Quantitative Determination of Formaldehyde and Benzaldehyde and Their Bisulfite Addition Products L. €I. DONNALLY, Chemical Laboratory of the University of California, Berkeley, Calif.
T
H E quantitative determination of aldehydes b y their reaction with sodium bisulfite has long been known. The method consists of adding an excess of bisulfite to a solution of the aldehyde, allowing reaction 1 t o take place, back-titrating the excess of bisulfite, and calculating the amount used by the aldehyde by difference. RCHO
+ HSOa-
=
RCH(0H)SOs-
(1)
The method is open to criticism, however, on account of the instability of bisulfite and, in the case of most aldehydes, the fact that the reaction is not complete. The direct determination of aldehyde bisulfite furnishes a simple and easy method for determination of the corresponding aldehyde. I n the method developed here the reaction of the aldehyde and bisulfite is allowed to take place a t the most favorable p H range, as determined by the work of Stewart and Donnally (4). After the reaction, the p H of the solution is ohanged to the range where the dissociation reaction RCH(0H)SOa-
=
RCHO
+ HSOa-
solution was therefore added, first 1 cc. a t % time, then 0.5 cc., and finally drop by drop. The permanent end point with 1 drop of iodine in the solution lasted at least 10 minutes. The results of these experiments are given in Table I, as well as results obtained by the method of difference. It will be seen that in 15 minutes the reaction goes to completion within two parts per thousand. I n order to check the method, the same solution was analyzed, using iodine and alkali according to a modified method of Romijn (3) and Fresenius (2). The modification consisted of increasing the alkali concentration to 1.6 M and allowing the reactants to stand for a t least 20 minutes. The results obtained with this method are also given in Table I. The agreement between the two methods is well within two parts per thousand. TABLEI. RESULTSOF DETERMINATION OF FORMALDEHYDE SOLUTION DIRECTTITRATION METSODOF WITH IODINE DIFFBRENCE Iodine Iodine
(2)
Minutes
cc .
cc.
78.00 79.10 79.33
is slow, and the excess bisulfite is titrated with iodine solution ( 5 ) . Following this titration, the pH of the solution is again adjusted so that dissociation reaction 2 is fast, permitting titration of the aldehyde bisulfite compound with iodine solution. The rate of reaction of iodine and aldehyde bisulfite depends only upon reaction 2 and is independent of the concentration of iodine. The suitable pH ranges depend upon the particular aldehyde used.
OXIDATIONOF FORMALDEHYDE Iodine
cc .
...
79:ZO
DETERMINATION OF BENZALDEHYDE AND BENZALDEHYDE BISULFITE Reaction 1 in the case of benzaldehyde is not as complete as the corresponding reaction with formaldehyde. It is possible, however, to apply a correction to overcome this DETERMINATION OF FORMALDEHYDE AND FORMALDEHYDE difficulty. The following analyses of benzaldehyde in water BISULFITE solution are presented as an example: SOLUTIONS REQUIRED Thirty cubic centimeters of sample (approximately 0.04 M ) , 1. Standard iodine solution, 1 M 5 cc. of 1 M sodium bicarbonate, and 25 cc. of sodium bisulfite 2. Sodium bisulfite solution, about 0.1 M were put in a flask and allowed to stand a measured length of 3. Sodium bicarbonate solution, about 1 M time. At the end of that time 10 cc. of 1M phosphoric acid were 4. Acetic acid solution, about 1 M added and the excess bisulfite not used by equation 1 was titrated 5. Sodium carbonate solution, about 1 M as before with standard iodine solution. The end point here was 6. Phosphoric acid solution, about 1 M a slowly fading one. Twenty-five cubic centimeters of 1 M sodium bicarbonate were then added and the solution further The dissociation constant of the sodium bisulfite solution titrated with iodine to the permanent end point. This second for formaldehyde bisulfite is 1 X lo-’ a t pH 6 (I), so that titration represents the amount of benzaldehyde bisulfite. the reaction is sufficiently quantitative as long as the conResults of the series of measurements which were made by centration of formaldehyde to be measured is not extremely varying the time of contact of bisulfite and benzaldehyde are low. given in Table 11, as well as results obtained with the method The following experiments illustrate the method used: of difference. These results show that the method based Five cubic centimeters of a sample of formaldehyde in water of upon difference is unreliable, giving a high result which is approximately 0.4 M concentration were transferred b a pipet to probably due to oxidation of bisulfite by oxygen. .a 250-cc. flask. Five cubic centimeters of 1 M so2um bicarbonate and 30 cc. of 0.1 M sodium bisulfite solution were then TABLE11. RESULTSOF DETERMINATION OF BENZALDEHYDE added (50 per cent excess bisulfite), and the solution mixture was SOLUTION allowed to stand a measured length of time. A t the end of this DIRECTTITRATION METHODOF time, 10 cc. of 1 M acetic acid and 2 cc. of starch solution were WITH IODINE DIFFERENCE BISWLFIT~ added. The excess bisulfite was then titrated and standard Iodine Iodine DISCREPANCY iodine solution was added to the first permanent end point. Minutes cc* cc . Next, 20 cc. of 1 M sodium carbonate were added, so the formal2 23.40 23.50 0.1 5 23.35 23.95 0.6 dehyde bisulfite compound could be titrated. In this titration 10 23.45 24.05 0.6 it was desirable to keep the concentration of iodine as low as 24 23.40 24.80 1.4 possible because of side reactions with formaldehyde, and the 30 23.50 25.20 1.7 91
