An Ostwald viscometer is a simple glass apparatus used to measure the viscosity of a liquid. Viscosity is the resistance of a liquid to flow. The Ostwald viscometer does not measure the absolute viscosity directly; instead it measures the relative (comparative) viscosity of a liquid with respect to a reference liquid such as water. It works by measuring the time taken by a fixed volume of the liquid to flow down through a fine capillary tube under gravity. The method is based on Poiseuille's equation for the flow of liquids through a narrow tube.
An Ostwald viscometer measures the relative viscosity of a liquid compared with a reference liquid (water).
It is based on Poiseuille's law of flow of liquids through a capillary.
Working formula: η₁/η₂ = (ρ₁ t₁)/(ρ₂ t₂).
It measures the time for a fixed volume of liquid to flow through a capillary under gravity.
It is a U-shaped glass tube with two marks above and below a small bulb.
Temperature must be kept constant because viscosity depends on temperature.
The same volume of liquid must be used for both the reference and the test liquid.
The Ostwald viscometer is based on Poiseuille's law, which describes the flow of a liquid through a capillary (narrow) tube.
For a given viscometer, the same volume of liquid flows through the same capillary under the pressure of its own column. Under these conditions, the viscosity (η) of a liquid is directly proportional to its density (ρ) and the time of flow (t):
η ∝ ρ × t
So, comparing a liquid with a reference liquid (usually water):
η₁ / η₂ = (ρ₁ × t₁) / (ρ₂ × t₂)
Where: • η₁, η₂ = viscosities of the liquid and the reference liquid (water) • ρ₁, ρ₂ = densities of the liquid and the reference liquid • t₁, t₂ = times of flow of the liquid and the reference liquid
By measuring the flow times and knowing the densities, the viscosity of the unknown liquid can be found if the viscosity of the reference liquid is known.
Construction: An Ostwald viscometer is a U-shaped glass tube. One limb has a wide bulb at the bottom; the other limb has a small bulb above a fine capillary tube. There are two marks (an upper mark A and a lower mark B) above and below the small bulb.
Working: Step 1: A fixed volume of the reference liquid (water) is introduced into the wide limb. Step 2: The liquid is sucked up the capillary limb until it rises above the upper mark A. Step 3: The liquid is then allowed to flow down freely under gravity. The time taken for the liquid level to fall from the upper mark A to the lower mark B is measured with a stopwatch. This is t₂ (for water). Step 4: The viscometer is cleaned and dried, and the experiment is repeated with the same volume of the experimental liquid to find its flow time t₁. Step 5: The densities ρ₁ and ρ₂ are measured (for example with a relative density bottle). Step 6: The viscosity of the liquid is calculated using η₁ = η₂ × (ρ₁ t₁)/(ρ₂ t₂).
The whole apparatus is usually kept in a constant-temperature water bath, because viscosity changes with temperature.
Precautions:
Uses:
An Ostwald viscometer is based on Poiseuille's law of flow through a capillary. For the same volume flowing through the same capillary, the viscosity of a liquid is proportional to its density and time of flow (η ∝ ρt). Comparing with a reference liquid gives η₁/η₂ = (ρ₁t₁)/(ρ₂t₂), so the viscosity of an unknown liquid can be found relative to water.
A fixed volume of liquid is sucked up above the upper mark of the capillary limb and then allowed to flow down under gravity. The time taken for the level to fall from the upper mark to the lower mark is measured. This is done for both the reference liquid (water) and the test liquid. Using their densities and flow times in the formula η₁ = η₂(ρ₁t₁)/(ρ₂t₂), the viscosity of the liquid is calculated. The apparatus is kept at constant temperature.
The formula is η₁/η₂ = (ρ₁ × t₁)/(ρ₂ × t₂), where η is viscosity, ρ is density and t is the time of flow. Subscript 1 is for the test liquid and subscript 2 is for the reference liquid (water). If η₂, ρ₁, ρ₂, t₁ and t₂ are known, η₁ can be calculated.
Viscosity depends strongly on temperature — the viscosity of most liquids decreases as temperature rises. If the temperature changes during the experiment, the flow time and viscosity values would not be reliable. So the viscometer is kept in a constant-temperature water bath to get accurate, comparable results.
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