Density measurements check the purity and concentration of a sample and gives insight into its composition. The measurement of density is crucial in different industries to ensure quality for both raw materials and finished goods.
For example, the density of ultrapure water at 20.00 °C is known to be 0.998203 g/cm3: any deviation from this value ± tolerances would imply that the water sample contains impurities.
On this page, you will gain essential knowledge about density measurement.
Jump to one of the following sections to explore and learn more about density of liquids.
Density is a physical parameter that provides information on the mass of a sample or body divided by its volume: in other words, how tightly a substance's molecules are packed together in space. The density is usually represented by the Greek letter rho "ρ". The Latin letter "d" is also commonly used.
Have you ever wondered why some oils stay on the ocean's surface when there is an oil spill?
The difference in densities makes some substances rise above others, in the image you can see examples of typical density values demonstrated.
A - Lamp Oil (0.80 g/cm3)
B - Rubbing Alcohol (0.87 g/cm3)
C - Vegetable Oil (0.91 g/cm3)
D - Water (0.99 g/cm3)
E - Dawn Dish Soap (1.33 g/cm3)
F - Honey (1.36 g/ccm3)
Hydrometers
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Measuring principle
Strengths
Weaknesses
Pycnometers
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Measuring principle
Strengths
Weaknesses
Digital Density Meters
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Measuring principle
Strengths
Weaknesses
Watch this video to check if you are following the correct steps for a successful density measurement with a pycnometer. Furthermore, we compare density measurement with a pycnometer vs a digital density meter.
Did you know that a temperature change of 0.1 °C can have an impact of 0.0001 g/cm3 on the measured density?
The temperature influences the space necessary to fit atoms in a molecule. The vibration increases with higher temperature, moving the atoms further apart and therefore reducing the density value.
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Molecule at a given temperature
(slight movements)
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Same molecule when temperature increases
(moving further apart)
Therefore, the higher the temperature, the higher the volume and the lower the density. If the temperature decreases, the volume becomes lower and the density becomes greater. But, the mass of the substance does not change.
The only exception to this rule is liquid water, which reaches its density peak at 3.98 ºC, above this point the volume of water increases and it becomes less dense. The opposite applies when water is cooled.
Note: The relation among temperature, volume and density is not a linear function and it depends on the specific heat capacity, heat of vaporization and other factors of each substance.
A simple air bubble present in a liquid measured can cause a massive difference to its density and the same applies for impurities.
For instance, a glass pycnometer relies on the mass for the calculation of the density value. If there is an air bubble present or a contamination e.g. due to improper cleaning, the mass measured and displayed by the balance will be incorrect. This leads to an incorrect density value.
If you are in Mexico City at 3.930 meters above the sea level, the local air pressure will be lower than in Rio de Janeiro, which is at sea level or 0 meters. This means the air pressure is directly related to the altitude.
As gases and liquids are fluids, the following information must be considered:
In a density measurement using a manual instrument (e.g. pycnometer), the air pressure value is not taken into consideration.
Modern digital density meters feature a built in barometer (pressure sensor) to measure the local air pressure, which automatically sets the reference air density value. This value is important for 2 reasons:
If a substance is viscous, is it denser than a less viscous liquid?
Viscosity describes the resistance of a liquid in flowing, informally described as the "thickness" of a fluid, and in principle, it has no direct relationship with density.
However, the viscosity may influence the density determination depending on the method used.
Measurements using:
Relative density definition
Relative density is the ratio between absolute densities of two substances, where the divisor is considered the reference substance. If this substance is not specified, it is assumed to be water at 3.98ºC and therefore has a density of 0.999972 g/cm3 or 999.972 kg/m3. As per the equation, the relative density has no units.
Specific gravity definition
Specific gravity (SG) is the ratio between the density of a substance and the density of water. If the temperature is not specified, it is assumed to be water at 3.98ºC and therefore has a density of 0.999972 g/cm3 or 999.972 kg/m3.
True density or absolute density definition
True density is the ratio between the mass and volume of a substance at a given pressure and temperature, corresponding to its weight in a vacuum. This is the concept also used in density measurement by digital density meters.
Apparent density definition
Apparent density is a property of partitioned solids, such as powders and granules, and is often used in the mining, food and chemical industries. By definition, apparent density is the ratio between mass and volume, but it corresponds to weight in air.
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Mass in vacuum = True density |
To illustrate the difference between true density and apparent density, we have a pycnometer placed onto a balance. When it is filled with a liquid, it weighs less in air than in a vacuum due to the buoyancy effect of air.
Many official density tables are still based on apparent density. Digital density meters deliver results in different units and concentrations, check out their specifications.
