Automated colour endpoint titrations

Together with gravimetry, titration is one of the oldest analytical techniques, and both belong to a group of analytical methods that are based on chemical reaction.

In titration one determines the volume of a standard solution (titrant) that is necessary for complete chemical reaction with the analyte.

The titrant contains a known amount of a particular substance.

Since Loschmidt and Avogadro we know that one gram molecule of a substance contains a defined number of particles.

A standard solution is produced by dissolving a particular weight of a substance in a solvent.

Each volume fraction of this standard solution contains a defined number of particles of the dissolved substance.

This means that measuring the volume of a standard solution is a method of counting particles, and this is the fundamental basis that is titration.

Despite many new, mainly physical instrumental analytical methods, titrimetry as a 'wet chemistry method' still remains a standard procedure for quantitative analysis today.

This is because it has a number of specific advantages: Titration is one of the absolute content determination methods, ie, the result of the analysis provides direct information about the substance to be determined, without instrument or method specific factors having to be calibrated (eg with HPLC, atomic spectroscopy or UV/vis photometry).

Titrations are easy to carry out.

The equipment and the procedures to be performed are simple.

Titrations are carried out rapidly.

Titration is a versatile method.

Numerous titration methods have been developed and analyte concentrations can range from 100% down to ppm range.

Titration supplies highly reproducible and correct results.

A typical reproducibility is <1% and in high precision titrations of 0.1% is demanded and also achieved.

Titrations can be automated.

Titrimetric determinations can be automated to a high degree, and this means they are suitable for analysing the content in routine operation.

Titration Reactions: there are essentially five types - acid/base reactions, aqueous; acid/base reactions, non-aqueous; precipitation reactions; redox reactions; and complexing reactions.

Indicator methods.

The equivalence point is reached when titrant and analyte have reacted completely with each other in the corresponding stoichiometric ratio.

If titration is regarded as being 'counting molecules or ions', then this point must be detected as accurately as possible.

This is done by using the properties of the solution or added indicator which, at its equivalence point, changes as abruptly as possible and in a well defined manner.

The volume of titrant consumed up to the point that this change occurs is the endpoint volume.

The endpoint should be as close to or identical to the equivalence point.

This means that the indicator method is very important.

In addition to the accuracy of the titrant addition, it makes an important contribution to the reproducibility and, above all, the correctness of the result.

There are essentially three important indicator methods (less important ones include conductivity and calorimetry).

1 Potentiometric indication.

2 Indication with polarised electrodes.

3 Visual or photometric indication.

Potentiometric indication: potentiometry is one of the most frequently used methods for the endpoint indication of titrimetric analyses.

Numerous sensors are available (some of which have been specifically developed for a particular application).

They cover the whole range of titrations - aqueous and non aqueous acid - base titrations, redox titrations, precipitation titrations and complexometric titrations.

The measuring arrangement for potentiometric measurements always consist of two electrodes - an indicator electrode and a reference electrode.

The quantities measured are not potentials, but differences in potentials (voltages).

Polarized electrodes: these methods are primarily used for redox titrations in dilute solutions or if dilute titrants are used, as in such cases potentiometric indication is not satisfactory (titration curves are flat - loss in precision).

There are four such indicator methods using polarised electrodes.

Amperometry - potential polarisation/current measurement with one polarisable electrode.

Biamperometry - potential polarisation/current measurement with two polarisable electrodes.

Voltametry - current polarisation/potential measurement with one polarisable electrode.

Bivoltametry - current polarisation/potential measurement with two polarisable electrodes.

Visual or photometric indication.

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.

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 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 eg 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 Spectrosense benefits from the advantages of photometric titration in an optimal way, says Metrohm.

Depending on the application, choose one of the two wavelengths available: 523nm or 610nm.

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.

Spectrosense.

The new Spectrosense is not only outstandingly robust, it is also very easy to handle, says the company.

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.

Photometric titrations can now be fully automated.

The 855 Robotic Titrosampler is an automated titrator that combines the highest quality standards in titration - based on the Titrando technology - with top-performance automation.

Its requires 40% less lab space and has easy handling thanks to features like plug-and-play.

This synthesis of automation and titration processes series of up to 140 samples on the smallest footprint ever.

Save 40% of lab space without any loss in convenience, accuracy or reproducibility.

Nevertheless the new 855 Robotic Titrosampler is an all-rounder with all the features of automation.

Modern automation means not just moving around samples - its key elements are sample preparation and sample processing.

A decisive advantage is that the determination is carried out by one and the same instrument.

In accordance with Metrohm's high standards in titration, the 855 Robotic Titrosampler covers the full range of titrations and measurements - dynamic equivalence point titration (DET), monotonic equivalence point titration (MET), endpoint titration (SET), enzymatic and pH-stat-titration (Stat) and direct measurement of pH values and ion concentrations.

As a complete package the system is said to fulfill all requirements of routine analysis with an impressive sample throughput on the smallest possible footprint.

It also fully implements the plug-and-play principle.

The 855 Robotic Titrosampler is the complete solution for routine analysis, says Metrohm.

The packages include everything that is required to set up a ready-to-use system.

Data management and system control is performed by Tiamo titration software.

Conclusion.

Although one of the oldest analytical techniques, titration is still very much an important tool in today's modern laboratory for analysing a whole host of different products.

Titration is still extensively used in many diverse industries from pharmaceutical to chemical, food to environmental.

Colour has been used in titration to determine the endpoint of the reaction, since 1750 when GF Venel used violet excerpt as an indicator for mineral water analysis.

Since then many synthetic indicators have produced and are common place in analytical laboratories all over the world.

The Metrohm Spectrosense removes subjectivity from this colour endpoint indication.

Bench space in the laboratory is at a premium.

The 855 robotic titrosampler is said to be the world's first all-in-one titrator and autosampler, and because of this, the combined system needs 40% less bench space than other modular titrators and autosamplers.

The 855 robotic titrosampler can of course be used in conjunction with the Spectrosense to fully automate colour titrations and/or other traditional electrode systems can also be used.

For many years, Metrohm has been at the forefront of advances in titration techniques and instrumentation, and the company says itwill remain so for the foreseeable future.

The Metrohm monograph 'Practical Titration' Professor by Dr Leo Gros, Peter A Bruttel, Marcus Von Kloeden, 2005, is available from the company, as is Metrohm Application Bulletin Number 33, 'Photometric determination of total hardness, calcium and magnesium hardness of water'.