Problem: During a titration, a titrant that reacts with the analyte is added until an equivalence point is reached. At this equivalence point (EP) there is an equal amount of analyte and titrant. Since the reaction between analyte and titrant is known, the amount of the analyte can be quantified using the volume of titrant that has been added.
Historically, titrations were performed manually with hanging burettes. The indication of the titration end point was determined visually by the addition of a color indicating compound that changes the color of the solution at the EP. There are several challenges when performing manual colorimetric titrations. The determination of the equivalence point is subjective and can vary from operator to operator. A neutral background is needed to accurately distinguish the color change. The original color of the sample can also inhibit the operators’ ability to view the end point. If the EP is exceeded, a back titration with additional calculations is necessary to determine the concentration of the analyte.
To overcome these obstacles, titration systems with potentiometric equivalence point indication were developed. These systems accurately dose the titrant with microliter accuracy and utilize potentiometric electrodes that electrochemically determine the EP. This type of end point indication increases accuracy and repeatability. Although potentiometric titrations have become quite popular, there are a considerable amount of industries with standard methods (United States Pharmacopeia, European Pharmacopeia, American Society for Testing and Materials, et al.) that require colorimetric EP indication.
Solution: A photometric sensor can be used to automate colorimetric titrations. The use of a photometric sensor offers the operator all of the advantages of potentiometric titrations with the mandated color indication. The photometric sensor works as an in-beaker spectrophotometer to determine precisely the equivalence point based on detected light absorbance.
Metrohm offers the Optrode photometric sensor. It is equipped with LEDs that produce light at eight different wavelengths ranging from 470 to 660 nanometers. The electrode shaft is completely made of glass making it 100 percent solvent resistant. It requires no maintenance and can be used on all of Metrohm’s titration platforms.
Prior to a titration, the desired wavelength is selected on the electrode or within the instrument software. During a titration, the Optrode’s optical window is immersed in the sample solution. As the titration proceeds, titrant is automatically added to the sample solution containing the color indicator via an accurate dosing device and burette. The absorbance is measured within the optical window and converted to a millivolt signal. This signal is then plotted versus the titrant volume added. At the equivalence point there is a drastic change in measured absorbance that corresponds to an inflection point in the titration curve. The software can then use the volume of titrant added to calculate the concentration of the analyte of interest.
The Optrode allows for automated titration with colorimetric equivalence point detection. With this sensor, higher accuracy and repeatability can be achieved. The sensor is robust and requires no maintenance.
For more information, go to: www.metrohm.com