Improved Apparatus for Isolation of Fluorine - Analytical Chemistry

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Improved Apparatus for Isolation of Fluorine Willard and Winter Method W. K. GILKEY, H. L. ROHS, AND H. V. HANSEN Calumet Chemical Company, Joliet, Ill.

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tacks rubber, all stoppers are of cork and must be selected and drilled carefully if leaks are to be avoided. (Corks could be eliminated by using ground-glass joints. The cork through which the steam enters the flask can be eliminated by using a glass seal a t this oint. The obvious disadvantage is the increased danger of breafing the joint when assembling the apparatus for a determination.) In this laboratory the distilling flask used is of the Claissen type, but probably a simple distilling flask would be satisfactory. The tetrachloroethane used in B is the Eastman technical grade. It is noninflammable and boils at 145' C. A depth of about 2 cm. in the bottom of the vessel is sufficient.

N THE determination of small amounts of fluorine accord-

ing to the method of Willard and Winter @), hydrofluosilicic acid is distilled from a sulfuric or perchloric acid solution of the sample, the temperature of the solution being held at about 135" C. by the addition of water at the proper rate to maintain this temperature.

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According to Willard and Winter, there is danger of violent oxidation of organic matter b y perchloric acid at temperatures exceeding 135" C.; they recommend the use of sulfuric rather than perchloric acid if the sample contains much organic matter. This substitution should be observed when tetrachloroethane is used as a heating liquid.

Procedure The procedure in making a distillation is as follows: Flask A is disconnected from the copper tube a t the rubber connection, G (Figure 1). The distilling flask, which contains a few glass beads, the sample, and the perchloric or sulfuric acid, is placed in position, all joints (except G ) are made tight and the burner, D, is lighted. Since the sample is usually added as an aqueous solution or suspension, the dilution of the acid is ordinarily so great that the contents of the flask will begin boiling at a relatively low temperature, but the boiling point will gradually rise as the water distills off. When the rate of flow from condenser C becomes small, flask A , which has been made ready and is now delivering steam, is connected to the copper tube and the steam distillation allowed to continue until a sufficient quantity of distillate is collected.

FIGURE 1. DIAGRAM OF APPARATUS This method of distillation requires the undivided attention of the analyst if the temperature is to be maintained approximately constant. Moreover the addition of water, as liquid, to the hot contents of the flask results in considerable bumping with the attendant danger of contamination of the distillate. This is a source of error when determining fluorine in phosphates (I), as the phosphate carried over will react like fluorine in the subsequent titration and will thus lead to high results. Where such entrainment is suspected, i t is customary to subject the distillate to another distillation under the same conditions. The phosphate carried over during the second distillation ordinarily will be negligible. By the use of the apparatus herein described, superheated steam is substituted for water, thereby eliminating the danger of bumping; also the contents of the distilling flask are maintained at a constant temperature by being immersed in the vapors of a boiling liquid. The above-mentioned difficulties are thus avoided.

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Apparatus Referring to Figure 1, steam is generated in flask A which is provided with a safety tube, the purpose of which is to prevent the contents of the distilling flask from being sucked back in case the pressure in A falls below atmospheric. The steam passes from A into a copper tube which is wound around the inside of vessel B. ( B was made of brass, but co per or steel would undoubtedly have been equally satisfactory.? This vessel, containing boiling tetrachloroethane, is heated by gas burner D and is provided with condenser E and thermometer F. In passing through the copper tube, the steam becomes superheated to the temperature maintained within the vessel. This superheated steam is then passed through a glass tube into the bottom of the distilling flask where it bubbles through the acidified sample and is subsequently condensed in C. Figure 2 shows a detailed section of B and its contents. The heavy, removable lid has a machined groove in which a cork gasket is inserted. Since the organic vapor contained in B at-

FIGURE 2 . SECTIONOF B 150

AND

ITS CONTENTS

MARCH 15, 1936

ANALYTICAL EDITION

151

A leaky cork will allow steam to pass into the organic vapor contained in vessel B. Even a small leak of this kind manifests itself as a noticeable drop in the temperature indicated by thermometer F.

tained with it, exercising only the precaution mentioned, have repeatedly checked those obtained with the original method of distillation using extreme care,

Results

Literature Cited

around corks has given no trouble whatever.

