Visual indication with coloured indicators is certainly the oldest method for recognising the endpoint of the titration; and is still frequently used today
Visual indication with coloured indicators can be realised without any complicated instrumentation and at a low cost.
Coloured indicators react with the analyte: a small amount of indicator is added at the start of the titration and together with the analyte, forms species A; when the analyte has almost completely reacted with the titrant, the titrant displaces the indicator from species A which releases the indicator.
This takes place with a change in colour (in acid - base titrations phenolphthalein reacts with protons to give a colourless substance.
Under alkaline conditions the proton is split off and the colour changes to red - violet).
With the titrant: a small amount of indicator is added at the start of the titration.
It does not react with the analyte, and the colour remains unchanged until the first excess titrant is present.
The titrant combines with the indicator.
This takes place with a change in colour.
If the correct indicator is chosen then good results are obtained.
The chief disadvantage of this method is that it cannot be automated and cannot be validated.
(The colour sensitivity differs from person to person and also depends upon the lighting conditions.
In addition, difficulties occur with coloured and/or turbid solutions).
Photometric indication can bring an improvement.
The (individual) human eye is replaced by a (neutral) sensor.
The method can be automated and validated - provided the correct colour indicator is chosen and that the solutions are not too turbid or no intense turbidity occurs during titration.
Coloured indicators do not have a transition point - they have a transition range.
This fact can considerably influence the correctness of the results of titrations with visual endpoint recognition.
An empirical rule is that the human eye recognizes a change in colour when the concentration ratio of the two indicator species changes from a ratio of e.g 1/10 to 10/1.
For pH indicators this means that the pH must have altered by two units.
A compact sensor for photometric titration is the new Metrohm Spectrosense.
The Metrosensor Spectrosense benefits from the advantages of photometric titration in an optimal way.
Depending on the application, you choose one of the two wavelengths available, 523 nm or 610 nm.
Modern LEDs serve as the light source; they have an average operating time of 50 000 hours and their light intensity remains high even after long operating times.
The new Spectrosense is not only outstandingly robust, it is also very easy to handle.
It can be connected to a Metrohm Titrino or Titrando like an ordinary electrode.
Power is supplied from the stirrer connection of the Titrino or the MSB connection of the Titrando.
If required, an external power adapter can also be used.