ANALYTICAL EDITION
92
These results give within one per cent the total amount of benzaldehyde in the solution, and if this accuracy is sufficient, nothing more need be done. However, a correction is easily applied in the following manner: (3) K = (PhCHO) (HSOa-) (PhCH (0H)SOa-) ,
and at pH 4 to 6, K equals 1.1 X The benzaldehyde bisulfite is 0.0195 M , and HSOs- is 0.0296 M. Solving for benzaldehyde in equation 3 we obtain 7.25 X M or
Vol. 5, No. 2
4.35 X 10-6 moles of benzaldehyde, equivalent to 0.087 CC. of iodine. The correction to be applied is therefore 0.09 CC. of iodine. LITERATURECITED (1) Donnally, Diseertation, University of California, 1932. (2) Fresenius and Grunhut, 2. anal. Chem., 44, 13 (1905). (3) Romijn, Ibid., 36, 18 (1897). (4) Stewart and Donnally, J. Am. Chem. floc., 54, 3555 (1932). ( 5 ) Stewart and Donnally, Ibid., 54, 2333 (1932). RECEIVED October 31,1932.
A Mechanical Device to Agitate Analytical Solutions by Swirling ARTHURF. SCOTT AND ELTON F. REID,JR.,Rice Institute, Houston, Texas
I
N THE course of some experiments it was found necessary
to agitate analytical solutions over a long period of time without the introduction of a stirrer. Since none of the conventional means of agitation met the necessary requirements, a device was developed whereby a swirling motion could be imparted to a solution analogous to that so common in the manual manipulation of analytical solutions. This device proved satisfactory for its intended use and has been otherwise serviceable in the laboratory. The general appearance of the apparatus is shown in Figure 1. The base A carries a heavy vertical driving shaft, resting on a large ball bearing in the bottom socket. The plate C is bolted to the top end of the shaft and supports, by means of three ball bearings, the wooden platform B, the bottom of which is covered with a steel plate.
C
B
FIGURE 1 The driving element between the pIate and the platform is a pin screwed to the plate slightly off-center and fitting into an opening in the center of the platform. Rotation of the platform is prevented by having the tail held in position by a pin, D, which passes through a small slot. The springwasher arrangement, shown in the figure on D, serves to stabilize the platform. With this arrangement the rotation of the shaft gives to the head of the platform a circular motion, and a solution in a flask, clamped in position as shown, quickly picks up the desired swirling motion. Most of the constructional details can be understood from the diagram, which is drawn to scale-the platform is 14.5 inches (36.8 cm.) long. A few additional observations, however, may be useful. The shaft is rotated at
about 150 r. p. m. Two interchangeable positions for the pin are convenient, one 0.31 inch and the other 0.19 inch off-center. The three steel ball bearings are 0.625 inch and are held in position by suitable sockets bolted to the plate. I n order to have a single race for the ball bearings, they must be equidistant from the pin for both positions of the pin. For this reason the plate is drilled and tapped so that the positions of the sockets can be changed. As a matter of fact, if the positions of the pins and sockets are taken as shown in the top view of the plate, it is only necessary to move one end of two sockets when the pin is changed from one hole to the other. Although no systematic study of the question was made, it was observed that the degree of mixing attained is influenced to a large extent by the speed of rotation of the head of the platform. If the rotation is slow, the liquid in the vessel mounted on the platform gains only a simple, circular motion. However, when the speed of rotation is sufficiently great, the motion of the liquid becomes considerably more complex. On the surface there is a t least one wave crest going around the wall of the flask, and internally, Judging from the observed movements of particles of precipitate, there is a mode of motion best described as spiral. It is this last type of motion that produces most of the mixing. The efficiency of the mixing process varies somewhat with the bulk of the liquid, the shape of the container, and the eccentricity used. This last factor, it should be noted, is determined solely by the first two of the conditions just named. Although the efficiency of the mixing obtained by the present device is probably not equal in all. cases to that obtained by manual manipulation, where the motion of the liquid is still more complicated, the method has the advantage that it can be continued over a long period of time without the attention of the operator. The apparatus described has been used successfully in agitating 300 cc. of solution contained in a beaker and also 2 liters of liquid in a 3-liter Erlenmeyer flask. The apparatus described above was constructed from parts and material readily available in most laboratories. The mechanical details could, of course, be modified. RECFAVED October 10, 1932.
CORRECTION. In the article on “Shorter Method for Determining Copper Iodometrically” [IND. ENG.CHEM.,Anal. Ed., 5, 15 (1933)) 10 0 0 . of 1.8 M potassium iodide solution should be added just prior t o titrating with standard sodium thiosulfate solution. T. H. WHITEHEAD
