Construction and Use of a Berman-Type Balance Laurence Huestis and Joseph Gormley Pacific Lutheran University, Tacoma, WA 98447
As part of our analytical scheme for identification of minerals, metals, and other inorganic compounds, we needed agood method to measure the density of small, solid samples. Chemists have traditionally used pycnometers for this purpose, whereas mineralogists have used Berman balances.' The underlying principle of the Berman balance is essentially the reverse ofthe familiar Westphal balance.= (Instead of measuring the density of an unknown liquid by measuring the buoyancy of a solid of known density, the Berman balance measures the buoyancy of a solid of unknown density using a liquid of known density). After using both methods, we found that a Berman-type balance has several advantages over a pycnometer.
was usually less than 1%.Very careful work gave reproducibility that justified four significant figures for the same volume range. An additional advantage for making such a balance in the 1990's is that mechanical single-pan balances have been largely replaced by the more rapid top-loading electronic balances, leaving many older siugle-pan balances readily available for modification to other purposes.
1. Errors due to evaporation of liquid and trapped air bubbles are much smaller. 2. The density of much smaller samples can be measured. 3. The required measurements are simpler and more rapid.
Since such a balance was not originally available to us, we made our own Berman balance by modification of a single-pan mechanical b a l a n ~ eAfter . ~ calibration, the ac-, curacy of the resulting balance was checked against 10 analytical-grade standards for densities 1.74(Mg)11.35(Pb) gImL, giving an average standard deviation of 0.02 glmL from the literature value^.^ Our students were able to quickly obtain very good density measurements to three significant figures with this balance for sample volumes of 0.34.03 mL. The relative error for student work 'Klein, C.: Hurlbut. C. ManualofMineralogy,20th ed.; Wiley: New York, 1985;p 206. 'Shoemaker, D.; Garland, C.; Nibler, J. Experiments in Physical Chemistry, 5th ed.: McGraw-Hill: New York, 1989;p 764. %koog, D.; West. D.; Holler, F. Anal. Chem., 5th ed.; Saunders: Philadelphia, 1990;p 519. 4Handbook of Chemistrv and Phvsics. 50th ed.: Weast. R.. Ed.: CRC: Boca Raton, FL, 19f0.
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Figure 1. A Berrnan-type balance forthe determination of the density of solids.
The f r s t three items a r e shown i n more detail in Figure 2. The immersion liquid for the density determinations was redistilled, reagent-grade toluene with a density of 0.867 g/mL (20 OCh6
Calibration The orieinal .. sinele- an balance was zeroed. Then t h e van and holder were removed and replaced with the counterweieht holder. the two lead counterweiehts. and the wire sample pan. h e total apparent weigh
