Potentiometric titration of buffer CAPS, Bicine, HEPES, etc.

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There are many methods for the determination of buffer content, among which potentiometric titration is a relatively simple and quick one. Strong acids and strong bases are strong electrolytes, which are completely ionized in aqueous solution and can be directly completed by acid-base titration, but buffers such as CAPS, Bicine, HEPES, etc., usually use potentiometric titration of concentration or content.

Why buffers need potentiometric titration

Buffers are both amphoteric weak acid and weak bases. They cannot be completely ionized like strong electrolytes in the solution, and they also have a buffering effect on pH changes during titration. The color of the indicator does not change significantly, so it is difficult to determine the end of the titration. The potentiometric titration test is used to determine the end point of the titration by the change of the potential. This detection method is suitable for situations where the reaction equilibrium constant is small, the titration jump is not obvious, or the reagents are colored and turbid.

Reagents for buffer potentiometric titration

Most buffers such as CAPS, Bicine, HEPES, etc. are a kind of organic weak acid (except Tris), which can be titrated with sodium hydroxide standard solution (strong electrolyte titration, and some with sodium carbonate titration, but it is a weak electrolyte. You can use it first A buffer with constant concentration or content, such as CAPS standard solution, is titrated with NaOH potentiometrically to make a standard curve, and then CAPS can be measured with unknown concentration.

The buffer is partially dissociated in the solution. The degree of dissociation can be calculated by the equilibrium constant and the amount of weak acid and strong acid added to the solution. The formula is pH=pKa+log([A]/[HA]), where pKa Is the dissociation constant of the buffer at the experimental temperature, [A]/[HA] is the degree of dissociation, taking CAPS as an example, it is expressed as the ratio of CAPS anions to CAPS molecules. This equation is very useful in the buffer of weak acid titration. The pKa is -logKa under the ionic strength of a certain solution.

 It should be noted that the NAOH solution is a strong alkali, which has a strong corrosive effect. Avoid contact with hands or clothing, and it will corrode the glass. Glass instruments must be cleaned in time. Other buffers may have more than one jump point. For example, PIPES has two ethanesulfonic acid groups that are not monobasic acids. After the propanesulfonic acid group of CAPS has reacted, continuing to add NaOH may react with the amino group on CAPS to change the potential. .

In addition to potentiometric titration, the buffer is often determined by liquid chromatography. For example, Tris buffer produced by Desheng, the laboratory uses HPLC to determine its main content. Of course, different measurement methods have their own advantages, and different buffer solutions may be determined by different detection methods.