NOTES ON ANALYTICAL PROCEDURES M e r c u r y Cleaning Apparatus for Continuous Operation FREDERIC E. HOLMES, 6515 Blueridge A v e . , Cincinnati 13, O h i o
Tmeans
H E present contribution is concerned with the mechanical of bringing &boutthorough agitation and fine division of mercury in contact with any chosen cleaning sohtions rather than with the chemical process itself. The apparatus consists of as many units, arranged in series, as are needed to acthe desired number of reagents and washes. A single unit is shown in Figure 1. Each unit is prepared for operation in the following manner. In order to prevent escape of solution, a small amount of mercury is introduced into the outlet trap through the outlet by of a funnel and short piece of rubber tubing, and a felv drops are also placed in the inlet cup and trap. The solution the reservoir is filled to Tithin about 100 ml, of its capacity
Figure 1.
Single Unit of Apparatus
Splash trap and $ohtion chamber Outlet trap (part surrounding ascending tube) Arcendin and descending tubes Inside &meter$ (bore) Pump tube Vertical distance D to L Vertical distance: D to M Inside diameter of chopping bulbs Dimenrlonr u e outside diameters except as noted. ratio of deliver, to recirculation.
P5-28 mm.*
15-16 mm.* 9 * 0.5 mm.* 3-3.5 mm. 1 5 mm. 11-13 mm. 90-99 mm. 9-11 mm.
* Calculated for desired
desired cleanlng solution and connected with the unlt by rubber tubing as shown. The air inlet is connected by pressure tubing t o a source of air under pressure, Cleaning solution is poured in through the splash trap until the level reaches about halfway between bottom and top of the pump tube. The connections to the solution reservoir are freed of air by pinching the rubber tubing. Air under pressure is admitted cautiously through the air inlet until it is bubbling freely from the top of the ascending tube a t J in the solution chamber. Mercury is then allowed to flow into the inlet cup of the first unit, and the rate of flow is adjusted to the desired rate Of Air entering the ascending tube a t A makes the column of fluid above this point lighter than in the descending tube and produces a rapid flow in the direction indicated by the arrows. Mercury entering the solution chamber through the inlet trap flows into the descending tube a t M in large drops interspersed with solution from the chamber and thence down this tube to the ascending tube. The bulb below A retains enough mercury to act as a valve, retarding back-ivard flow and favoring normal flow. Above A , the mercury is broken into smaller droplets by the air. The chapping bulbs in the ascending tube promote division, COalescence, and redivision of the mercury, thus frequently renewing the surface exposed to action of the solution and greatly extending its area. During their progress upmard in this tube, these fine droplets are falling in relation to, and through, the more rapidly moving solution and air. Air, solution, and mercury are ejected into the solution chamber from the jet a t J. The air passes out through the pump tube, carrying solution over with !t and maintaining circulation of the solution between resyvoir and solution chamber. The mercury impinges on the wall of the chamber and falls back in a shower of droplets to the dividing point, D, where a small part enters the top of the outlet trap. The larger portion falls to the bottom of the chamber and reenters the cleaning cycle. The ratio of mercury delivered from the outlet t o that recirculated is approximately proportional to the areas of the annular space between the outside of the ascending tube and the top of the outlet trap and that between the top of the trap and the wall of the solution chamber, indicated by I and 0, respectively, in the insert. The ratio is fixed in the construction of each unit. A ratio of approximately 1 to 5 has been used. Adjustments during operation are not critical within a wide range. Normal function is maintained by rates of flow of air from that of a “lazy” stream of bubbles t o that which produces vigorous action and maximum turbulence. The limiting factor is back-pressure in the ascending tube, The rate of flow of mercury may be from the minimum obtainable t o as much as 8 kg. per hour attained in one unit tested, Because of this wide range in rate of performance, each unit of the series mill accommodate varying amounts delivered from the preceding unit, and no elaborate balancing of adjustments is required. Air under pressure may be obtained from the usual laboratory system or from a pump delivering a t a pressure of 700 grams per sq. cm. (10 pounds per square inch). Each unit requires a separate valve in a line carrying full pressure, so that the unit having lowest back-pressure cannot steal air from the others. I n order t o use one pump or one outlet of the laboratory air supply for several units, a metal manifold having a sufficient number of metal valves is suggested. If this is made of large pipe capped a t one end, it can be filled with cotton or other material to serve as a filter to remove oil and dirt from the air. To prevent escape of cleaning solution when a unit is not in use, the pressure tubing may be disconnected a t the metal valve and the free end raised and hung on the supporting frame by means of a pinchclamp. 45 1
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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
Batch operation of single units of a similar design, using succesqive changes of cleaning solution, has been tested over a period of two years a t Wright Field, Dayton, Ohio, and a t Bushnell General Hospital, Brigham, Utah, and proved convenient and satisfactory for processing very small amounts of mercury. A single unit of the present apparatus may be used similarly when economy of space is more important than the advantages of simultmeous multiple operations. The advantages of the npparatus are fine division of mercury and prolonged contact I\ ith the solution, elimination of fine orifices which tend t o become clogged with dirt, automatic operation requiring little attention, relatively small size, light weight, adaptability to few or many simultaneous cleaning operations and to very small and very large capacity requirements, and small holdup. Cleaning solutions that have been used satisfactorily in this type of apparatus are 10% lactic acid, 5% sodium hydroxide, 5Tc sulfuric acid, 3 to 10% nitric acid, and mater, usually in the order named when more than one is used. Control of operations has been based on such criteria as speed of coalescence of droplets, freedom from blemish of the mercury surface, freedom from stain on glass surfaces in prolonged contact with the mercury, and clean appearance during distillation. More specific data h a w not been sought. I t has been assumed that quality of product is inversely related to its rate of flow.
