Air Displacement with Dry Ice in Iodometric Titrations. Frederick G. Strong, Bryn Mawr College, Bryn M a w , Pa.
readily available and a routine is established in a series of titrations to use the time required for its sublimation, the total time and effort required are no more than for preparing acid of the right concentration and weighing out the proper amount of carbonate (which must be done fairly accurately). Occasions may arise when it would be undesirable to increase the acidity of a solution in order to perform an iodometric titration. The amount of solid carbon dioxide can soon be estimated without weighing; about 15 grams per 100 ml. are suggested, but less may be sufficient. Bringing iodide and oxidizing agent into contact with each other before all carbon dioxide has sublimed seemed to give equally accurate results. L-se of too much solid carbon dioxide caused the pieces to stick to the bottom and become covered with ice, and slowed down the rate of sublimation considerably.
of frequent iodometric analyses on hot sumTmerperforming days suggested that air displacement and simultaneous HE
cooling by use of dry ice might be substituted for chemical generation of carbon dioxide in the reaction flask, if sodium bicarbonate or sodium carbonate was added to excess acid before addition of iodide ion. Scott (“Standard Methods of Chemical Analysis,” Furman, N. H., ed., 5th ed., Vol. I, p. 1211, Kew York, D. Van Kostrand Co., 1939) says that in standardizing sodium thiosulfate with potassium dichromate, “the acid solution should be freed of dissolved air (previous boiling or treatment u ith carbon dioxide).” Pierce and Haenisch (“Quantitative Analysis,” 3rd ed., p. 244, Kew York, John Wiley & Sons, 1948) suggest the use of excess sulfuric acid and sodium carbonate. Willard and Furman (“Elementary Quantitative Analysis,” 3rd ed., pp. 267-8, New York, D. Van Nostrand Co., 1940) advise the use of excess hydrochloric acid and sodium bicarbonate. Experiments were carried out w t h both boiled and air-saturated solutions, using dry ice to effect air displacement and cooling by the procedure of Pierce and Haenisch.
A Sintered-Glass Manostatic Valve for Maintaining an Inert Atmosphere. J. C. Cavagnol’, Johns Hopkins Cniversity, Baltimore, hld. HE
of a manostat or sucking it back into the attached system. It has also served to maintain an inert atmosphere during a reaction or distillation and subsequently to keep the isolated product under the same conditions. The problem of replacing air with other gases in an apparatus of large volume is simplified by having this valve link the gas and vacuum lines with the system.
I. Standard Method.
One hundred milliliters of freshly boiled 2 S sulfuric acid and 25.00 ml. of 0.02538 N potassium dichromate were placed in a 500-ml. Erlenmeyer flask, 2.0 grams of sodium carbonate were added in portions while the contents were swirled, 5.0 grams of sodium iodide dissolved in 10 ml. of freshly boiled water were then added, and the flask was stoppered and allowed to stand for 10 minutes in subdued light. The temperature of the solution x a s 28” C. Two hundred milliliters of freshly boiled vater were then added to reduce the acidity and the solution was titrated with sodium thiosulfate. 11. No Air Displacement. I n a 500-ml. Erlenmeyer flask were placed 75 ml. of distilled water a t 28” C., saturated with air (by drawing air through it for several hours), 25.00 ml. of 0.02538 N potassium dichromate, 5 ml. of 6 AT hydrochloric acid, and 5.0 grams of sodium iodide in 10 ml. of water. After being stoppered and allowed to stand for 10 minutes in subdued light, the solution was titrated with sodium thiosulfate solution. 111. Dry Ice. In a 500-ml. Erlenmeyer flask were placed 75 ml. of distilled water a t 28” C., saturated with air as in 11, 25.00 ml. of 0.02538 N potassium dichromate, and 5 ml. of 6 AT hydrochloric acid. Dry ice was added (30 grams each for solutions 1 and 2, 20 grams each for 3 and 4) and alloxed to sublime (1 and 2 took rather long, 3 and 4 about 5 minutes). The temperature was taken a t this point ( 9’ C. for 1 and 2, 14’ for 3 and 41, 5.0 grams of sodium iodide in 10 ml. of water 17-ere added, and the flask was stoppered, allowed to stand (10 minutes for 1, 2, and 4; 5 minutes for 3), and titrated. I
‘4-7.
I1
hll. of Na2920a 33.28 32.83 33.23 32.85 33.20 32.88 33.26 32.87 32.86 33.24 Normality of NazSzOs 0.01931 .....
all-Pyres apparatus shown in the accompanying sketch
Twas designed to remedy the nuisance of blowing mercury out
Only four regular seals and one ring G s seal are required to a s s e m b l e the 13mm. tube with m e d i u m porosity fritted disk, 22 X 175 mm. test tube, s 2-mm. straight-bore stopcock, and 2-mm. three-way T s t o p c o c k . An 8-mm. side arm on the body functions as a mercury overflow. With stopcock A open and stopcock B in the ‘(fill” position, gas is allowed to enter slo~vlya t G. S o w S is connected to the system to be filled and b’ is attached to a vacuum pump. By turning B back and forth through a 90” arc between the tlvo positions it is possible alternately to evacuate and fill any apparatus as often as necessary. With reaction mixtures evolving gas, the air is first replaced with an inert atmosphere, A is closed, and B is placed in the fill position with the gas and vacuum lines disconnected, so that the manostat functions as a one-way valve.
I11 32,87 32.84 32,83 32.83 32 85 0.01931
The utilization of the impermeability of sintered or porous glass plugs to mercury has resulted in several elegant designs [Lewis, F. hl., IND. EXG.CHEW,ANAL. ED., 13, 418 (1941); Taylor, R. C., and Young, W.S., Ibid., 17, 811 (194511. Although no originality is claimed for any of the principles involved, this multipurpose arrangement may provide simplification or additional flexibility to existing setups.
Results I and I11 check well, sholving that dry ice can replace chemical evolution of carbon dioxide as a method of air displacement in iodometry. Neglect of this factor gives high results (11). That the lowered temperature does not seriously decrease the rate of reaction between dichromate and iodide is shown by the general agreement and the fact that solution 2 (111) stood only 5 minutes before being titrated. The use of dry ice is suggested as an alternative method of iodometry, rather than as a significant improvement. If dry ice is
1 Present address, Department of Chemistry, University of Kentucky, Lexington, Ky.
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