What is Measuring Density in Chemistry | Measurement

Definition of Measuring Density

Measuring density of a sample is an important quality parameter of both raw materials and finished products.

measuring density
  • Various techniques enable the density of solid, viscous and liquid materials to be accurately determined, e.g. metals, plastics, chemicals, lubricants and food. Using a vernier calliper or ruler, measure the object’s length, depth, and width in centimetres. Multiply these three measurements to find the volume in cubic centimetres. Divide the object’s mass by its volume to determine its density.
  • Density is expressed in grams per cubic centimeter or grams per milliliter.
  • Density is a physical parameter that plays a vital role in all material states, whether solid, liquid or gaseous. It is measured throughout the industry to gain insight into materials, for example, their purity, concentration of components, and composition. The density (and concentration) of liquid products greatly impacts their quality, behaviour, and use.

Other Definitions of Measuring Density

  • The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass per unit volume.

The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume :

ρ = m / V

where ρ is the density, m is the mass, and V is the volume.

  • In some cases (for instance, in the United States oil and gas industry), density is loosely defined as weight per unit volume. However, this is scientifically inaccurate – this quantity is more specifically called specific weight.

For a pure substance, the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging.

Osmium and Iridium are the densest known elements at standard conditions for temperature and pressure.

To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity “relative density” or “specific gravity”, i.e. the ratio of the density of the material to that of a standard material, usually water.

Thus a relative density of less than one relative to water means that the substance floats in water.

Relation of Density with Temperature and Pressure

relationship of density with temperature and pressure

The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases.

Increasing the pressure on an object decreases the volume of the object and thus increases its density.

Increasing the temperature of a substance (with a few exceptions) decreases its density by increasing its volume.

In most materials, heating the bottom of fluid results in the convection of the heat from the bottom to the top due to the decrease in the density of the heated fluid. This causes it to rise relative to more dense unheated material.

The reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics.

Density is an intensive property in that increasing the amount of a substance does not increase its density; rather, it increases its mass.

History of Measuring Density

Density measurement has a long history – it actually dates back 2200 years to the times of King Hieron II of Syracuse. In a nutshell, King Hieron II had a crown made of pure gold. The goldsmith was accused of having added some silver to the crown.

Archimedes, a great mathematician, physicist, engineer, inventor and astronomer, was asked to prove – or disprove – that Hieron’s crown consisted of nothing but gold without destroying it.

In a well-known but probably apocryphal tale, Archimedes was given the task of determining whether King Hiero’s goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.

Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass, but the king disapproved of this baffled.

Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water.

Upon this discovery, he leapt from his bath and ran naked through the streets shouting, “Eureka! Eureka!” (Εύρηκα! Greek “I have found it”).

As a result, the term “eureka” entered common parlance and is used today to indicate a moment of enlightenment.

The story first appeared in written form in Vitruvius’ architecture books, two centuries after it supposedly took place.

Some scholars have doubted the accuracy of this tale, saying, among other things, that the method would have required precise measurements that would have been difficult to make at the time.

Measurement of Density

Density can be measured in different ways, which are listed below

1. Hydrostatic Pressure

hydrostatic pressure

This type of density measurement relies on a constant height of liquid and measures the pressure differences.

Principle

Since level may vary, the principle of operation works on the difference in pressure between any two fixed elevations below the surface. Because the height between these two points does not change, any change in pressure is due to density variations. The distance between these points is equal to the difference in
liquid head pressure between these elevations.

2. Pycnometers

pycnometer

A pycnometer consists of a glass flask and a stopper (sometimes with an integrated thermometer). It is placed on a balance. After determining the weight of the empty pycnometer, you can calculate its volume by filling in a calibration liquid of known density (e.g. water) using the corresponding definition of density (volume = weight/density). Afterwards, by weighing the pycnometer filled with sample, the density of the model can be determined (density = weight/volume).

  • Using a pycnometer can yield accurate and reliable results if the temperature control and the balance are equally precise.
  • Pycnometers are affordable, but can break easily.
  • The method is rather slow and time-consuming and a skilled operator is needed. Another drawback is the large sample volume that is required, usually 10 mL to 100 mL.

