p,
i
Equation 1 may now be written as IC
1,
= -kK ~
f 1%
(3)
The current, I, is imagined t o be the algebraic sum of two opposing currents
I, = -I +I,
(4)
where
The inner part of the double layer is amumed to consist predominantly of suppressor ions; this part constitutes is, the effective double layer-that the part connected with the suppression of the maximum. The thickness of the effective double layer may be approximately expressed by Equation 2 (C = ionic concentration of suppressor ion in the bulk of the solution).
The hypothetical negative current, -1. has meaning only in so far as it expresses the fact that reducible nickel ions are repelled by the effective double layer towards the interior of the solution. This negative current may be expected t o approach inverse proportionality with A. Combination of Relationships 2 and 5 leads directly to the empirical equation. This explanation emanates from the modern theories on polarographic maxima (2, 3). I t is assumed that the physical causes responsible for the appearance of the maximum keep the solution around the electrode in fairly good constant motion, and as a consequence the nickel concentration near the fixed effective double layer is constant. The current now depends on the rate at which nickel ions penetrate the effective double layer. This current is not diffusion-controlled. When the applied voltage increases to the point where the electrokinetic phenomena causative of the maximurn cease to exist, the solution near the electrode cannot be stirred, and eventually becomes depleted in nickel ions. The current from now on will have to depend on the rate a t which nickel ions diffuse to the vicinity adjacent to the effective double layer (migration current has been eliminated by the presence of the supporting electrolyte, potassium chloride). The nickel ions still must penetrate the double layer, but the rate of diffusion is slower than that of penetration of the layer, and as a
SIR: Samples containing 0.1 to 1.0mg. of isopropyl. alcohol in 5 ml. of aqueous solution are assayed by oxidation and direct colorimetric determination of the acetone. Several methods reported in the literature use a similar oxidation to acetone, but the intense
color of the oxidizing agent, usually potassiuni dichromate, either requires that the acetone be distilled before a colorimetric method can be applied (1, Q, b ) , or the subsequent method of determining acetone ii3 limited to accommodate this color (8). If such color-
-110 -120 -130 APPLIED EMF VS 8C.E
-100
Figure 1.
Typical polarograms of 2.3
X 1Q-3MNi ion in Q.1M KCI with varying amaunts of Ca ion as maximum suppressor Concn. of suppressor ion, M
A. 2.5 X B. 7.5 x 10-4 C. D.
E.
1.25 X 10-3 2.5 x 10-3 3.25 x 10-8
where h = thickness of layer C = ionic concentration in the bulk
K
1
of the solution
= a constant
e
ANALYTICAL CHEMISTRY
, 00
IO
20
30
40
50
60
c % x 158XIO.* Figure 2. Plot of current at peak of maximum of nickel ion vs. square root of molar concentration of ions employed as maximum suppressors in 0.1 M KCI solution
4.6 X lQ-aM Ni solution 5. 2.3 X 10-‘M 0 Ba A Ca Mg A.
result, the rate of diffusion is the determining step in regard t o production of current. The HlkoviE equation can now be applied. LITERATURE CITED
(1) Emelianova, N. V., Heyrovskp, S., Trans. Faraday SOC.24 257 (1928). (2) Frumkin, A. N., Zkur. Fia. Khim. 29, 1318 (1955). (3) Stackelberg, M. V., Fortschr. Chem. Forsch. 2, 229-72 (1951).
(4) Taylor, H. S Glasstone, S., “Treatise on Physical 2henlistry,” Vol. 2, p . 635-6, Van Nostrand, Yew Yo& 1952.
DEMETRIOS KYRIACOU Aerojet-General Corp. Sacramento, Calif. RECEIVEDfor review August 29, 1960 Accepted October 7, 1960.
producing reagents are eliminated, the determination of isopropyl alcohol is more rapid and more accurate. Feigl (3) suggesk that “suitable oxidants such as alkaline persulfate readily convert isopropyl alcohol to acetone.” This reaction was utilized
:“h\ 24
11 \ \ 20
WAVE L E N G T H ,
mp
Figure 1 . Visual spectra of colored solution obtained by reaction of 0.8 mg. of isopropyl alcohol A. B. C.
