Quality control of tablets

Tablets may be evaluated for their physical characteristics, drug contents, dissolution sites, rate of release of drug etc. Certain physical properties of drugs like uniformity of weight, hardness, friability, thickness, disintegration and dissolution times are important characteristics having a bearing on their handling and use.

(1) Weight Uniformity:

It is desirable that every individual tablet in a batch is uniform in weight, and the weight variation, if any, is within the permissible limits (generally ± 5% for tablets weighing more than 325 mg). Non-uniformity in weights can lead to variation in dosaging. Hence, all finished bathes of tablets should be sampled and tested for weight uniformity. Generally 20 tablets are weighed collectively and individually. From the collective weight average weight per tablet is calculated. The weights of individual tablets are then compared with the average weight to ascertain whether the variations in weights are within permissible limits or not. Some of the basic causes of weight variation of tablets are faulty incorporation of glidants and resultant poor flow of granules, wide variations in granule sizes, differences in lower punch length, improper lubricants etc.

(2) Hardness :

hardness of a tablet is indicative of its tensile strength and is measured in terms of load/pressure required to crush it when placed on its edge. A number of handy hardness testers such as, Mosanto type or Pfizer type are currently in use. A tablet hardness of about 5 kg is considered to be minimum for uncoated tablets for mechanical stability. The hardness is a function of physical properties of granules like their hardness and deformation under load, binders and above all the compressional force. The hardness has influence on disintegration and dissolution times and is as such a factor that may affect bioavailabilities.

(3) Thickness :

The thickness of a tablet depends mainly upon die filling, physical properties of materials to be compressed and compressional forces. There is bound to be a small variation in the thicknesses of individual tablets in a batch but it should be of such an order that it does not immediately become apparent to unaided eye. The thickness can be easily measured by micrometers or in holding trays with sliding caliper scale. Thickness should not vary beyond ± 5% of the standard value.

(4) Friability :

Friability generally refers to loss in weight of tablets in the containers due to removal of fine particles from their surfaces. However, in wider sense chipping and fragmentations can also be included in friability. Friability generally reflects poor cohesion o tablet ingredients. Standard devices have been fabricated to measure friabilities. Generally such instruments, marketed as `Friability Test Apparatus’ or `Friabilators’, consist of a circular plastic chamber, divided into 2-3 compartments. The chamber rotates at a speed of 25 r.p.m. and drops the tablets by a distance of 15 cms. Preweighed tablets are placed in the apparatus which is given 100 revolutions after which the tablets are weighed once again. The difference in the two weights represents friability. The weight loss should not be more than one percent.