cept that a 1-cm. cell and a wave length of 360 mp were used. In Table I are given the data for one set of determinations. Absorbance values were corrected for the reagent blanks a t the chlorosities in question. Reagent blanks vary very little and have no relation to chlorosity. The average values of the micromolar absorbance coefficients a t different chlorosities and the salt factors obtained are given in Table 11. Salt factor as a function of chlorosity is shown in Figure 1. It is simpler to utilize the functional relation between micromolar absorbance coefficient and chlorosity to correct for the salt effect. I n Figure 2, the micromolar absorbance coefficient is plotted as a function of chlorosity. Above a chlorosity of 5 grams of C1-
per liter, a linear relationship is obtained. Between chlorosities of 0.5 to 5 grams of C1- per liter, another straight line may be approximated without seriously sacrificing accuracy. When the concentration of silica is in micromoles per liter, two empirical linear relationships are obtained: kcl = 0.0058 - 0.00006 C1; for C1 from kcl =
0.5 to 5 grams of C1- er liter 0.0056 - 0.0000185 81. for C1 greater than 5 grams 0 ) C1- per liter
where k,, stands for the micromolar absorbance coefficient a t chlorosity C1. The silica content of the solution at this chlorosity may be calculated directly from the absorbance at 360 nip by Si02 (pmole/liter)
=
A360/kciX 1
where I = 1 cm. LITERATURE CITED
(1) Brujewicz, S. V., Blinov, L. K., State Oceanog. Inst., Moscow, U.S.S.R., Bull. 14 (1933). (2) Chow, D. T. W., Robinson, R. J., ANAL. CHEM.25, 646 (1953). (3) Dienert, F., Wandenbulke, F., Compt. rend. 176, 1478 (1923). (4) Lyman, J., Fleming, R. H., J . Marine Research (Sears Foundation) 3, 134
(1940). (5) Robinson, R. J., Spoor, H. J . 9 I S D . ENG.CHEM.,AXAL.ED. 8 , 455 (1936). (6) Saeki, A,, J . Oceanog. SOC.J a p a n 6, 39 (1950). RECEIVED for review November 18, 1957. hccepted May 5, 1958. Work supported by a grant from the American Petroleum Institute, Project 51. Contributions from the Scripps Institution of Oceanography, New Series.
Iodometric Estimation of Small Quantities of Acetaldehyde SAMEER BOSE'
M. M. V. laboratories,
labalpur, India
,The need for a precise but simple method for the determination of acetaldehyde in the range from 3 to 5 p.p.m. in water led to a study of the procedure hypoiodite method. A based on substitution of iodine in acetaldehyde in dilute solutions by alkaline iodine solution has been suitable. The analytical precision is to 2.2%. The method is applicable to very dilute solutions only, and the maximum amount that can b e estimated is 1 mg. Ammonium salts in low concentrations do not interfere. The procedure is applicable in the presence of large quantities of ethyl alcohol.