VS. HzO Vs. reagent blank Reagent blank vs. HzO
to yield a colorless solution suitable for direct colorimetric determination of the acetone produced. In our procedure, the interference of persulfate is removed by reacting it with sodium bisulfite to maintain a colorless solution. The colorimetric method applied utilizes alkaline salicylaldehyde to yield dihydroxybenzal acetone, which imparts a distinct orange-red color to the solution (6).
MILLIGRAMS ISOPROPYL ALCOHOL
Figure 2.
reading in a DU spectrophotometer, but may not be necessary if another type of instrument or visual comparison is used.
APPARATUS
A Beckman DU spectrophotometer with tungsten light source was used; however, any colorimeter operating in the range of 470 t o 540 mp can be used. PROCEDURE
Using a closed weighing bottle, accurately weigh a sample of absolute isopropyl alcohol and dilute with wat,er to prepare a solution containing 1mg. of isopropyl alcohol in 5 ml. of solution. Use 1, 2, 3, 4, and 5 ml. of standard and sufficient water to total 5 ml. Use 5 ml. of water as a blank. To 5-ml. aqueous solutions of unknown, standards, or blank each in 25ml. glass-stoppered volumetric flasks, add 1 ml. of 1% potassium persulfate solution, stopper to prevent loss by volatilization, and heat 15 minutes in a water bath at 80” C. Cool to room temperature, add 1 ml. of 5% sodium bisulfite, and mix. Add 4 ml. of 40y0 sodium hydroxide solution followed by 1ml. of 20% ethanolic salicylaldehyde and quickly shake t o prevent precipitation. Heat as above for 15 minutes, cool to room temperature, and dilute the contents of each flask to volume. Further dilution of 1 to 4 or I to 5 volume is normally carried out before
Absorbance
of colored solution vs. amount
of isopropyl alcohol a t indicated wave lengths
DISCUSSION
Figures 1 and 2 are absorption curves \vhich show the color characteristics of both the blank and the standard solutions. The blank has a distinct yellow color absorbing below 480 mp, while that produced by the standard is orange, but this latter solution shows no distinct peak when read against water in the reference cell. If the standard is read against the blank solution in the reference cell, a peak occurs a t 460 mp but the high background absorption decreases the accuracy of the readings at this point. As can be seen from Figure 2, good analytical data can be secured by reading the colored solution at any convenient wave length from 470 to 530 mp. Actual spectrophotometric readings when plotted indicate good reproducibility and approach conformity to Beer’s law. The method as first set up was applied visually against a set of standards to establish the knit of concentration such as 20, 10, or below 10 p.p.m. of isopropyl alcohol in the 5-ml. sample used. The graduated shades of yellow to orange Rere sufficiently distinct to be compared readily.
Any acetone present in the original solution, or any material, such as hydroxybutyric acid, which on oxidation will yield similar compounds, will cause high results. Methyl, ethyl, or n-propyl alcohols were tried in the same procedure; none yielded significant colors. Isobutyl and tert-butyl alcohols produced slight colors from 5 to 10% of that produced with corresponding weights of isopropyl alcohol. LITERATURE CITED
(1) ASSQC.Offic. Agr. Chemists, “Official Methods of Analysis,” 8th ed., p. 325, 1955. (2) Critchfield, F. E., Hutchinson, J. il., ANAL-CHBM. 32,862 (1960). ( 3 ) Feigl, Fritz, “Spot Tests. Organic 9 plication,” 4th ed., p. 282, Elsevier, ew York, 1954. (4) Frisone, G. J., ANAL.CHEX 26, 924 ( 1954). (5) Snell, F. D., Snell, C. J., “Colorimetric Methods of AnalyEis,” 3rd ed., Vol. 111, p. 50, Van Nostrand, New York, 1953,. ( 6 ) Ibzd., pp. 286, 292.
2
GORDON B.GINTHER RAYMOND C. FINCH
Eaton Laboratories Norwich Pharmacal Co. Norwich, N. Y. PITTSBURQH Conference on Analytical
Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1960. VOL. 32, NO. 13, DECEMBER 1960
1895
