Product category:
Materials Testing
News Release from: Metrohm UK | Subject: Titrator
Edited by the Manufacturingtalk Editorial
Team on 08 January 2007
Photometric measurement uses titration
method
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 is certainly the oldest method for recognising the endpoint of the titration; and is still frequently used today It can be realised without any complicated instrumentation and at a low cost
This article was originally published on Manufacturingtalk on 31 Jul 2007 at 8.00am (UK)
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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.
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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 can hardly 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 recognises 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 (3).
The 662 Photometer allows exact measurement of absorption and transmission as well as their direct conversion to concentration values.
The analog output is used in combination with a titrator for the photometric endpoint determination.
Display of transmission, absorption and concentration: Transmission and absorption are the traditional measured quantities.
Additionally, the 662 Photometer determines the concentration of sample solutions after previous calibration.
Glass fibre light guide: Measurements can be performed in any sample container, be it a beaker, a test tube, a titrating vessel, etc.
The long, flexible light guide is just as convenient to use as a pH electrode; its resistance to solvents is excellent.
Gradual filter: Thanks to the continuous wavelength scale with stepless adjustment, measurements can always be performed at the point of maximum absorption.
Accordingly, there is no loss of sensitivity due to non-optimal selection of the wavelength.
Expensive cells are no longer needed: the long, flexible glass-fibre light guide of the 662 Photometer makes photometric determinations just as easy as, for example, a pH measurement - in a wide range of different vessels and over the entire visible wavelength range (400 ...
700 nm). Request a free brochure from Metrohm UK ...
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