The Westphal Balance was first described by German chemist Carl Friedrich Mohr in 1832. Although often referred to as a density balance, a more concise definition would be that it is a specific gravity balance. Modifications were made by the good Dr. Westphal who replaced a tray method with an adjustable arm counterweight.
Depicted in the diagram below, the Westphal Balance operates by suspending a glass tube (with a mercury thermometer contained within it) into a sample of a solution of unknown density via a thin platinum wire. The scale relies on Archimedes' Principle of buoyancy and is balanced by an array of horseshoe shaped counterweights which come in 5 g, 0.5 g, 0.05 g, and 0.005 g masses. These counterweights (sometimes called riders) respectively signify the ones place of the specific gravity of the sample solution, the tenths place, the hundereths place, and the thousandths place. The numerical value each rider represents is equal to the numbered notch of the arm which it sits in when the scale is balanced.
To operate a Westphal Balance care must be taken to first calibrate the balance by means of the leveling screw at the bottom of the body. With no weight on the arm of the balance the two pointers must be aligned before the balance can be used. Since the Westphal Balance is measuring specific gravities of our sample we can proceed to divide the numerical result of all samples' specific gravities by ρref (the density of water at 4oC in a vacuum, ρref = 0.999973 g/cm3) in order to get ρsample. It is in this way that we ensure we are finding the exact density of each sample we examine. This step may be skipped, however, if the density of water at current air conditions is assumed to be 1 g/cm3.
As alluded to earlier, reading the Westphal is quite simple. Placing a 5 g counterweight (1/1 counterweight) in the #1 position should balance the scale for pure water. This means that there would be a 1 in the ones position of water's density, and the scale's pointers being aligned would apply that no further digits after the decimal place is necessary which we know to be true. Two 0.5 g counterweights (1/10 counterweights) may also be placed in the #5 position to equal a density of 1 g/cm3. Supposing an organic solution of unknown composition was balanced with a 1/10 rider in the #8 position, a 1/100 rider in the #3 position, and a 1/1000 rider in the #9 position will have a specific gravity of 0.839. Assuming the density of water for this experiment was a perfect 1.00 g/cm3 the density of this organic solution would be 0.839 g/cm3 as well.
The Westphal Balance is incapable of highly accurate density readings, as large temperature ranges in laboratories can affect is ability to duplicate results, and the alignment of its pointers is based on human sight and therefore full of potential for human error. For extremely accurate density readings one may prefer to use a pycnometer although for speed and ease of use a Westphal can quickly provide a very close approximation to the true density of any liquid solution.
________
/ ______ _ __ __ __ __ __ __ __ __ __~
/\ / |~~| V V V V V V V V V |
|)=(XX> {________| | 9__8__7__6__5__4__3__2__1___|
\/ | | |
/ \____=______|__| | C ---hook
/ | | | |
pointers | | | |
| | arm |---platinum
| | | wire
| | |
| | |
_ | | _|__
/ | | | | |
/ | | |_|_|
body | | | | |
| | | | |
|=|=(| | | |
| | | | | -tube of
| | | p | solution
| | glass bulb-- | d |
| | | p |
| | | d |
| | | |
| | | |
| | ____ |___|_____
| | leveling |
| | screw |
| | / |
| | / |
| | / |
| | |
| | = |
____| |__|_ |
|___________| |
_____n________n______________________|==========
Sources:
http://www.photolib.noaa.gov/ships/shind35.htm
Ch 421 Laboratory at Stevens Institute of Technology