Hardness Test of Paracetamol Tablet

Hardness test of Paracetamol Tablet

The resistance of the tablet to chipping, abrasion, or breakage under conditions of storage, transportation, and handling be fore usage depends on its hardness. In the past, a rule of thumb described a tablet to be of proper hardness if it was firm enough to break with a sharp snap when it was held between the 2nd and 3rd fingers and using the thumb as the fulcrum, yet didn’t break when it fell on the floor. For obvious reasons and control purposes a number of attempts have been made to quantitative the degree of hardness.

A small and portable hardness tester was manufactured and introduced in the mid-1930s by Monsanto. It now is distributed by the Stokes Div (Pennwalt) and may be designated as either the Monsanto or Stokes hardness tester. The instrument measures the force required to break the tablet when the force generated by a coil spring is applied diametrically to the tablet. The force is measured in kilograms and when used in production, a hardness of 4 kg is considered to be minimum for a satisfactory tablet.

The Strong-Cobb hardness tester introduced in 1950 also measures the diametrically applied force required to break the tablet. In this instrument the force is produced by a manually operated air pump. As the pressure is increased, a plunger is forced against the tablet placed on anvil. The final breaking point is indicated on a dial calibrated into 30 arbitrary units. The hardness values of the Stokes and Strong-Cobb instruments are not equivalent. Values obtained with the Strong- Cobb tester have been found to be 1.6 times those of the Stokes tester.

Another instrument is the Pfizer hardness tester, which operates on the same mechanical principle as ordinary pliers. The force required to break the tablet is recorded on a dial and may be expressed in either kilograms or pounds of force. In an experimental comparison of testers the Pfizer and the Stokes testers were found to check each other fairly well. Again the Strong-Cobb tester was found to give values 1.4 to 1.7 times the absolute values on the other instruments.     –

The most widely used apparatus to measure tablet hardness or crushing strength is the Schleuniger apparatus, also known as the Heberlein, distributed by Vector. This and other’s newer, electrically operated test equipment eliminate the operator variability inherent in the measurements described above. Newer equipment is also available with printers to provide a record of test results. Manufacturers, such as Key, Van Kel, Erweka, and others, make similar hardness testers.

Hardness (or more appropriately, crushing strength) deter nations are made throughout the tablet runs to determine the need for pressure adjustments on the table ting machine. If the tablet is too hard, it may not disintegrate in the required period of time or meet the dissolution specification; if it is too soft, it will not withstand the handling during subsequent processing such as coating or packaging and shipping operations.

A tablet property related to hardness is friability, and the measurement is made by use of the Roche friabilator. Rather than a measure of the force required to crush a tablet, the instrument is designed to evaluate the ability of the tablet to withstand abrasion in packaging, handling, and shipping. A number of tablets are weighed and placed  in the tumbling

Apparatus where they are exposed to rolling and repeated shocks resulting from freefalls within the apparatus. After a given number of rotations the tablets are weighed. and the loss in weight indicates the ability of the tablets to withstand this type of wear

Recent research has proposed that there are at least three measurable hardness parameters that can give a clue to the compatibility and. intrinsic strength of powdered materials. These include bonding strength, internal strain, and brittleness..

The higher the bonding index   the stronger a tablet is likely to be. The higher is the strain index, the weaker the tablet. Since the two parameters are opposite in their effect on the tablet, it is possible for a material (such as Avicel) to have a relatively high strain index, but yet have superior compaction properties.

A similar approach is taken by many manufacturers when they evaluate a new product in the new market  package by sending the package to, distant points and back using various. Methods of transportation this is called a shipping test. The condition of the product on its return indicates its ability to withstand transportation handling.


Method of hardness test:

Title: Hardness test for Tablet (paracetamol)

Objective: The objective of this experiment is to measure the hardness of tablets (paracetamol) by using a hardness tester.

Theory: Too ‘soft’ tablets can disintegrate in transport. Too ‘hard’ tablets could damage teeth. An acceptable ‘hardness’ is required and tablet strength testing is necessary for both, research & development of new formulations, and for quality control the test instruments should provide accurate results and output these results in standard units.


Materials Required:

  • Hardness tester
  • Tablets (paracetamol)


  1. Zero the sliding scale of the hardness tester.
  2. Place one tablet vertically between two jaws.
  3. Apply force with a screw thread and spring until the tablet fractured.
  4. Take reading in Kg from the sliding scale


Measurement Units:

Most materials testing are performed using the International System of Units. The Newton is the preferred unit of force as is recognized by the SI system. However the kilogram can also be used.

Kilogram (kg) – The kilogram is recognized by the SI system as the primary unit of mass.

Newton (N) – The Newton is the SI unit of force and is the unit that should be used for tablet hardness testing. 9.807 Newtons = 1 kilogram.

Pound (lb) – Technically a unit of mass but can also be used for force and should be written as pound force or lbf in this case. Sometimes used for tablet strength testing in North America, but it is not an SI unit. 1 kilogram = 2.204 pounds.


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