3. Radiation

radiation

Density measurement by radiation is based on the increased absorption of gamma radiation for an increase in specific gravity of the material being measured.

  • The main components of such a system are a constant gamma source (usually radium) and a detector. Variations in radiation passing through a fixed volume of flowing liquid is converted into a proportional electrical signal by the detector.
  • This type of measurement is often used in dredging where the density of the mud indicates the effectiveness of the dredging vessel.

4. Vibration

vibration

Damping of a vibrating object in a fluid will increase as the density of the fluid increases. An object is vibrated from an e xternal energy source. The object may be an immersed reed or plate.

There are two methods to measure density by vibration-

  1. Changes in the natural frequency of vibration can be measured when the object is energized constantly.
  2. Changes in the amplitude of vibration can be measured when the object is struck periodically, like a bell.

5. Differential Pressure

differential pressure

Constant level overflow tanks are the simplest for measuring as only one differential pressure transmitter is required. Howev er applications with level or static pressure variations require compensation.

6. Temperature Effects

temperature effect
  • Increases in temperature cause expansion of a liquid, altering its density.
  • Not all liquids expand at the same rate. A specific gravity measurement must be corrected for temperature effects to be completely accurate in terms of reference conditions for density and concentration. Although in most cases, this is not practical.
  • In applications where specific gravity is extremely critical, it is possible to control the temperature to a constant value. The necessary correction to the base temperature can then be included in the density instrument calibration.

Tools Used to Measure The Density

One of the basic terms used in many natural sciences is density, a physical property defined as an object’s mass divided by its volume. This means that to measure density, you typically need to measure an object’s mass and volume separately, then calculate its density by dividing the mass by the volume.

To measure mass and volume, you need the use of several basic laboratory tools.

A number of techniques as well as standards exist for the measurement of the density of materials. Such techniques include the use of a hydrometer (a buoyancy method for liquids), Hydrostatic balance (a buoyancy method for liquids and solids), immersed body method (a buoyancy method for liquids), pycnometer (liquids and solids), air comparison pycnometer (solids), oscillating densitometer (liquids), as well as pour and tap (solids).

However, each individual method or technique measures different types of density (e.g. bulk density, skeletal density, etc.). And therefore, it is necessary to understand the type of density being measured and the type of material in question.

Followings are the methods and tools generally used to measure the density

1. Scale

Mass is one of the most easily obtained measurements.

  • Use a scale or electronic balance to determine the object’s weight or mass.
  • This measurement is usually represented in ounces or grams for English and metric systems, respectively.
  • When measuring the mass of a liquid, first weigh the container and then tare the scale before adding the liquid.

2. Graduated Cylinder

The most accurate way to determine an object’s volume, especially in the case of an irregularly shaped object, is to immerse it in water and measure the amount of water it displaces.

  • A graduated cylinder large enough to hold both the object and enough water to immerse it is the best tool for this job fully.
  • A graduated cylinder can also tell you the volume of a liquid simply by pouring it into an empty cylinder.
  • Don’t use a beaker to determine volume since the scale printed on the side of a beaker may be less accurate than on a graduated cylinder, which is specially designed for measuring.

3. Calculating Density

Once you’ve measured the mass and volume of an object, you find density with a simple calculation. Divide the mass by the volume to get the density.

Example – You measure a quantity of pure water in a graduated cylinder and see it comes to 11.5 ml. You place a plastic weighing dish on a scale and find its mass is 3.2 grams. When you add the water, the total comes to 14.7 grams. Subtract the mass of the dish from the total to get the mass of the water alone:

14.7 – 3.2 = 11.5 grams
Divide 11.5 grams by 11.5 ml to get the density, 1.0 grams per ml.

4. Hydrometer

Density is tough to measure directly and accurately, as it depends on two separate properties, mass and volume. However, density comes into play with fluids and floatation, as a denser object will always sink into a less dense fluid and float on a liquid of greater density.

  • A hydrometer is a special device used to determine the density of liquids. Rather than measuring the volume in a graduated cylinder and then needing to weigh the liquid to determine its mass (and, of course, subtracting the weight of its container), a hydrometer will float at a different level within a liquid based on its density.
  • Different hydrometers use different scales for measuring density, so follow all instructions and measure carefully.

So this is all about the measuring density. I hope you like the article.

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