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Mass in air = Apparent density |
Ensuring accurate density measurement results requires taking direct action to avoid inaccuracy throughout the workflow process. Below you will find important tips for the success of your density determination.
Before you begin your measurement, it is important to understand the necessary precautions to take depending on your sample type.
Tips:
Viscous
Aggressive / toxic
Volatile
Sample with dissolved gases
Non-homogeneous or with suspensions
Regularly verify the measurement performance of your system by measuring a sample of accurately known density (e.g. ultrapure water or a liquid standard). This procedure is called test, calibration or check. After completing the check or test, the measured density is compared to the known nominal value of the sample.
Additionally, it is good practice to check that the measuring cell is fully clean and dry between the analyses of different substances. It is recommended that on a daily basis you complete a test of your density meter with ultrapure water (0.998203 g/cm3 at 20.00 °C).
Some common questions are:
Which tolerance should be applied for a test with deionized water?
And for organic standards?
What to do to if the test fails?
Did you know that a bubble of 2 mm diameter can cause an error of 0.0008 g/cm3 on your result?
If you are measuring with a syringe:
When filling automatically with a pump or autosampler:
Still converting your results manually?
Looking up in or interpolating from a table is error-prone and time-consuming. In addition, hand-written results bear the risk of transcription errors. Depending on the environment, there is no guarantee that this kind of documentation fulfills regulatory requirements.
The most convenient solution is an automatic conversion using built-in tables (e.g. alcohol, Brix, temperature compensation according to API), or also user defined, which prevents reading or calculation errors and saves time. A digital density meter allows the use of built-in conversion tables to show the result directly in the desired unit.
Inappropriate cleaning is the most common source of erroneous results!
Make sure that the measuring cell does not contain any residue from previously measured samples or rinsing solutions
To prevent this, the cell should be cleaned with suitable rinsing solutions and dried – preferably after each measurement
Some general recommendations for the rinsing solutions:
Cleaning solution 1
Cleaning solution 2
Finally, make sure to dry the cell completely with dry air. In many cases, there is no need to use 2 solvents when using METTLER TOLEDO Density Meters. Thanks to the powerful DryPro pump, you can save time and solvent consumption.
Important: Adjusting a density meter
Frequent adjustments of the instrument does not necessarily guarantee accurate results. Any adjustment results in changes being made to the instrument’s internal settings. If an adjustment is performed incorrectly, this will then lead to incorrect measurements.
Therefore, we recommend the adjustment of the density meter only if the test has failed several times repeatedly
Mass is a measure of how much matter there is within an object or liquid while density expresses how much mass there is per a certain amount of volume.
For example, 10 kg of steel and 10 kg of feathers have the same mass, but different volumes therefore they have different densities.
Density can easily be used to identify a pure sample because each element has a unique density. After a measurement, the density of the sample in question can be looked up to see what it corresponds to.
Let's take a solution of ethanol in water as an example.
As shown before, at 20 °C pure water has a density of d = 0.9982 g/cm3, and pure ethanol has a density of d = 0.7892 g/cm3 at 20 °C. A solution of ethanol/water will have a density value which depends on the concentration of the solution.
Density is directly proportional to the local air pressure but indirectly proportional to temperature. At a constant temperature, when pressure increases density increases. Learn more about the relationship between density of liquids and pressure here.
Some applications of density measurement includes the determination of alcohol concentration in spirits, control of fermentation process in wine and beer production, Brix (sugar content) measurement of intermediate and final products in food and beverages products. Density and other concentration as API gravity in heavy oils, paraffin and lubricants in the Petrochemical segment. Density (specific gravity) in battery acid in the automobile industry, as well as other solvents, acids and bases in the chemical industry. Lastly, there are many applications in the pharmaceutical industry, such as the density measurement of specific gravity in cosmetics, personal care products, and many more. Click on the links above to read our detailed applications or access our expertise library to find the right application note according to your sample.
The density of air is 0.00120 g/cm3 at 20°C and under atmospheric pressure of 101.325 kPa (i.e. at sea level). This atmospheric pressure changes with the weather conditions (lower pressure when rainy or snowy) and with the altitude (lower pressure at high altitude than at sea level). At an elevation of 440 m above sea level, for example, the atmospheric pressure (yearly mean) is 96.12 kPa only and the mean air density becomes 0.00114 g/cm3 at 20°C.
The density of water is 0.99820 g/cm3 at 20°. The density of water changes with the temperature. It increases from 0°C to 4°C (where it is nearly 1) and then decreases from 4°C to higher temperatures.
Sample viscosity has an impact on the density measured with a digital density meter. The higher shear force which occurs between the fluid and the tube wall results in slowing down the oscillation frequency, thus showing a higher density value. Modern digital density meters therefore have a built-in viscosity correction, which compensates for this effect to show correct results.