the reaction is not usually stoichiometric and depends on the compound. Among the compounds compared, acetaldehyde gave the smallest reaction, 60%. But precision or reproducibility was consistently within a few tenths of 1%. I n a colorimetric method ( 3 ) of estimating acetaldehyde and acetone, the iodoform produced is extracted with chloroform, and the absorbancy is measured a t 347 mp in silica cells. Data are presented on the applicability of the hypoiodite method in estimating 98% of acetaldehyde in dilute solutions. THEORY
T
inherent usefulness of the hypoiodite method of estimating formaldehyde as described by Romijn ('7) led to the reinvestigation of its applicability to acetaldehyde. Hatcher and Mueller (4) reported that acetaldehyde gives 60% yield when treated with a n alkaline solution of iodine. Mitchell and his associates ( 5 ) ,in a summary of the hypoiodite reaction in volumetric analysis, state that the reagent reacts with all compounds containing the CH3.CO or CH=C.CO group giving iodoform as the primary product, but HE
Present address, 310 Napier Town, Jabalpur, India. 1526
ANALYTICAL CHEMISTRY
Iodoform is the primary reaction product when acetaldehyde is added to a n alkaline solution of iodine :
+
+ +
+
312 4NaOH -,CHI3 3 NaI HCOONa 3Ht0 (1)
CH,CHO
+
The intermediate product CHIsCHO, which has not been isolated, reacts with the alkali to give iodoform and sodium formate. If this is carried to completion, every molecule of acetaldehyde will consume six atoms of iodine. But a part of acetaldehyde undergoes oxidation to acetic acid, which escapes iodine substitution. This oxidation process is analogous to the formaldehyde reaction:
HCHO
+ NaOI + NaOH HCOONa + NaI + H 2 0 +
(2)
Every molecule of acetaldehyde which undergoes oxidation consumes only two atoms of iodine. The 60% reaction reported by previous workers means that an average of 3.6 atoms of iodine are consumed per molecule of acetaldehyde. Thus, 60% of the acetaldehyde undergoes oxidation and the rest is converted to iodoform. Two competing reactions take place. No suitable catalyst could be found to suppress oxidation. On decreasing the concentration of acetald3hyde and iodine and increasing that of sodium hydroxide, the amount of iodine consumed increased and rose to 98% of the expected value. I n a kinetic study of the iodoform reaction of acetone, Bell and Longuet-Higgins (1) reported that the reaction is bimolecular, depending on the concentrations of acetone and sodium hydroxide. I t is zero order with respect to iodine concentration. Acetaldehyde probably follows a similar course, kinetically. B y decreasing the concentrations of acetaldehyde and iodine and increasing that of sodium hydroxide say 100 times, the iodoform reaction is not greatly affected. However, the velocity of the oxidation reaction, which appears to be bimolecular depending on the concentrations of acetaldehyde and iodine, falls.
EXPERIMENTAL
Equipment and Reagents. A 10ml. microburet graduated in 0.01 ml., 2-, 5-, lo-, 25-, and 100-ml. pipets, and a graduated 5-ml. pipet were used. All solutions were made from analytical grade chemicals in water, which was redistilled in an all glass still aftw adding 1 crystal of potassium permanganate and barium hydroxide. Iodine, 0.05 and 0.0125N. Sodium thiosulfate, 0.05 and 0.025P1. Sodium hydroxide. To prepare a 4 N carbonate-free solution, 16.5 granis of the alkali were dissolved in a little water, left overnight, and filtercmd through a sintered crucible. The vclume was made up to 100 ml. Sulfuric acid, 4N. The solution was prepared by diluting 104 ml. of conceitrated acid to 1 liter with water. Acetaldehyde. As the boiling point of acetaldehyde is very low (21” C,), a standard solution could not be prepared by weighing. Approximately 0.01 to 0.03M solutions were prepared and standardized by the hydroxylamine sulfate method of Schultes (8) and the bisulfite method recommended by Curnming, Hopper, and Wheeler ( 2 ) . Both methods gave results which were accurate to 1.5%. Preliminary Experiments. The dilution of the reaction mixture wii,h water has a marked effect on the amount of iodine absorbed. T o eig i t 500-ml. volumetric flasks, 2 ml. of 0.0279M acetaldehyde and 10 ml. of 0.05N iodine Tvere added. The contents of each flask were diluted by adding varying quantities of distilled water and made alkaline with 5 ml. of 4 S sodium hydroxide. The flasks were corked and left standing for 1 hour a t 20” C. The contents were acidified with 5.5 ml. of 4N sulfuric acid, and the liberated iodine was titrated. A blaik was carried out using 200 ml. of waiter only. The results obtained a t 20” C. are shown in Figure 1. By carrying out the experiments a t 5’ C., up to 98% of the acetaldehyde could be estimated. Procedure. From 10 to 25 ml. of the sample containing 0.5 t o 1.0 rrg. of acetaldehyde mas added to a 500ml. volumetric flask which containvd 200 ml. of ice cold distilled mater. Then 6 to 12 ml. of 0.0125Ar iodine solution was introduced from a buret. This mas about 10 t o 20% in excess, because 1 mg. of acetaldehyde consumes 10.9 ml. of 0.0125N iodine. The mixture was made alkaline by adding 4 ml. of 4 N sodium hydroxide with swirling. The flask was corked and kept immersed in ice cold water for 2 hours, during ivhich 95yo of the reaction took place. The reaction was completed by taking the flask from cold nater and immersing it in water a t about 20” C. for 1 hour. The contents of the flask n-ere acidified with 5 ml. of 4 S sulfuric acid. The iodine liberated as titrated with 0.025&V thiosulfate
INTERFERENCES
solution in a 10-ml. microburet, using 1 ml. of starch solution as indicator. A blank was carried through this procedure.
