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Úvodní stránka podle aplikace Laboratorní analytické aplikace Aplikace pro měření hustoty What Is Specific Gravity?

What Is Specific Gravity?

All You Need to Know About Specific Gravity

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What is specific gravity

Specific gravity, also known as relative density, measures the density of a substance compared to the density of water.

Specific gravity is related to density, as both are physical properties used to determine how "dense" a particular material is. This material can be a gas, liquid, or solid.

Specific gravity and density are used to identify a material, determine the concentration of a liquid solution e.g., alcohol or sugar in a drink), or test whether a product is within specifications.

This page will cover the basics of specific gravity and density.

1. What Are Density and Specific Gravity?

The density of a sample is defined as its mass divided by its volume.

The formula for density:

Density formula

Specific gravity, also known as relative density, is used to describe the density of a substance compared to the density of water. To calculate specific gravity, divide the sample's density by the density of water.

The formula for specific gravity:

Specific gravity formula

2. What Is the Difference Between Specific Gravity and Density?

Oil on water

The main difference between specific gravity and density is that specific gravity is dimensionless, meaning it has no units, while density has a unit (g/cm3, kg/m3, g/mL, oz/US fl, ...).

Specific gravity tells you whether something is floating or sinking in water. A specific gravity below 1 means that the sample is less dense (lighter) than water and will, therefore, float. For example, an oil with a specific gravity of 0.825 will float on water.

3. How to Convert Density to Specific Gravity?

Specific Gravity of the Substance = Density of the Substance / Density of Water

 

Example (at 20°C):

Density of the Sample = 1.3265 g/cm3

Density of Water = 0.9982 g/cm3

Specific Gravity of the Sample = 1.3265 / 0.9982 = 1.3289

4. How to Convert Specific Gravity to Density?

Density of the Substance = Specific Gravity of the Substance x Density of Water

 

Example (at 20°C):

Specific Gravity of the Sample = 1.0627 g/cm3

Density of Water = 0.9982 g/cm3

Density of the Sample = 1. 1.0627 x 0.9982 = 1.0608

5. How Does the Temperature of Samples Affect the Specific Gravity and Density?

The temperature of a sample affects both the density and the specific gravity. The higher the temperature, the higher the volume and the lower the density. If the temperature increases, the volume increases, and the density decreases. However, the mass of the substance does not change with temperature.

The most notable exception to this rule is liquid water, which reaches its maximum density at 3.98 ºC; above this point, the volume of water increases, and it becomes less dense. The opposite is true when water is cooled.

Since specific gravity is the density of the sample divided by the density of water, both densities will decrease with increasing temperature, but not by the same magnitude. The effect of the temperature will usually be slightly less important for specific gravity than for density.

moving molecules - low temperature
moving molecules - low temperature

Molecule at a given temperature
(slight movements)

moving molecules - high temperature
moving molecules - high temperature

Same molecules when temperature increases
(moving further apart)

Density vs. Temperature
Density vs. Temperature

Example:

d ethanol at 20°C = 0.7895 g/cm³

d ethanol at 40°C = 0.7724 g/cm³

 

d water at 20°C = 0.9982 g/cm³

d water at 40°C = 0.9922 g/cm³

 

SG ethanol at 20°C = 0.7909 g/cm³

SG ethanol at 40°C = 0.7785 g/cm³

6. Which Instruments Are Used to Measure Specific Gravity and Density?

There are different types of instruments to measure both specific gravity and density:

Hydrometer

Hydrometer
Hydrometer

A hydrometer is a cost-effective instrument used to determine the specific gravity/density of liquids. Made of blown glass, it consists of a bulbous bottom weighted with lead or steel shot and a long, narrow stem with a scale. The hydrometer is immersed into the sample liquid until it floats. The density reading is taken by looking at the scale, where the level of the sample liquid aligns with a marking on the hydrometer scale. Most hydrometers measure the specific gravity of samples: in simple terms, a hydrometer tells the user if a liquid is denser or less dense than water. It will float higher in a liquid with a greater specific gravity, such as water with sugar dissolved, compared to one with a lower specific gravity, such as pure water or alcohol.

When using a hydrometer, the user has two options:

  1. Use the hydrometer at its calibration temperature (usually 16 °C or 20 °C). Depending on the sample volume, it can take some time for the sample to reach this temperature.
  2. Simply record the measurement value at the surrounding temperature. Both measurement and temperature values must be recorded. If needed, a correction factor can be applied later to obtain the temperature-corrected measurement value.

