Determining Density by Using an Analytical Balance

Archimedes of Syracuse, (c. 287 to 212 BC), a scientist, mathematician and reputed coiner of the word Eureka! developed the principle of buoyancy. It states that an object immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced by the object. On a small scale, a 1 carat diamond displaces 200 milligrams. On a much larger scale the 30,000 ton RMS Oceanic launched in 1899 displaced 30,000 tons of seawater.

Analytical balances are used in laboratories not only to weigh small quantities but also, with the use of density determination kits, to calculate the density of liquids and solids. Depending on the particular model these precision instruments have a maximum weighing range of 83 to 320 grams and readout of 0.1 milligram. This makes them eminently suitable for density determination exercises. Once programmed the analytical balance exhibits the density or specific gravity on its large liquid crystal display. Data can be transferred to a PC or printer using an RS 232 interface for record-keeping.

Setting up a Density Determination Kit

Cotton gloves should be worn by laboratory technicians setting up and operating a density determination set. This is because oil on fingers can transfer to the object being studied and influence weight, and because oil can cause bubbles as small as 1 mm3 that can increase buoyancy on small specimens, resulting in a reading error.

Different analytical balances will have different procedures for density determination, so instructions should be carefully followed. Many balances can be programmed to measure solids or liquids. Here is a brief on setting up the balance for density determination:

1. Turn off the analytical balance and disconnect it from its power source.

2. Remove the balance grading ring, weighing plate and platform.

3. Place the density determination kits weighing platform and vertical rack assembly into the weighing area of the balance. The rack assembly supports a sample dish used when weighing in air and a suspended mesh sample holder for weighing in liquid.

4. Center the kits glass beaker platform over the weighing platform in such a way that it does not touch the weighing platform and center the beaker on its platform.

5. Carefully install the weighing platform assembly consisting of the sample dish and mesh sample holder so that it is suspended in the beaker, then install the thermometer.

The analytical balance may then be connected to its power supply and density calculations can proceed by following the instruction manual.


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