Volumetric Test for Analytes Determination

As a classical quantitative wet chemical analysis, volumetric analysis is also known as titrimetric method. Titration is one of the oldest methodologies in analytical chemistry. However, its simplicity, universality and speed make titration the most reliable and versatile method of quantitative analysis. In quantitative analysis, volumetric analysis is widely employed to determine the quantity of a chemical by using the volume of a liquid.

Principle of Volumetric Test

In a volumetric test, a reagent with a known volume and concentration is added to a solution with an unknown substance and concentration. The amount of reagent needed for a change to occur is proportional to the amount of unknown substance, thus determining the exact amount that has been consumed by the time the end point is reached. Titration often uses visual indicators, such as a change in color of the reactant mixture, to indicate the end point of the reaction. If there is no visible change, an indicator is added to the solution. The indicator changes color depending on the pH of the solution. The exact point where the color change occurs is called the end point.

Many methods can be used to indicate the endpoint of a reaction, and titrations often use a visual indicator (the reactant mixture changes color). For example, an oxidation-reduction titration using potassium permanganate (pink/purple) as the titrant does not require an indicator. When the titrant is reduced, it becomes colorless. After the equilibrium point, there is excess titrant present. The equilibrium point can be determined from the persistence of the first faint pink color in the solution being titrated. In simple acid-base titrations, a pH indicator, such as phenolphthalein, can be used.

Our Capabilities

At BOC Sciences, the most commonly used measurement in volumetric methods is to determine the amount of analyte present in a solution by measuring the absorbance or transmittance of UV/Visible light at a specific wavelength versus the absorbance or transmittance of a solution of known concentration of the analyte. If a solute absorbs light of a specific wavelength, the absorbance is proportional to the concentration of the substance in solution. When a standard substance with the same absorbance and known concentration is identified, measuring the absorption of light by the sample gives accurate information about the concentration. We can reasonably ensure that the substances with the same absorbance also have the same concentration.

• Acid/base titration

Acid/base titration method utilizes the neutralization reaction that occurs between an acid and a base. We employ acid/base titration to quantify the concentration of an unknown acid or base solution. In addition, pKa and Ka (the acid constant) can be determined from the pH titration diagram. We also use acid-base titrations to determine the percentage purity of chemicals.

• Redox titration

A redox titration is a titration based on a redox reaction between an analyte and a titrant. At BOC Sciences, we support the following types of redox titrations:

• Redox titration - Titrant
• Iodometric - Iodine
• Bromimetry - Bromine
• Colorimetric method - Cerium (IV) salt
• Potassium permanganate assay - Potassium permanganate
• Dichromate method - Potassium dichromate

Figure 2. A schematic illustration for the chemical titration procedure of the Mg/Al ratios of the MgAl-LDHs. (Zhang, Z; et al. 2014)

• Complexation titration

We use complexometric titrations for the volumetric analysis of compounds, especially for the determination of mixtures of different metal ions in solution, in which an indicator capable of producing an unambiguous color change is used to indicate the end point of the titration.

• Potentiometric titration

Potentiometric titration is a technique similar to the direct titration of redox reactions. We use potentiometric titration to characterize acids, where the potential is measured across the analyte, usually an electrolyte solution. We use two electrodes, an indicator electrode (glass electrode and metal ion indicator electrode) and a reference electrode. The reference electrodes typically used are the hydrogen electrodes, calomel electrodes and silver chloride electrodes.

• Precipitation titration

Precipitation titration involves the formation of a precipitate during the addition of a titrant. The titrant reacts with the analyte to form an insoluble substance and the titration continues until the last amount of the analyte is consumed. The first excess drop of titrant reacts with the indicator, causing a color change and announcing the termination of the titration.