Pycnometer

Pycnometer
Pycnometer

Typically made of glass, a pycnometer is a flask of a pre-defined volume used to measure the specific gravity/density of a liquid. It can also determine the specific gravity/density of dispersions, solids, and even gases. When used correctly, pycnometers provide very precise results, with accuracy up to 10-5 g/cm3 – this correlates with the accuracy (number of decimal places) of the digital balance used. A thermometer is also required to measure the temperature. User training is required to guarantee accurate measurements with the pycnometer.

Portable digital density meter

Portable digital density meter
Portable digital density meter

Portable digital density meters are used to quickly and accurately determine the specific gravity/density of liquids. Determination of density or specific gravity using digital meters is based on two factors:

  1. The oscillation, or vibration, of a U-shaped glass tube (U-tube).
  2. The relationship between the liquid sample mass and the frequency of oscillation of the U-tube. Filling the U-tube with sample liquid affects its frequency of oscillation: due to factory adjustment with samples of known densities, this frequency of oscillation can be directly correlated with the density of any liquid sample with an accuracy of 0.001 g/cm3 or a specific gravity with an accuracy of 0.001. Handheld digital density meters measure the sample at ambient temperature. If a result is needed at a certain temperature, the digital density meter can apply a correction factor to the measured result to compensate the result to a defined temperature. Each measurement takes only a few seconds, allowing users to move on to the next sample quickly. The measured density can be automatically converted into other units and concentrations for specific applications, such as specific gravity, API, alcohol%, °Brix, etc.

Benchtop digital density meter

Benchtop digital density meter
Benchtop digital density meter

Benchtop digital density meters use the same technology as portable digital density meters, the oscillation of a U-shaped glass tube (U-tube). In addition, they feature a built-in Peltier temperature control, which brings the sample to the selected temperature (e.g., 20°C). The temperature control can range from 0 °C to 95 °C. These density meters can reach an accuracy of 0.000005 g/m3 for density, or 0.000005 for specific gravity.

Some benchtop digital density meters can be connected to sample automation solutions for single or multiple samples, which offer automated sampling, rinsing, and drying. These density meters can often be upgraded into a dedicated automated multi-parameter system combining density, refractive index, pH, color, conductivity, and more to save time, increase data quality, and prevent any alteration of samples between individual analyses.

One of the benefits of digital density meters using the U-shaped glass tube is the small volume of sample required (typically 1.5 mL), which allows for a faster temperature equilibrium of the sample.

7. Examples of Specific Gravity and Density of Some Materials

At Different Temperatures

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FAQ

What is the specific gravity of air?

The specific gravity of air is 0.00121 at 20°C and under an atmospheric pressure of 101.325 kPa (i.e., at sea level). This pressure changes with the weather (lower pressure when it rains or snows) and with altitude (lower pressure at high altitude than at sea level). For example, at an altitude of 440 m above sea level, the atmospheric pressure (annual average) is only 96.12 kPa, and the average specific gravity of the air at 20°C is 0.00114.

 

What is the specific gravity of water?

The specific gravity of water is 1.00000 at 20°. Note that 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. However, the specific gravity of water doesn’t change with the temperature; it will still be equal to 1.00000.

 

Why can specific gravity be used to identify a sample?

Specific gravity and density can easily be used to identify a pure sample because each substance has a unique specific gravity/density. After a measurement, the specific gravity/density of the sample in question can be looked up to see what it corresponds with.

 

How can density/specific gravity be used to determine concentrations?

The amount of solute in a solvent usually affects the density and specific gravity of the resulting solution. By measuring the specific gravity of a series of solutions with known varying amounts of dissolved solute, a conversion table or formula can be found that correlates the specific gravity to the concentration for a given solution.

 

What are some typical applications for specific gravity measurement?

Here are some typical applications of specific gravity measurement:

  • Determination of alcohol concentration in spirits, control of the fermentation process in wine and beer production
  • Brix (sugar content) measurement of intermediate and final products in food and beverage products
  • Specific gravity, density, and other concentrations such as API gravity in heavy oils, paraffin, and lubricants in petrochemicals
  • Specific gravity and density in battery acid in the automotive industry and other solvents
  • Acids and bases in the chemical industry
  • Many applications in the pharmaceutical industry, such as specific gravity in cosmetics, personal care products, and many more