Ubbelohde viscometer

Ubbelohde viscometer

(1877-1964).

ASTM and other test methods are: ISO 3104, ISO 3105, ASTM D 445, ASTM D 446, IP 71, BS 188 [ [http://www.rheotek.com/scat.asp?id=69 ASTM Ubbelohde Viscometer ] ]

The Ubbelohde viscometer is closely related to the Ostwald viscometer. Both are u-shaped pieces of glassware with a reservoir on the right and a measuring bulb with a capillary on the left. A liquid is introduced and a pressure head forces this liquid from the bulb through the capillary to the reservoir. The time it takes for the liquid to pass through two calibrated marks is a measure for viscosity. The Ubbelohde device has a third arm extending from the end of the capillary and open to the atmosphere. In this way the pressure head only depends on a fixed height and no longer on the total volume of liquid.

The determination of viscosity is based on Poiseuille's law:

: frac{dV}{dt} = v pi R^{2} = frac{pi R^{4{8 eta} left( frac{- Delta P}{Delta x} ight) = frac{pi R^{4{8 eta} frac{ |Delta P{L}

where t is the time it takes for a volume V to elute. The ratio dv/dt depends on R as the capillary radius, on the average applied pressure P, on its length L and on the dynamic viscosity η.

The average pressure head is given by:

:Delta P = ho g Delta H ,

with ρ the density of the liquid, g the Standard gravity and h the average head of the liquid. In this way the viscosity of a fluid can be determined.

Usually the viscosity of a liquid is compared to a liquid with an analyte for example a polymer dissolved in it. The relative viscosity is given by:

:eta_r = frac{eta}{eta_0} = frac{t ho}{t_0 ho_0}.

where t0 and ρ0 are the elution time and density of the pure liquid. When the solution is very diluted

: ho simeq ho_0 ,

the so-called specific viscosity becomes:

:eta_{sp} = eta_r - 1 = frac{t - t_0}{t_0} ,.

This specific viscosity is related to the concentration of the analyte through the Intrinsic viscosity [η] by the power series:

:eta_{sp} = [eta] c + k [eta] ^2 c^2 + ....... ,

or

:frac{eta_{sp{c} = [eta] + k [eta] ^2 c + ....... ,

where ηsp/c is called the viscosity number.

The intrinsic viscosity can be determined experimentally by measuring the viscosity number as function of concentration as the Y-axis intercept.

References


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