AN ELEMENTARY CHEMISTRY LABORATORY BALANCE
A need seems to exist in elementary and general chemistry laboratories for a new type of small demountable balance. To date, the balances on the market for use in chemistry classes seem to include one or more of these disadvantages: they are flimsy and poorly constructed; they are either not demountable or consist of too many small parts; weights are required which the student easily loses; they are limited to use in only a few experiments; and in some cases they fullil other requirements but are too expensive. Description and Operation With the above defects in mind a balance has been designed to fill the need and is shown set up in the accompanying illustration. It is composedof fivesubstantial, well-made units. The base is lacquered cast iron of a size, 9" X 33/4" X '/&". The upright post screws into the base two and one-half inches from the left side and the rigidity and sturdiness of the balance are assured by the weight of the base firmly holding the post. The latter is of lacquered cast iron, 5" long by 3/8" in diameter. On too of this is the knife edge rest and card, made of brass and with contact points of inlaid tool steel. The card, rest, and post is 8 3 / 4 v long and comprises the second unit of the balance. The beam is the third part and consists of a brass counterpoise on a Z 3 / & " threaded arm; a large fine-grade tool steel knife edge fulcrum mounted in the beam, which is extended downward to stop swinging too far; a blackened steel pointer; the rider arm, carrying two riders; a knife edge; and hanger hook. The rider arm is made of German silver and the riders of brass. The knife edge is mounted in the rider arm Z1/18" from the fulcrum of the beam and supports the hanger hook. The 127
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JOURNAL OF CHEMICAL EDUCATION
JANUARY,
1930
beam is notched so that a small rider between the fulcrum and the pan knife edge covers a scale of one gram in tenths of grams and a large rider covers a scale of twenty-five grams in grams. A small screw inserted a t the extreme right of the beam, eleven inches long, prevents the removal of the riders. The fourth and fifth parts of the balance are the pan hanger of brass and a fine German silver pan, 23/&"in diameter. When set up the scale stands 9'/2" high and is as wide as the beam is long. The five separated parts are packed away in their respective pockets of a heavy flannel case 4" X 12" X l'/%", which offers the best protection for the parts and occupies a minimum of space when not in use. When a weighing is to be made the balance need not be leveled other than by placing on a horizontal surface, because the beam should be balanced each time it is set up. It is ready for use after balancing with the counterpoise. Due to the lever principle of the balance the rider scales read from right to left. The only device approximating these principles is the westphal balance. We feel that this scale meets the requirements, in that it is strong, solid, and substantial, that i t is demountable and made of only five large parts, that there are no weights to be lost and that i t is adaptable to many uses in classrooms, laboratories, and field work. It is sensitive to ten milligrams or 0.01 gram and when weighings are desired to that accuracy a third rider consisting of a small loop of wire should be used, the sensitivity is nearly uniform regardless of the weight on the pan. With the aid of the counterpoise aucibles, dishes, beakers, and watch glasses may be balanced before weighing substances in them. By using a small weight and a'glass bob hung from the hanger hook, specific gravities of liquids lighter and heavier than water up to 2.500 may be measured. The weight similar to the large rider and having a small book on the under side is attached to the hanger hook to compensate for the weight of the pan and hanger removed. The glass bob is made of solid glass rod rounded off a t the top and with a small loop of glass annealed to it. The bottom is ground and in the grinding is standardized to lose exactly ten grams of weight when immersed in distilled water 70" Fahrenheit. The bob is 25/*" long by 9/16" (15 mm.) in diameter and is hung by means of No. 36 platinum wire one inch from the weight. The weight and bob are placed on the beam, the large rider on the twenty-five-grammark. and balanced with the counterpoise. Then whw the bob has been immersed in a liquid each notch to the right on the large scale will be equivalent to 0.1 of one degree specific gravity. The rider must be moved to the fifteen gram mark for water. Each notch to the left with the small rider detracts one-hundredth of a degree from the reading on the large scale. With the weight and bob is provided a scale showing the equivalent specific gravities for the readings on the beam scales. The third rider may be incorporated for reading to three decimal places.
VOL. 7, No. 1
ELEMENTARY CHEMISTRY LABORATORY BALANCE
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Use of Balance in Elementary Chemistry A study of laboratory manuals for elementary chemistry has shown that ten per cent of the experiments require a balance or scale as part of the apparatus. Another fifty per cent can for definite quantities of substances or for solutions of definite proportions of salts which are best obtained by using a balance. In other words, in elementary chemistry laboratories the aim is more nearly fulfilled in sixty per cent of the experiments by employing a balance. Then, too, the student is learning by actual experience something of the accuracy of modem science which might otherwise be entirely missed. Uses for such a balance in qualitative analysis are quite frequent. We have found by experiment that with ten or twenty-gram samples, rough quantitative analyses can be made with the above described balance showing accuracy of one tenth to one half of a per cent.
