February 15, 1941
ANALYTICAL EDITION
the cast iron, which showed in one determination an error of SO.05 per cent, the greatest error for any determination was 0.02 per cent. Most of the analyses show either an error of 0.01 per cent or no error at all. The certificate values of the second column are rounded and listed as percentage of carbon present in column 3. It is Obthat are 'Ompared with the these latter tained, because the magnitude of the weight change in the
117
absorber restricts expression of results to ti$fo significant figures. Literature Cited (1) Bright, H. A., and Lundell, G. E. F., Bur. Standards J. Reseamh, 5 , 943 (1930).
(2)
Brinton, P. H. M. P., J . Am. Chem. SOC.,41,
1151 (1919).
OPINIONBexpressed in this article are those of the writers and do not necessarily reflect the ideas of the Naval Service.
Simple Tests to Indicate the Condition of an Analvtical Balance J
LEONARD C. KREIDER, Bethel College, Pr'orth Xewton, Kansas
I
T OFTEX falls to the lot of a teacher of quantitative analysis or an industrial chemist to determine whether a balance is in satisfactory weighing condition. This paper presents a simple and rapid method of balance testing that is sufficiently exacting for most purposes. The fundamental principles upon which this brief series of tests is based are well recognized, but the method of application is believed to be new. A recent paper (1) gives valuable additional information from the instrument repairman's point of view. The following points should be considered: 1. The general condition of the balance must be acceptable. 2. The rest point should be constant for any particular load
when the mass on each of the pans is equal. 3. The balance must be of the proper degree of sensitivity. (Sensitivity is understood in this paper to be numerically equal to the deflection on the pointer scale caused by the addition of a 1-mg. load to a single pan of the balance.) 4. The balance must give weighings that are closely reproducible. 5 . The balance must have lever arms of nearly equal length. These points are intimately interrelated-for example, a n imperfect knife-edge may cause the balance to perform poorly in respect to points 2, 3, and 4. Point 1 is not readily tested quantitatively, but a careful inspection will usually suffice. Make sure that the beam releases, pan rests, rider carrier, chain weight devices, and other moving parts are in good mechanical condition. See that the knife-edges are separated from their bearing plates by the beam lift to a gap of about 0.1 mm. and that when the beam lift is released all three knife-edges make contact with their plates over the whole edge gently and simultaneously. This is essential to the life of the edges. Metal parts well finished and free from corrosion are desirable, but do not necessarily indicate an accurate balance. The information sought in points 2 and 3 can be obtained quantitatively by the following method: Place the balance on a firm support in a part of a room where a fairly constant temperature prevails (away from radiators, open windows, and other drafts, out of direct sunlight, and removed from other hot light sources). Level the balance with the setscrews provided in the base. If the balance has recently been moved from another location, open the door of the case and allow at least an hour for the balance to attain room temperature. Bfter the balance has been brought to the same temperature as its environment, determine the data required to construct a table similar to Table I. For this purpose select two sets of analytical weights, W1 and WB. I t is convenient, but not necessary, to have the sets agree within fairly narrow limits. They need not be calibrated. If two sets of weights are not available, one can make shift with only one set-for example, if the data a t 20 grams' load are to be determined, one could call the two 10-gram weights together W1 and the 20-gram weight W2.
Table I records data obtained in applying this method to a typical student balance. The principle involved is that of double weighings first devised by Gauss. TABLEI. DATAFOR STUDENT BALANCE Weight of
WI and Wv2 (Each) Grams 0 10 20 50
Rest Point", A
Rest Pointh, B
Average Rest Point, C (A B)/2
9.0 9.0 9.5 9.1 8.6 10.2 9.7 9.5 100 9.3 11.1 WIon left pan Wz on right pan. b Ws on left pan: WI on right pan. c W I on left pan, WI 1 mg. on right
+
+
9.0 9.3 9.4 9.6 10.2
Rest Point",
Sensitivity, E (B D)
6.9
2.1 1.8 1.4 1.0 0.8
D
7.3
7.2 8.5 10.3
-
pan.
Constancy of the values in column C would satisfy point 2. If the rest point in C should shift by as much as two or three pointer scale divisions between loads of zero weight and 100 grams' weight on each pan, the balance would not be acceptable for determining absolute mass values, but might prove acceptable for certain types of gravimetric analysis where the determination of small differences in mass only is required. The balance tested (Table I), where the rest point shifts 1.2 scale divisions between zero weight and 100 grams' weight load on each pan on the basis of a sensitivity of 0.8 at 100 grams' load, would cause an error of 1.5 mg. in determining a 100gram load. This amounts to a deviation of only 0.0015 per cent, which would be negligible for most work. It is worth while to test the effect of changing the position of the masses from the centers of the pans to the edges and see if the value of the rest point is thereby changed. Defects of the end knife-edges may sometimes be detected by this method, whereas they may remain unnoticed when the masses on the pans are perfectly centered. The point of rest should also be checked by using swings of small amplitude and then swings of considerably greater amplitude. Difference between the two values indicates worn, nonparallel, or otherwise faulty knife-edges. If the balance is to be used where the requirements are only moderately exacting (point 3), the sensitivity (column E ) should have a numerical value of at least 2 and preferably 3 or 4 a t zero load on the balance pans. The sensitivity of a balance should remain nearly constant or should decrease slowly and regularly with increasing load on the balance pans. The fall in sensitivity is usually due to a difference in level between the middle and the end knife-edges, and may be caused by bending of the beam under the load, by wear of the knife-edges, or by not sharpening them uniformly. I n general, it is not safe to use a balance for loads that reduce the sensitivity to less than 40 per cent of the value with zero load.
118
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. I n 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).
___
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
