The determination of viscosity of newtonian liquids is carried out by means of a capillary viscometer, unless otherwise specified; Methods A and B described below are recommended. For non-newtonian liquids Method C using the rotating viscometer may be used. For measurement of viscosity, the temperature of the substance being measured must be accurately controlled, since small temperature changes may lead to marked changes in viscosity. For usual pharmaceutical purposes, the temperature should be maintained to within ± 0.1°.
The apparatus consists of a glass U-tube viscometer (see Fig.) made of clear borosilicate glass and constructed in accordance with the dimensions shown in the figure and in Table 1.
ProcedureFill the viscometer, previously washed and completely dried, with the liquid under examination through tube L to slightly above the mark G, using a long pipette to minimize wetting the tube above the mark. Place the tube vertically in a water-bath maintained at the temperature indicated in the monograph and allow to stand for not less than 30 minutes to allow the temperature to reach equilibrium.
Adjust the volume of the liquid so that the bottom of the meniscus settles at the mark G. Suck or blow the liquid to a point about 5 mm above the mark E. After releasing pressure or suction, measure the time taken for the bottom of the meniscus to fall from the top edge of mark E to the top edge of mark F.
Calculate, as required, either the kinematic viscosity (v) in square millimetres per second (mm2S-1) from the expression
or the dynamic viscosity (h) in millipascal seconds (mPa s) from the expression
where, t = time in seconds for the meniscus to fall from
P = mass/volume (g cm-3) obtained by multiplying the relative density, of the liquid under examination by 0.998203.
The constant (K) of the instrument is determined on a liquid of known viscosity.
NOTE - For Dextran Injections, the viscosity ratio is calculated by dividing the time taken with the liquid under examination by the time taken by saline solution for the meniscus to fall from E to F.
Method B: (Using the Suspended-level Viscometer)
The apparatus consists of a glass suspended-level viscometer (see Fig.) made of clear borosilicate glass and constructed in accordance with the dimensions shown in the figure and in Table 2.
ProcedureFill the viscometer through tube L with a sufficient quantity of the liquid under examination to ensure that bulb A is satisfactorily filled without blocking the ventilation tube M.
After the tube has been placed vertically in a bath maintained at the specified temperature allow it to stand for not less than 30 minutes to allow the temperature to reach equilibrium, close tube M and apply suction to tube N until the liquid reaches a level about 5 mm above mark E. Hold the liquid at this level by closing tube N and open tube M. When the liquid is clear of the capillary end of tube N and the lower end of tube M, open tube N. Measure the time taken, to the nearest 0.2 of a second, for the bottom of the meniscus to fall from the top edge of mark E to the top edge of mark F. If the end of tube M becomes blocked by the liquid at any time
while the flow time is being measured, the determination must be repeated.
The result is not valid unless two consecutive readings do not differ by more than 1 per cent. The average of not fewer than three readings gives the flow time of the liquid under examination.
Calculate the kinematic viscosity (v) or the dynamic viscosity (h) as given under Method A.
Related: Calibration of Viscometer
Related: Calibration of Viscometer
Method C: (Using the Rotating Viscometer)
The rotating viscometer measures the shearing forces in a liquid medium placed between two coaxial cylinders one of which is driven by a motor and the other is caused to revolve by the rotation of the first. Under these conditions, the viscosity becomes a measurement of the angle of deflection of the cylinder caused to revolve, expressed in newton meters.
ProcedureOperate the Rotating Viscometer in accordance with the manufacturer's instructions and carry out the determination of viscosity of the liquid under examination, at the temperature and angular velocity or shear rate specified in the individual monograph.
If it is not possible to obtain the indicated shear rate exactly, use shear rates slightly higher and slightly lower than the indicated value and interpolate.
Calculate the dynamic viscosity (η) in pascal seconds (Pa s) from the expression
where, L = the angular momentum in newton meters,
w = the angular speed in radians per second.
The constant (K) of the instrument is determined using a liquid of known viscosity or by reference to tables supplied by the instrument manufacturer.
A convenient type of instrument is a rotational utilizing a spindle immersed in the test preparation and measures the resistance to movement of the rotating part. Several spindles and rotational speeds are generally available for given viscosity ranges. Since small temperature changes may lead to marked changes in viscosity, the temperature should be held to within ± 0.1°.
Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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