Compounds which are oxidized or undergo iodine substitution with hypoiodous acid will interfere in this procedure. Ammonium salts do not interfere unless the amount present, on a molar basis, is more than 10 times that of acetaldehyde. The interference produced by ethyl alcohol may be prevented by using barium hydroxide as the alkali, provided the molal concentration of alcohol is not more than 100 times that of acetaldehyde. This procedure is analogous to that described by Rakshit (6) for estimating ethyl alcohol and acetone mixtures with alkaline iodine solution.
The reaction when carried out a t 20’ C., gave 2 to 3% lower results, although the time for its completion was only 1 hour. The preliminary experiments to determine the amount of iodine required were carried out a t room temperature. Six solutions which were st,andardized by the bisulfite and the hydroxylamine sulfate methods were estimated after diluting them ten times with water. All results in Table I are averages of four determinations.
t
-1
.
P
w
0
2a
,OOt
300t
CONCLUSION
i
200
The method described is suitable for microestimation of acetaldehyde. Results obtained when sodium hydroxide is used as a n alkali are about 2% low, hence a n empirical correction of +2% should be employed for greater accuracy.
ATOMS OF I2 CONSUMED PER MOLECULE OF C H 3 C H 0
ACKNOWLEDGMENT
The author wishes to thank R. L. Xirula and V. V. Gore for providing the laboratory facilities.
Figure 1. Effect of diluting reaction mixture a t 20” C.
Table 1.
Comparison of Methods
Acetaldehyde Found, hIg. per 1000 M1. Bisulfite
Hydroxylamine
rlv.
Present Method
% of Found
429 450 511 565 622 820
436 455 517 5iO 628 826
432.5 452. 514 567.5 625 823
422 442 504 558 614 810
9i.8 97.9 98 0 98.4 98.2 98.4
I n the procedure described, the reaction mixture was diluted with 200 ml. of water. It was possible to use very dilute solutions of acetaldehyde (2.5 to 5.0 p.p.m.) and to omit this dilution. I n Table 11, the results of estimating six dilute solutions are recorded. The solutions were prepared by diluting a 0.04M solution of acetaldehyde, which was standardized with hydroxylamine.
Table 11. Estimation of Very Dilute Solutions of Acetaldehyde
Acetaldehyde, P.P.i’v1. Present Found 2.5 3.0 3.5 4.0 4.5 5.0
2.45 2.94 3.44 3.92 4.41 4.89
Error, % -2.0 -2.0 -1.8 -2.0 -2.0 -2.2
Av .
LITERATURE CITED
(1) Bell, R. P., Longuet-Higgins, H. C., J . Chem. SOC.1946, 636. (2) Cumming, W. M., Hopper, I. V.,
Wheeler, T. S., “Systematic Organic Chemistry.” 4th ed.. v. 492. Constable, London, i950. (3) Dal Nogare, S., Norris, T. O., Mitchell, J., Jr., ANAL. CHEM.23, 1473-8 I
_
(1951). (4) Hatcher, W. H., Mueller, W. H., Trans. Roy. SOC.Can. 111 23, 35-44 (1929). (5) Mitchell, J., Jr., Kolthoff, I. M., Proskauer, E. S., Weissberger, A., eds., “Organic Analysis,” Vol. I, p. 268, Interscience, New York. 1953. (6) Rakshit, J., Analyst 41, 245-6 (1916). (7) Romijn, G., Z. anal. Chem. 36, 18-24 (1897). (8) Schultes, H., Angew. Chem. 47, 258 (1934).
RECEIVEDfor review April 9, Accepted April 19, 1958. VOL. 30, NO. 9, SEPTEMBER 1958
1957.
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