Item Description

TH-091 Gaseous Diffusion Coefficients Apparatus

The diffusion of a vapour ‘A’ from a volatile liquid into another gas ‘B’ can be conveniently

studied by confining a small sample of the liquid in a narrow vertical tube, and observing its rate

of evaporation into a stream of gas ‘B’ passed across the top of the tube. Normally, for simple

instructional purposes, ‘B’ is air and ‘A’ is an organic solvent such as acetone or methyl alcohol.

The apparatus consists essentially of a glass capillary tube placed in a transparent-sided

temperature controlled water bath. A horizontal glass tube is fixed to the upper end of the

capillary tube and air is blown through this by a small air pump included within the unit. This

arrangement allows the maintenance of a partial pressure difference within the capillary tube

between the evaporating liquid surface and the flowing air stream. A travelling microscope, with

sliding vernier scale, is mounted on a rigid stand alongside the thermostatic bath and is used to

measure the rate of fall of the solvent/air meniscus within the capillary

The relation between the measured molar mass transfer rate (‘NA’ per unit area), the partial pressure gradient and the diffusion coefficient D is deduced from the one dimensional steady state

version of Fick’s Law with bulk flow:

Na= -D (C_A +C_B) / C_B   dC_A/dy

where ‘CA and ‘CB ’are the molar concentrations of the vapour ‘A’ and air ‘B’ respectively

TH-092 Liquid Diffusion Coefficient Apparatus

Sci-tech has developed a unique diffusion cell which overcomes the traditional problem of

slow diffusion rates in liquids requiring long observation times, but without sacrificing accuracy or introducing convective effects.

Essentially, the cell consists of a honeycomb of accurately dimensioned capillaries, positioned

between two liquids of differing concentration of the solute whose diffusion coefficient is to be

determined. In practice, a small volume of concentrated solution is placed on one side of the honeycomb, whilst the other side consists initially of a large volume of pure solvent (water). As diffusion of the solute occurs, the concentration within the larger volume increases, and is monitored with a conductivity sensor and meter. The mixture is continuously stirred with a magnetic stirrer to

ensure uniform concentration within the bulk liquid. Whilst the conductivity sensor may be readily calibrated for any required aqueous

system, for introductory studies, dilute solutions of sodium chloride are recommended, for which

conductivity data are provided.

TECHNICAL DETAILS

Diffuser vessel: capacity 1.0 litre

Conductivity meter: 3 ranges 199.9µS to19.99mS

Computer output: RS232

RECOMMENDED ACCESSORIES

Stop clock

Cartridge deionizer

MEASUREMENT AND INSTRUCTIONAL CAPABILITIES

➤ accurate measurement of mass transfer rates in the absence of convective effects

➤ use of Fick’s Law to deduce diffusion coefficients from measurements of mass transfer rate and concentration difference

➤ simple analysis of a first order unsteady state process

➤ effect of concentration on diffusion coefficients

➤ gaining familiarity with the use of laboratory instruments to achieve accurate measurements of

data required for industrial process design

➤ Windows data logging software included

Experimental Capabilities

➤        Accurate measurement of mass transfer rates in the absence of convective effects.

➤        Use of gas laws to calculate concentration differences in terms of partial pressures.

➤        Use of Fick’s Law to deduce diffusion coefficients from measurement of mass transfer rate

and concentration difference.

➤        Simple analysis of a first order unsteady state process.

➤        Effect of concentration on diffusion coefficients.

➤        Gaining familiarity with the use of laboratory instruments to achieve accurate measurements of data required

for industrial process design.

• Software is included to allow the temperature and conductivity in the diffusion vessel to be

displayed, logged and recorded on a customer supplied PC, using a RS232 interface.