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THE LABORATORY HOOD By J. C. Witt PITTSBURQE. PENNSYLVANIA Received January 10, 1920
Though the hood is an important part of the equipment of the chemical laboratory, its design often receives surprisingly little consideration. A number of excellent hoods may be found, but really satisfactory ones are comparatively rare, and for every even fair hood there may be found several t h a t are decidedly faulty. Why this should be is difficult t o explain. It may be partly due t o the fact t h a t the subject has not received much general discussion and little has been published concerning it.l The inconvenience t h a t may be caused by a faulty hood, and the factors which make one good or bad are known t o everyone with laboratory experience, yet i t often happens t h a t in installing a laboratory the chemist does not give the matter the thought i t deserves, or from motives of economy does not spend the amount necessary for satisfactory construction. Then, too, many laboratories are installed in rooms not built for the purpose and the space available and the general arrangement are not favorable. The design will depend to some extent upon the character and amount of work carried on in the laboratory for which i t is built, but the essential points are as f OlIOWS: sIzE--The hood should be sufficiently large so t h a t in case a hot plate is used, the walls will not be so close t o the source of heat t h a t the glass will be broken, the woodwork burned or the paint on the interior blistered. The hood should also be large enough t o accommodate the work for which i t is intended, but no larger than is necessary, as its efficiency may be considerably affected by the size. FLOOR-The floor should be of some substance which is not readily affected b y heat or by acids. Care should be taken t o insulate the portion under the hot plate from the heat thrown down from the burners. One satisfactory way is t o first place a heavy sheet of asbestos several inches longer and wider t h a n the plate, then a sheet of iron sufficiently heavy t o support all its load without bending. The iron plate is supported by small bricks or cement briquettes about one inch above the asbestos t o provide an air chamber as a further insulator. White tiles are well adapted for use in most hoods. They are acid and heat resisting and assist in keeping the interior light. 'They are somewhat more expensive t h a n most other materials used for the purpose, and considerable care must be taken in setting them, since otherwise shrinkage cracks, caused by heat inside the hood, are likely t o appear. LIGHT-when space permits the best location for a hood is in the center of a room, so t h a t all the walls may be of glass. When this arrangement is not possible, the walls should be painted or ecameled white and sufficient electric lights should be placed in the ceiling, or on t h e side walls high enough t o be out of the direct 1
Chem-Eng., 9, 21; 12, 183; Chem.-Zlg., 86, 254; J . A m . Chem. S O C ,
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line of vision, t o keep the place well lighted. Even when the walls of the hood are of glass the electric lights should not be omitted because they are needed a t night and on cloudy days. Since white paint will gradually grow yellowish or brownish from the effect of fumes, the walls should be repainted frequently. The paint should be inspected from time t o time t o see t h a t it is not blistering or scaling off. METAL INSIDE THE HOOD-AS far as possible, metal inside the hood-especially iron or steel-should be avoided. I t is not necessary t o have any exposed metal above the level of t h e hot plate. Porcelain electric light fixtures may be used, and i t is often possible t o place these above the hood, allowing the light t o pass through the glass roof. No matter how iron is plated or painted a t the start, the metal is likely t o be exposed in time t o the action of fumes, after which i t is not long before rust is dropping into vessels on the hot plate. O U T L E T PIm-This should be made of tile or earthenware or possibly of lead, but never of iron. It is preferable for the pipe t o leave the hood in a horizontal position rather t h a n vertical, t o lessen the chance of dust and dirt falling into the hood. If the pipe is vertical the lower end should be protected by a glass baffle plate. For the ordinary sized hood, having natural draft, the outlet pipe should be a t least 8 or I O in. in diameter, and there should be as few turns a s possible. Round bends are, ol course, preferable t o right angles. It is best t o connect the outlet pipe with a high chimney or stack when possible. When this is not possible a special stack high enough t o give a satisfactory draft should be erected. FORCED DRAFT-when i t is possible i t is much more satisfactory t o construct a hood with natural draft only, and if the hood itself and outlet pipe are properly made and proportioned, as a rule no forced draft is necessary. The gases will move more readily t o the outlet pipe if t h e roof slants in t h a t direction t h a n if i t is in a horizontal plane. When the hood will not draw sufficiently unaided, a gas jet inside t h e stack or t h e outlet pipe or some form of exhaust fan may b e employed. The former is t o be recommended when i t is practicable. The fan has the disadvantage of the expense of operation, and deterioration due t o corrosive fumes. In building, where there are a number of hoods, i t is often necessary t o depend entirely on forced draft, and i t is customary t o connect all the hoods with one large exhaust fan. I n many cases such a n arrangement is not satisfactory, since i t is very likely t h a t the draft cannot be kept equal in all the hoods even with a system of dampers. A pipe may become clogged or a leak develop and thereby cut off t h e draft from a number of hoods. It is better t o have a small exhaust f a n for each hood. Down-draft hoods are sometimes made. These are, of course, better adapted t o heavy gases such as sulfur trioxide t h a n the lighter gases, but are often not satisfactory. G A S A N D W A T E R CONNECTIONS-valves COntrOlklg the gas and water supply should be placed in the front, below the level of the floor of the hood. Otherwise i t is awkward t o reach t h e valves and one is likely to
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upset apparatus and spoil determinations. There should be a small sink in the hood so t h a t liquid giving off fumes may be emptied witfiout removal from t h e hood. I n case the hood is of considerable length, there should be a slightly sloping tile trough or gutter connecting with the sink. Fuming or corrosive liquids if accidentally spilled may be washed into this and thus removed. wmDows-It is sometimes recommended t h a t the windows be made small. This arrangement has advantages especially when heavy fumes are produced. However, i t is a sacrifice of working space
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because the available capacity of the hood is thereby considerably lessened. If the draft is sufficient, i t is better t o have the windows take up the whole front of the hood. When there are two windows i t is convenient t o connect the weight chains a t t h e middle of the top of each one and connect the windows directly with each other, with no fixed support between. In this way when all are u p there is no support t o interfere and more room is available for work. Another advantage is t h a t one window may be pulled down a n d substances likely t o spatter into the eyes placed behind the glass.
