Many reactions do not conveniently produce gas for measurement and a variety of techniques have been developed to follow reactions in other ways.
When some kind of visible change occurs as the products are formed this may be used to determine the progress of a reaction.
One example is the formation of an opaque precipitate. This may be used to gauge the extent of reaction and the time recorded for a precipitate to obscure a mark on the opposite side of a piece of glassware.
Example: An experiment to investigate the reaction between sodium thiosulfate and dilute acid.
During the course of the reaction a yellow/white precipitate of sulfur forms that makes the solution opaque. The time taken for a mark to become obscured is recorded. animation
During the course of a reaction there may be changes that can be measured electronically. These include:
- 1 Conductivity
- 2 Acidity
- 3 Colour
Nowadays there are a number of probes and electronic measuring devices available that allow an experiment to be followed in this way.
Example: The course of a reaction that changes colour may be followed by using a colorimeter.
The instrument can be set at a specific wavelength to measure the transmittance of light during the reaction. The colour absorbance is related to the concentration of absorbing solute by the Beer Lambert law. This says that the absorbance of light is proportional to the product of the distance the light has to travel through, the concentration and a constant.
If the length of the light path through a solution remains the same and the coefficient is constant then:
A graph of absorbance against time will give the rate as the gradient of the graph.
This means to stop a reaction at a specific point to allow analysis to take place. It may be that addition of a substance reacts so fast with one of the reactants that the other is left unaffected and may then be determined at leisure.
Another possibility is that the reaction is frozen by rapid cooling, effectively stopping it in its tracks, and once again providing an opportunity for analysis of the reactants (or products).
Example: The hydrolysis of an ester is a slow process, catalysed by dilute acid. The products are an acid and an alcohol.
Thus, as the reaction proceeds the number of moles of acid increases. Samples of the reaction mixture may be extracted and titrated using dilute base. This not only destroys the acid catalyst, quenching the reaction, but also allows determination of the total moles of acid in the mixture.
From this the rate of the reaction may be calculated by the determined concentrations of acid against time.