Titrators – Measuring Principle

This section contains some information about the basic principles of titration. The information is divided in several chapters. Please click on the desired section in the menu.




What is Titration?

Titration is a quantitative chemical analysis. It is used to determine an unknown concentration of a known substance in a sample. The basic principle of the titration is the following: A solution - a so called titrant or standard solution - is added to sample to be analyzed. The titrant contains a known concentration of a chemical which reacts with the substance to be determined. The titrant is added by means of a burette. A burette is a device which allows to exactly measure the quantity (volume) of the titrant added. Due to the chemical reaction taking place in the sample to be analysed, the characteristics of the sample changes.

This change of the characteristics can be detected either by a so called color indicator or a sensor:

  • A color indicator changes its color as soon as all the substance contained in the sample has reacted with the titrant added.
  • A sensor shows a significant change in the signal measured as soon as all the substance contained in the sample has reacted with the titrant added.

The concentration of the substance contained in the sample can then be calculated based on the volume of the titrant which was required to add until all the substance had reacted.

titration_principle.gifindicator_color_change.png

The animation on the left shows an example of a titration, illustrating the color change of an indicator and the signal change of a sensor. This could for example be a titration of Hydrochloric Acid (HCL) with Sodium Hydroxyde (NaOH), using Bromothymol Blue as indicator and a pH electrode as sensor. During the titration the pH value changes from about 1 to 7 (endpoint) and raises then further if an excess of the titrant is added. The color of the indicator used changes from yellow (pH value < 5) to green (pH value 5.5 … 7.5) and finally to blue (pH value > 7.7), see illustration on the right.

What is a Titrator?

A Titrator consists basically of an electric burette, a sensor whose signal is amplified with an preamplifier and a microcomputer. During a titration, the Titrator measures the signal of the sensor and uses this information to control the addition of the titrant with the electric burette. Once an endpoint is reached, the microcomputer calculates the volume of titrant added and converts this value to a result (e.g. a concentration like the concentration of table salt in soy sauce) based on formulas. The formulas needed for this calculation can be programmed and depend on type of analysis.

Let's have a closer look at some of the main components of a Titrator:

Burette

In modern titrators, so called piston burettes are used to add the titrant. These burettes (picture on the left) basically consist of a glass cylinder (1) with a piston (2) and a valve(3). To fill the burette with the titrant, the valve is opened towards the reagent bottle (4) and the piston moves down. To add the titrant to the sample, the valve is opened towards the sample and the piston moves up.

During a titration, the burette is automatically refilled if the regent consumption is higher than the volume of the burette. This automatic refilling should normally be avoided as it requires time (slower measurement) and leads to a (small) loss of precision. It is not advisable either to use a burette which has a much bigger volume than the amount of titrant required in the measurements, as bigger burettes do not allow to measure the amount of titrant added with the same accuracy as smaller ones due to the resolution of the drive used for the piston (see below). The amount of titrant used depends on the type of analysis. It is thus impossible to cover all possible applications with one size of burette. That's why burettes are available in different sizes. KEM offers burettes with volumes of 1, 2, 5, 10, 20 and 50 mL (picture on the right). A 20 mL burette is included in the standard delivery of each titrator.

It is obvious that the accuracy of the burette is a decisive factor for accurate results. The accuracy of the burette depends on the one hand on the resolution of the burette drive. In practice, however, there are other factors which are considerably more important for the accuracy of the burette than the resolution of the drive:

pistonBurette.png

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It is obvious that the accuracy of the burette is a decisive factor for accurate results. The accuracy of the burette depends on the one hand on the resolution of the burette drive. In practice, however, there are other factors which are considerably more important for the accuracy of the burette than the resolution of the drive:

  • Diffusion at the tip of the tube must be prevented by an anti-diffusion device. Diffusion means titrant addition which cannot be measured by the burette!
  • Worn out piston. The piston of the burette slightly wears off over time and needs to be replaced at regular intervals.
  • Corrosion of the burette. The burette is made of glass. Some titrants like sodium hydroxide slowly corrode glass which causes a loss of accuracy of the burette. The precision of the burette must thus be checked at regular intervals.
  • Ambient temperature fluctuations. Titrators measure the volume of titrant added. Accurate results can only be achieved, however, if a certain volume added always represents the same mass. This can be ensured only if the ambient temperature does not fluctuate. If the titrant contains organic solvents (high volume expansion coefficient) and if the ambient temperature fluctuates by more than ± 3°C, the temperature of the titrant needs to be measured and the volume added by the burette needs to be corrected based on the temperature and the volume expansion coefficient of the titrant.

To detect a loss of accuracy of the burette, regular system performance checks must be performed. This can be done by performing a titration of a known standard and/or by checking the precision of the burette with a balance. Many customers get their burettes checked at regular intervals by KEM.

