How to Determine Density of a Solid or Liquid Sample with Your Analytical Balance

Some models of analytical balances, also known as electronic scales are suitable for measuring the specific gravity, or density of objects. The density of a solid sample is calculated from the weight of the sample in air and the weight of the sample in a measuring fluid of known density, based on Archimedes Principle of buoyancy. According to this principle, an object immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced by the object.



For example, to determine the density of gemstones and minerals you first need to weigh the stone "in air" and then weigh it in water. Analytical scales which offer this function have a "weigh below" hook on the bottom of the scale or a special kit for density measurements, and in some cases both. From the under scale hook, you can suspend and immerse a "gemstone holder" in a beaker of water. The accuracy of density measurement of small, lightweight gemstones will be limited to the weighing capacity and readability of the scale. A minimum readability down to 0.001 to 0.002 grams (0.005 to 0.01 carats) is recommended for determining specific gravity of gem stones.



Let us now look at the Kern ABT Analytical Balance. The ABT has two methods for density determination: using under balance weighing or using a separate optional set for density determination offered with the balance. The optional set for density determination makes the process easy.



To start the measurement method using under balance weighing, unscrew the two fixing screws to remove the cover for the below-weigh hook cover on the floor of the balance. Then suspend the user-prepared hanging pan from the below-weigh hook and allow the hanging pan to sink into a tank containing liquid of known density.



To determine the density of solids, first change the display to "density of solids" by pressing the [UNIT] key several times. Weigh the sample, then immerse the sample in the temperature-adjusted reference liquid so that it does not touch the weighing pan floor and weigh again. The balance will calculate the density of the solid sample.



The density of a liquid sample is determined with the help of a reference body of known volume called a plummet. The liquid density is calculated from the weight of the plummet in air and in the liquid sample. For determining the density of liquids, change the display to "density of liquids" as described above. Then press the appropriate function keys (as described in the Operating manual) until the density for the solid reference body to be immersed appears on the display. Weigh the solid reference body in air, then immerse the plummet in the liquid sample, but take care that no bubbles develop while you are doing so. The balance will calculate the density of the liquid sample.



The Kern PES/PEJ series of analytical balances also has a density determination function. It has an underfloor weighing device which helps to determine density. To prepare the balance for this task, you need to do the following:



• Turn the balance over and screw in the hook for underfloor weighing.



• Position the balance on a table which has a slit or circular opening in it.



• Hook-in the sample support.



• Fill measuring liquid in a vessel, such as a beaker, and wait till its temperature is stabilized. The measuring liquid should be one whose density has been predetermined. If you are using distilled water, you also need to input its temperature.



After the parameters for the measuring liquid have been input into the balance, you can determine the density of your sample. You do this by first weighing the sample in air; and then in the measuring liquid, both as per the instructions given in the operating manual.



The density determination function is also present in the PRS/PRJ series of Kern balances. Density of solid materials can be determined in two modes: "Solid on bottom" and "Solid in air". In the first mode, the reference liquid of known density is placed on the weighing pan and tared. The solid sample is placed in the liquid and weighed. Then the solid is suspended in the liquid in such a way that, while being completely submerged, it does not make contact with the bottom of the container. It is then weighed. The balance can determine the density of the solid from the weights taken. The second mode is similar to the under balance methods described previously.