Requirements:
Ostwald viscometer, stop watch, Pipette, beaker, unknown liquid, distilled water.
Theory:
The force of friction which one part of the liquid offers to another part of the liquid is called viscosity. For measuring the viscosity coefficient, Ostwald viscometer method is used which is based on Poiseuille’s law. According to this law, the rate of flow of liquid through a capillary tube having viscosity coefficient, h, can be expressed as \( n=πr^4tp/8vl \)
where, v= vol. of liquid (in ml)
t= flow time (in sec.) through capillary r= radius of the capillary (in cm)
l= length of the capillary (in cm)
P= hydrostatic pressure (in dyne/sq.cm)
h= viscosity coefficient (in poise).
Since, the hydrostatic pressure (the driving force) of the liquid is given by P = d g h (where h is the height of the column and d is the density of the liquid);
\( nα p,t \) or \( nαd,g,h,t \)
If, h1 and h2 are the viscosity coefficients of the liquids under study, d1, d2 , are their densities and t1 and t2 are their times of flow of equal volume of liquids through the same capillary respectively, then,
\( n_1αd_1,g , h,t_1 \) and \( n_2αd_2,g,h,t_2 \)
Hence \( n_1/n_2=d_1t_1/d_2t_2 \)
Here, usually the viscosity of given liquid is measured with respect to water whose viscosity is known very accurately at different temperatures. The SI physical unit of viscosity is the pascal-second (Pa·s), (i.e., kg·m^{−1}·s^{−1}). This means: if a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance equal to the thickness of the layer between the plates in one second. The cgs unit for the same is the poise (P), (named after J. L. Marie Poiseuille). It is more commonly expressed, as centipoise (cP). [1 cP = 0.001 Pa·s]. Water at 20 °C has a viscosity of 1.0020 cP.
Procedure:
- 1. Note the laboratory temperature.
- Clean and rinse the viscometer properly with distilled water. Fix the viscometer vertically on the stand and filled with specific amount (say 20ml) of mixture (every time take the same volume).
- Time of flows were recorded for each solutions (water and the given liquid).
- Take 2 or 3 readings for each solution.
Observation Table:
Laboratory temperature =…. ^{°}C
Solution taken | Time of flow (sec) | Mean time of flow (sec) (t_{s}) | D_{sp }of solution | η_{s }/ η_{w }
_{ = }D_{sp }x t_{s / }t_{w} |
Water | i)
ii) |
1 | 1 | |
2% sugar soln. (say) | i)
ii) |
1.0024 | ||
4% sugar soln. (say) | i)
ii) |
1.0040 | ||
6% sugar soln. (say) | i)
ii) |
1.0044 | ||
Unknown sugar soln. | i)
ii) |
……. |
Calculations:
- 2% sugar solutions:
- 4% sugar solutions
- 6% sugar solutions:
- Unknown sugar solutions:
Result: The percentage of unknown sugar solution is……… (from graph).
Precaution:
i) There must be no air bubble within the Viscometer when it is in operation.
ii) Start of ‘stop watch’ & stop of watch must be coinciding with the start & stop of the flow of liquid trough the capillary tube of Viscometer.