 

The table below shows the features which offer KEM titrators to avoid measuring errors caused by the reasons mentioned above:


ModelAT-500NAT-510AT-610AT-700
Anti diffusion device yes yes yes yes
Automatic alert for system performance check yes yes yes yes
Automatic alert for piston replacement no no yes yes
Automatic system performance check with balance no yes yes yes
Automatic correction of ambient temperature fluctuations no yes yes yes 

Sensors and Preamplifiers

The type of sensor to be used depends on the kind of analysis to be performed:

  1. Acid base titrations like for example the determination of acidity in fruit juice require a pH electrode as sensor.
  2. Precipitation titrations as for example the determination of table salt (NaCl) in soy sauce require often a silver electrode.
  3. Redox titrations as for example the determination of vitamin C in orange juice normally require a platinum electrode.
  4. The determination of certain ions like for example calcium in water require an ion selective electrode.
  5. Certain applications like the determination of sulfur dioxide in wine require a double platinum electrode.
  6. In special cases, the endpoint of the titration is detected by measuring the electric conductivity with a conductivity cell.
  7. It is as well possible to use a color indicator and to detect its color change with a photometer cell.

 

The signal of the sensor is amplified with a so called preamplifier. The type of preamplifier used for this purpose must be compatible with the sensor. All KEM titrators come with a preamplifier with two inputs, one for a pH electrode and another one which is compatible with all electrochemical sensors which yield mV as a signal. This standard preamplifier is suitable for the type of applications 1 – 4 of the list above and covers thus most of the applications. For special applications several preamplifiers equipped with a third input (conductometric for conductivity titrations, polarized for titrations with double platinum electrodes or photometric to perform titrations with color indicators) are available (see picture on the left). The preamplifier is located in the stirrer or the sample changer (see picture on the right). It is thus easy to replace the preamplifyer as it is not required to open the titrator for this purpose. Furthermore, a short cable connection between the sensor and the preamplifier helps to avoid electrical disturbances.

preamplifier.jpg

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How does a Titrator work?

The shape of the titration curve (volume of titrant vs. signal of the sensor) depends on the type of sample being analyzed and the sensor being used.


  • Type of sample:
    A titration curve can have one or several endpoints. Example (see picture on the left): The titration of carbonic acid with sodium hydroxide yields two endpoints. In many cases, not all the endpoints detected in the titration are of interest.
  • Type of sensor:
    pH and metal electrodes normally yield an S-shaped curve (see picture on the left), whereas conductometric titrations yield an V-shaped one (see picture in on the right).

shapeOfTitCurves.png


A titrator must thus be able

  • to calculate the endpoints correctly for the different shapes of curves.
  • to filter out the endpoints which are not of interest.

 

In order to calculate the endpoints precisely the titrant should theoretically be added slowly which means in small steps. To shorten the time required per analysis, however, the titrant should be added fast which means in big steps. In order to perform the titration fast and precisely, the titrator should thus ideally add the titrant fast until coming close to an endpoint and then slow down the addition of titrant in order to be able to calculate the endpoint precisely. This means in other words that the speed of titrant addition should be related to the first derivative of the titration curve (dE/dmL): The smaller the signal change per mL of titrant added (i.e. the flatter the curve) the quicker the titrant should be added. When the curve becomes steeper, the titrant addition should be slowed down. It is required in any case, however, that the titrant is not added faster than it can react with the sample. If the reaction is slow, the ratio between the amount of titrant added per second and the first derivative of the titration curve should (s/mL / (dE/dmL) should thus be lower than for fast reactions. This ratio is called control speed (C.S.) in KEM titrators and can be selected according to the type of sample analyzed (see picture below).

 

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The evaluation of endpoint must be done according to shape of the titration curve. The pictures below illustrate some algorithms used to calculate the endpoint.

  • For S-shaped curves (see picture on the left) which are obtained normally if an electrode (pH or metal electrode) is used as sensor, the endpoints are calculated based on the first derivative of the titration curve. In certain cases, however, the measured potential is not stable which causes a noisy curve. In order to avoid errors caused by this noise, the endpoint can be evaluated as well based on a fix potential.
  • When using a conductivity cell as sensor (V-shaded curve) the endpoint is calculated based on the crossing point of two tangents (see picture in the center).
  • When using a color indicator and a photometric sensor the signal changes suddenly. In such cases, the endpoint is calculated with tangents (see picture on the right). According to the type of sample analyzed and the indicator used, either the point below or the point above can be taken as endpoint.

 

evalTitCurve.png

 

To filter out endpoints which are not of interest, several approaches are possible as the following two examples illustrate:

  • One possibility is to filter out endpoints by starting the titration only after the addition of certain volume of titrant or adding titrant until a certain potential has been reached, see pictures on the left and in the center.

 

  • Another possibility is to take into consideration endpoints only, which are located in a certain range of the signal measured (e.g. pH), see picture on the right.

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