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INDUSTRIAL AND ENGINEERING CHEMISTRY
Weights of 90 to 100 grams should be the maximum allowed on each pan of the balance that is serving as our example. Point 4 could be determined by again getting the rest point of the balance with pans empty after securing the data of Table I. If this checks.mith the corresponding value in the table within 0.2 pointer scale division, the balance is satisfactory. If further checks on this point are desired, one can repeat the weighings a t any pan load. Point 5 is easily tested as follows: An object of mass M (a 20-gram weight is convenient) is placed on the left-hand balance pan and counterbalanced with other weights from the set and their sum, S, is recorded. M is then transferred to the right-hand balance pan and again counterbalanced with weights from the set and their sum, s’,is recorded. If L equals the length of the left lever arm and R equals the length of the right lever arm of the balance, from the principle of the lever ML = RS and
IMR
=
LS’
If we divide Equation 2 by Equation 1 we get RIL
=
(3) On developing the quantity under the radical sign in terms of a series of powers of z and S, where z represents the difference between S’ and S (S’ = S * z),we get the series
Vol. 13, No. 2
In applying this t o the balance, z is always very small as compared to S; so the equation reduces essentially to
The upper sign is used where S’ is greater than S and the lower sign is used when S’ is less than S. In a good balance the R / L value should be 1.0 * 0.00002. In dealing with comparative values, as in gravimetric analysis, an R / L vaiue of 1.0 * 0.0002 can be tolerated without appreciable error in the final result. In deciding whether a balance is suitable for the work a t hand, one must also know the probable limits of error intrcduced by factors other than the balance. The balance may be used without hesitation if i t is twice as accurate as the least accurate of any of the other measurements involved. It is very probable that more errors in student and commercial work are due to uncalibrated or poorly calibrated weights than to inaccurate balances. Moreover, manipulative techniques, aside from weighing, usually introduce far larger errors than can be accounted for by the inaccuracy of weighing; and the percentage of error of many analytical methods, due to such things as end-point errors, solubility of precipitates, adsorption, deliquescence, inability to measure volumes accurately, etc., is far greater than most of us would tolerate in an analytical balance.
Literature Cited (4)
(1) Craig, A,, IND.ENQ.CHEM.,Anal. Ed., 11, 581 (1939).
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Useful Centrifuge Accessories
W
C. R. JOHNSON, University of Texas, Austin, Texas, AND HARVEY MILLER, Quartermaster Laboratory, San Antonio General Depot, Fort Sam Houston. Texas
HEN reagents are to be purified rapidly and efficiently by crptalliza-
tion, centrifugal draining of the crystals is essential. Unfortunately, convenient and inexpensive centrifuge accessories for this purpose are scarce, particularly for crystallizations which must be carried out on a small laboratory scale. The basket-head type of apparatus is expensive, cannot easily be made without special equipment, and is not very convenient for use with small quantities of material, or in any case where very high purity is required. Accessories which permit the adaptation of various sizes of Biichner funnels or Gooch crucibles as baskets are not quite so expensive, but are unsatisfactory in other respects. The perforated cup type of accessory, in which both crystals and liquid are completely enclosed during centrifuging, is especially suitable for small quantities of material. Made from platinum or gold, such an accessory is universally useful, but the cost is prohibitive for most laboratories. A plated apparatus might seem to offer a suitable substitute, but inquiries indicate that the cost of any practical design is rather high. Accessories of this type made from the methyl methacrylate resin Lucite are inexpensive and for many purposes fully as useful as if they were made from one of the noble metals. Lucite is almost completely insoluble in water solutions of salts, acids, and dilute alkalies, and in straight-chain hydrocarbons. However, it cannot be heated much above 70” C. and is soluble in many organic solvents. The authors have designed two accessories which may be machined from stock sizes of Lucite sheet, rod, and tubing to fit standard centrifuge cups. The inner cup of the small model has 61 holes drilled in a hexagonal pattern with a No. 70 B. & S. gage drill. The inner cup of the larger model has interchangeable bottom plates, each with 169 No. 60 or Yo. 70 holes. Eight complete accessories were made for $3.50 each, including the cost of labor and material. This is one eighth the lowest quotation obtained for a single set made from any suitable combination of base metals, and one fifteenth the cost of a small basket-head accessory made from manganese bronze. The economy is even greater, since the Lucite accessories are more generally useful with water aolutions than those made from base metal alloys.
--3l rnm. D.4
