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
