An Improved Dilatometer ARTHUR FURST Sun Francisco Junior College, S a n Francisco, California
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T IS possible to follow reactions that are accompanled by a change in volume by means of a dilatometer. This instrument is essentially a bulb to which is sealed a capillary tube, and i t is by the rise or fall of the liquid level En the tube that the volume change can be noted. To be efficient, a dilatometer must be filled easily in a relatively short space of time, and it must not introduce any extraneous errors; e. g., an m o r in volume readings due to the drainage of the capillary tube. In dilatometry the solutions are mixed before being introduced into the dilatometer. The time of filling becomes of prime importance when a reaction goes to apparent completion in about two or three minutes. To obtain true rates of volume change it is best that the capillary tube be dry. If the bulb is filled by means of the capillary, the first few readings must be neglected. This is of importance only when an increase in volume is the net result. A typical study of a physical-chemical reaction rate as outlined by Daniels, Matheks, and Williams1 is the hydrolysis of acetal. The dilatometer described is constructed by sealing a capillary tube to a bulb, and then, in turn, sealing a tube of larger diameter to the capillary. The instrument can be filled by evacuating the system and allowing a previously mixed reaction mixture to flow in. During this procedure the reaction is taking place. The bottom bulb is filled by means of the connecting tube. At times it is necessary - capillary to evacuate twice before the dilatometer is completely filled, since the capillary restricts the flow of incoming liquid. The excess liquid in the capillary tube is removed by a very fine capillary. Although the seals are so contructed that the error due to drainage is minimized, this source of error is still present; thus the first few readings must be neglected. The drawbacks in using this type of dilatometer are: (1) method of filling; (2) time of filling; and (3) drainaEe - error. ' DANIELS, MATUEWS, AND WILLIAMS "Experimental physical
chemistry," 2nd ed., McGraw-Hill Book Co., Inc., New York City, 1934, p. 143.
In an attempt to overcome these disadvantages, a modified dilatometer was designed. (See figure.) The advantage of this type lies in the fact that the drainage error is eliminated by a method of filling from the bottom. A capillary stopcock is used in order to prevent air bubbles from remaining in the tube leading to the bkb. The stopcock can be shut before the level of the solution rises above the reference point in the capillary tube. The reservoir may be of larger capacity than the main bulb. This is especially desirable iti' the case of' rapidly reacting solutions. Care must be taken that the level of the liquid in the reservoir above the stopcock does not get below the , seal, for this will introduce air into the system. The dilatometer may be left in the thermostat during the process of filling. The capillary tube can be graduated by pasting a s t ~ i p of millimeter graph paper on it, the glass in front of the paper acting as a magnifier. A fine capillary (not shown) may be inserted in the capillary of the dilatometer to remove any liquid and to adjust the level to a convenient reference point. For solutions that are reacting too rapidly the time of filling may be decreased by attaching the capillary to an aspirator, but if this is undesirable the reservoir can be filled, the stopcock opened, and pressure applied. The suggested dilatometer was used for the study of the hydrolysis of acetal and excellent results were obtained.
