What is Density?
Density, also called Mass Density, is a fundamental physical property of a material. It is defined as the mass per unit volume at a specific temperature. Mathematically, density is expressed as mass/volume where the SI units are kg/m3 but g/cm3 or g/ml are also widely used. The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases.
How is Density Measured?
Density is traditionally determined by a manual method called pycnometry but can also be measured by automated methods including, vibrational density meters or by hydrostatic weighing. Density measurement by pycnometry, is a fundamental measurement technique covered by ASTM D1480 and ASTM D1217 methods.
Measuring Density using Pycnometry
When using pycnometry, firstly, the liquid sample is introduced into a calibrated Bingham pycnometer, and allowed to equilibrate to the desired test temperature (±0.01°C) in a circulating bath. Then the volume of liquid is adjusted to mark on pycnometer, the pycnometer is removed from the bath and allowed to equilibrate to room temperature before weighing. The density of the liquid sample is then calculated based on the weight of liquid in the pycnometer and the calibration factor for the pycnometer which is a correction made for air buoyancy. This is an extremely accurate method but also very time consuming.
Density measurement by vibrational density
Density measurement by vibrational density is covered by the ASTM D4052 method. The liquid sample is injected into an oscillating U-tube through a port on the side of the instrument. The density is then calculated based on the change in frequency of oscillation of the tube. This is a fast, reliable and accurate method. The method has a temperature range from 0°C to 95°C, as little as 1ml of sample can be used, an autosampler can be used and there are several reliable manufacturers of such instruments.
Density measurement by hydrostatic weighing
The third method is density measurement by hydrostatic weighing. This is the method of measuring the density of liquids and solids, based on Archimedes’ law. The density of a sample is determined by weighing it twice – first in air and then in a liquid whose density is known, usually distilled water is the liquid of choice. The first weighing determines the objects weight and the second weighing determines the volume. The method is more commonly used for solids, but can also be used for liquids. The method is expensive to set up and is generally used in reference laboratories. To simplify the comparisons of density across the different systems of units that include kg/m3, g/cm3 or g/ml, it is sometimes replaced by a dimensionless quantity, for example specific gravity or relative density.
Specific gravity or relative density
So, relative density or specific gravity is the ratio of the density of a substance to the density of a reference substance which is nearly always water for liquids or air for gases. Therefore, for liquids, Relative density or Specific gravity usually means relative density with respect to water and a specific gravity less than one which means that the substance floats in water. So, relative density is the density of the test substance divided by the Density of the reference substance. As Specific gravity is a ratio of densities it is a dimensionless quantity.
In summary, specific gravity is the density of a material relative to the density of water, whereas relative density is the density of a material relative to a specified reference material.
Another expression of density used is American Petroleum Institute gravity or API Gravity, which is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter than, and floats on water.
Density in Industry
The measurement of density is carried out in a wide range of industries and is a widely performed and very useful test. It is used particularly widely as a quality control or identification test in sectors such as Beverages, Foodstuffs, and Petrochemicals, where the test solution under consideration is in liquid form. It is also widely carried out in the broad areas of Pharma, Life Sciences, Cosmetics, Power, and Pulp and Paper processing. Other areas of use are too numerous to mention, but range into Environmental, Mining, Electronics, and Textile Manufacturing. The vast majority of consumer products that are in liquid form, will have had density measurement carried out at some point in their production cycle.
Reagecon Density Standards
Reagecon offer two ranges of density standards, the premium range and the quality range. The premium range are tested to a higher level of accuracy, and therefore have a lower level of measurement uncertainty. The testing is performed in accordance with the ASTM D1480 method by Bingham Pycnometry and are specified with a measurement uncertainty of ±0.00077 density units or grams/ml. The products of which there is an extensive range can be used as control or calibration standards for density measurements by pycnometry, vibrational or hydrometer based techniques and are offered for any make or model of instrument in 100ml glass bottles.
On the other hand, the quality range is manufactured in accordance with the ASTM D4052 method for testing of density by a digital density meter. These products which have a specification of ±0.001642 g/ml (±0.16%), can be used as calibration or control standards for density measurement by vibrational or hydrometer based techniques.
The test results for some of these products are accredited to ISO 17025 and the density of each standard is verified using a high performance calibrated density meter.
Both ranges of products can be used for calibration and control, as already stated, but also method validation and instrument qualification in a regulated industry.
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