Results ob-

RECEIVED November 7, 1935.

Improved Micromanometer HARRY W. SMITH, JR., American Gas Association Testing Laboratories, Cleveland, Ohio

circle, affording a division mark on the instrument for every 0.001 inch of pressure. By interpolating between the divisions in fifths, accurate readings may be made to 0.0002 inch. Both openings of the glass gage system, the two pressure connections, are fitted with vent branches including glass stopcocks. All glass tubing clamps on the apparatus are fashioned of brass but lined with rubber to preclude breakage by shock or expansion. The Testing Laboratories employ methyl alcohol as the gage liquid, since i t does not affect the rubber tubing, possesses a n accurately known specific gravity, and exhibits a low surface tension. Distilled water or any other liquid may, of course, be used with proper care so long as the specific gravity is known. If the instrument is constructed as indicated, precise readings may be made with i t in 30 seconds. Additional speed of measurement may, of course, be attained if the slope of the meniscus tube is set at more than 3" from level, although some sensitivity is sacrificed thereby. A slope of less than 3" destroys the ease of adjustment necessary for routine usage and adds little or nothing to the accuracy of readings. When using the micromanometer it is essential that the meniscus be brought to the point of adjustment on the cross hair always by turning the micrometer head in the same direction (that which draws the meniscus tube upward). This procedure overcomes errors due to play in the micrometer screw threads and failure of the movable assembly to drop against the friction of its ways. Contrary to frequent belief, i t is not mandatory that the sides of the liquid reservoir be absolutely vertical at the risk of error. So long as the volume of the liquid in the gage remains the same, the volume between the hair line (when the meniscus is adjusted upon it) and the surface of liquid in the reservoir must be the same, and therefore the level of the liquid in the reservoir bulb is, at the completion of any adjustment, identical with that obtained with atmospheric

FIGURE1

A

SIMPLE, convenient, and rugged design of micromanometer, adaptable to routine measurements of minute

pressures and pressure differentials, has been developed by engineers of the American Gas Association Testing Laboratories and successfully used by both the Cleveland and Los Angeles laboratories for several years.

The American Gas Association micromanometer consists, as shown in Figures 1 and 2, primarily of a heavy metal base, equipped with suitable leveling screws and indicating levels, and supporting reasonably massive columns carrying, respectively, the micromanometer screw assembly and the gage liquid reservoir. The reading meniscus lies in a short length of straight 0.25-inch glass tubing inclined at a slight angle from horizontal (generally in the neighborhood of 3") and securely affixed to a thick-brass annular plate attached to the micrometer screw. This plate may be rotated and clamped to its carriage in order to permit adjustment of the slope of the meniscus tube. The other arm of the gage, comprised of a liquid reservoir bulb 3 to 4 inches in diameter, preferably with straight vertical sides, is clamped rigidly to the left brass column and connected to the movable meniscus tube by a length of thick-walled rubber P R E Sn suR70 tubing as short as possible and directed uniformly upward so that bubbles may not be trapped therein. At its mid-point, the meniscus tube is supplied with a cross hair or engraved hairline. The meniscus is observed through a magnifying eyepiece of the variety commonly used for reading precise 'calibrated mercury thermometers. It is often necessary to locate a light disk,, painted in flat white, behind the hairline and in line with the eyepiece, to facilitate reading in questionable light. Forty threads per inch, corresponding to a linear pitch of 0.025 inch, should be cut on the micrometer screw, the diameter of which should not be less than 0.5 inch. At the Testing Laboratories the screw head is divided 25 divisions t o the

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SCREW

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RUBBER TUBING

LEVELING SCREW

FIGURE2