant. Figure 1 shows the curves for the conductometric titration of three bases of different strengths. The decrease of the ionization constant causes larger angles a t the equivalence point, but the curves are still sharp enough to allow the location of the equivalence point with great accuracy. For a given base, the variables which determine the value of the equivalence point angle are the base and acetic acid concentrations. Varying the sample concentration from 50 to 150 meq. per liter and the acetic acid concentration from 200 to 900 meq. per liter the value of the equivalence point angle is not greatly influenced. It is possible to produce sharper angles, increasing the acid concentration a t constant base VOL. 32, NO. 8, JULY 1960
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