ADDRESSES AND CONTRIBUTED ARTICLES SELENIUM AND TELLURIUM’ By Victor Lenher UNIVERSITY os WISCONSIN, MADISON,WISCONSIN
The elements, selenium and tellurium, are not so rare as is generally supposed. Indeed the term “rare element” is a rather vague and indefinite expression. Titanium, for example, is commonly regarded as such though F. W. Clarke has shown i t to be more plentiful than carbon, sulfur, and phosphorus combined. Titanium is, therefore, very plentiful but the number of its chemical compounds is very limited. A recent estimate of the amount of selenium and tellurium that can be produced in the United States, without making any material additions to the present plants, has shown that this country can furnish more than 300,000 pounds of selenium and about 1~5,000pounds of tellurium annually. The elements are commonly marketed in elementary form although some of the refineries have produced small quantities of derivatives such as sodium selenite and tellurium dioxide. USES FOR THESE ELEMENTS
The chemical characteristics of these two elements closely follow those of sulfur; indeed, the types of selenium and tellurium compounds are in general those of sulfur, but owing to their higher atomic weights they are more metallic. Tellurium in elementary form looks much like antimony. It is white and so strongly crystalline that i t is quite brittle and can be easily powdered. Toward acids it is as refractory as antimony. Toward alkaline solutions i t is strongly resistant, while in water or in moist air i t does not rust or corrode appreciably. It is known that antimony can be electroplated and gives a durable plating. It would be interesting to study tellurium in this direction. A systematic study of the available electrolytes that can hold tellurium in solution could be carried out advantageously. Antimony has been successfully used for many years in antifriction alloys and is an essential constituent of stereotype metal. No recorded study is known of an attempt to utilize tellurium in these alloys, but the whole field of the metallic alloys of tellurium needs to be studied carefully, and unquestionably an element whose general characteristics are so close to antimony will be found a useful metal instead of having no practical applications. IRON AND STEEL-Another metallurgical line which has not been studied in any detail is the action of these two elements in the iron and steel industry. The effects of sulfur and phosphorus on iron and steel have been very carefully studied. The objectionable phosphorus has been most scrupulously removed by the basic open hearth process as a result of years of study and observations, yet to-day we are actually using iron phosphide to bring 1 Report of a Committee of the National Research Council, presented at the 59th Meeting of the American Chemical Society, St. Louis, Mo., April 12 t o 16, 1920.
up the phosphorus content of open hearth steel which is to be used for certain purposes. It is interesting to contemplate what careful experimentation might develop on the influence of selenium and tellurium on the various grades of steel. SELENIUM cELL-,!?,eleniurn in the so-called metallic form has long been characterized by its unique action toward light. Its conductivity of the current varies so greatly when brought from the dark into the light that this peculiar property has caused the development of the selenium cell. This cell or resistance apparatus has found numerous uses a t various times, such as automatically turning off city gas lights a t daylight. It has been used in lighting and extinguishing the lights in light buoys, in army signaling of various kinds based on the heliographic principle, as a control in chemical processes such as the contact sulfuric acid manufacture, and in wireless telephones. G u s s - I t has been known for a long time that selenium gives a red color to glass. During the war this principle was made use of in decolorizing glass on account of the shortage of manganese. Selenium is introduced into the glass either in the elementary form or as a salt. As the rose color which it imparts is not exactly the complement of the green of ferrous iron, it is common practice to add a small amount of cobalt oxide along with the selenium. Since both selenium and its compounds are very readily volatile a t such temperatures as are used in glass making, a large amount of the selenium decolorizer is volatilized. The loss of selenium is therefore high, and selenium can be used to decolorize glass only when the glass manufacturers are willing to pay the higher cost price. As soon as shipping was resumed after the war and manganese again became available the use of selenium in the glass business fell to almost nothing. LITHOPONE-A number of possible uses for these elements suggest themselves, none of which have received the attention that they deserve. Lithopone, the intimate mixture of barium sulfate and zinc sulfide made by bringing together barium sulfide and zinc sulfate, is full of suggestions. Various colored lithopones can be produced by using an antimony sulfate liquor with barium sulfide, when the white barium sulfate is colored by the orange sulfide of antimony. Similarly, cadmium or arsenic liquors give a corresponding yellow lithopone. The substitution of selenium and tellurium is very suggestive, inasmuch as it should be possible to replace the sulfur of either the sulfate or the sulfide by either of these elements. The selenate and tellurate of barium are white and insoluble like the sulfate, whjle the selenides and tellurides like many of the metallic sulfides are variously colored. MEDICINES-The use of the various compounds of selenium in medicine has received some attention, but the derivatives have had almost no systematic study. In a very careful study Prof. W. J. Gies, of Columbia University, has shown that tellurium compounds have a physiological action quite similar to