Vol. 18, No. 7
Figure 1 is drawn to scale, except that some diameters are slightly exaggerated for printing. Operable models can be made within a wide range of dimensions and proportions. Ascending tubes have been used having inside diameters up t u The descending tube should have the same inside diameter as that of the ascending tube; greater bore results in unstable operation, and smaller bore yields little if any advantage. The bore of the jet should not be tapered to less than half that of the rest of the ascending tube; if too small, the back pressure is excessive. The ascending and descending tubes should be built as close to each other as possible to avoid collection of excessive amount of mercury in the horizontal part of the loop. A slight funnel-like expansion of the top of the descending tube favors normal flow. A slight constriction at the bottom of the pump tube is intended to improve pumping action. The side tubes for connection with the solution reservoir are separated horizontally sufficiently to avoid difficult glass blowing and excessive fragility. The one carrying the return to the solution chamber is slightly funnel-shaped and inclined to provide for return of mercury droplets to the chamber. The mercury inlet cup and trap and the mercury outlet are actually in a plane perpendicular to that of the ascending and descending tubes and air inlet. The length of the splash trap may be increased for alkaline or soapy solutions, or caprylic alcohol may be used to break any foam. Any convenient bottle is suitable for the solution reservoir. Units of this apparatus may be obtained from Harshaw Srientific Company, Cincinnati, Ohio. 4 mm. and lengths up to 600 mm.
Assay of Hydroquinone 1. M. KOLTHOFF AND THOMAS S. LEE University of Minnesota, Minneapolis, Minn.
AIODOMETRIC .
COMPARISOS has been made of the iodometric and cerometric titrations in the assay of hydroquinone. METHOD. In this method an excess of standard iodine solution is added to a solution of the hydroquinone and the excess iodine is back-titrated (1, 4, 6, 6). The sources of error have been studied systematically with the following results:
1. In order to have the oxidation of hydroquinone go to completion the pH of the solution must be greater than 6. Many authors (1, 5, 6) recommend a solution of sodium hydrogen carbonate (pH 8.4) as a suitable reaction medium. 2. If the reaction i s carried out in a hydrogen carbonate solution, the solution should be protected from air to prevent air oxidation of hydroquinone (3). The back-titration of excass iodine must be made with arsenious acid, not with sodium thiosulfate. 3. Another source of error, not discussed in the literature, is that iodine reacts to some extent with quinone to form an orange or brown colored product. From a number of experiments it was concluded that the back-titration of iodine must be made relatively quickly in order to reduce the extent of reaction of iodine with quinone to a minimum. The orange or brown compound formed also obscures the end point. The following procedure, taken from the Laboratory Manual of the Rubber Reserve Company and modified slightly, was found to give results almost identical with the procedure recommended by Kolthoff (4). Procedure. Accurately weigh a sample of 0.15 to 0.2 gram and transfer it t o an Erlenmeyer flask. Add 25 ml. of water, dissolve the hydroquinone, and add 5 grams of solid carbon dioxide and 2 grams of sodium hydrogen carbonate. Cover the flask with a small watch glass and swirl the flask intermittently until the dry ice has disappeared. Then warm the solution to room temperature but not above. Do not allow the sample to stand any length of time a t this point. Add 1 ml. of starch indicator and titrate immediately with 0.1 N iodine solution to the starch-iodine end point. Add just 1 ml. of iodine solution in excess and back-titrate with 0.05 N arsenious acid solution to a light yellow color. The error in both Kolthoff’s procedure and that givr,ri above
was found to be +0.30/0,and the precision is of the same order of magnitude. CEROMETRIC METHOD. Titration of hydroquinone with ceric sulfate solution was recommended by Furman and Wallace (Z), mho found that the end point could be detected potentiometrically or R ith diphenylamine or diphenylamine sulfonate indicators. The authors have found that ferrous phenanthroline is a very suitable indicator. The average error of ten titrations with ceric sulfate (0.1 N ) wae
0.02%. In the titration of three commercial samples of hydroquinone of 96 to 100% purity, iodometric and cerometric methods gave the same results to within *0.3%. The cerometric method is recommended for the assay, ruutine or otherwise, of hydroquinone, except when the sample contains impurities, such as oxalate, which are oxidized by ceric sulfate but not by iodine. SUMMARY
The ceric sulfate piocedure for the titration of hydroquinone is accurate and precise to O.OZyo. Ferrous phenanthroline can be used as an indicator. Under the most favorable conditions the iodometric method gives an error of +0.3%, due to react,ion of quinone with iodine. LITERATURE CITED
(1) Casolari, A , Gam. chim. ital., 39, 589 (1909). (2) Furman, N.H., and Wallace, J. H., J. Am. Chem. Soo., 52, 1443 (1930). (3) James, T.H., and Weissberger, A.,Ibid., 60,2090 (1938). (4) Kolthoff, I. M.,Rec. trav. chim., 45,745 (1926). (5) Sorensen and Linderstrom-Lang, Comp. rend. trav. lab. Carlaberg, 14,No. 14,26 (1921). (6) Valeur, A., Bull. S O C . chim., 23, 58 (1900). THISinvestigation was carried out under the sponsorship of the O 5 o e of Rubber Reserve, Reconstruction Finance Corporation, in connection with the Government Synthetic Rubber Program.
