What Is a Dissociation Constant?

The dissociation constant (pK _a ) is a polarity parameter of a solute with a certain degree of dissociation in an aqueous solution. The dissociation constant gives a molecule a measure of acidity or basicity, with increasing K _{a, which} increases for the proton donor, and decreases K _a , which increases for the proton acceptor.

pK _a is a specific type

Dissociation constant (pK _a ) is a very important property of organic compounds, which determines the form of the compound in the medium, and then determines its solubility, lipophilicity, bioconcentration and toxicity. For drug molecules, pK _a also affects its pharmacokinetics and biochemical properties. ^[2]

Dissociation constant potential titration

Potentiometric titration is one of the most commonly used methods for determining the dissociation constant pK of a substance. Taking a monovalent weak acid as an example, its dissociation equilibrium formula in water is:

According to the above formula, the pK _{a of the} substance can be obtained by substituting the volume V of the added base and the measured solution pH. Usually, the pH of the solution is

The pK _{a of the} substance is obtained by mapping. Therefore, during the experiment, it is only necessary to record the volume of the alkali added at a certain temperature and the pH value of the solution measured after each volume of alkali added.

In order to overcome the problem that the substance is difficult to dissolve in pure water and it is difficult to determine its dissociation constant, a mixed solvent of organic solvent and water can be used instead of pure water. By measuring the substance in a mixed solvent of organic solvent and water with different volume ratios, Dissociation constant, and then apply a certain mathematical method to obtain the dissociation constant in pure water.

The ionic strength of a solution affects the dissociation of a substance in water. Therefore, in determining the dissociation constant of certain substances in water, the influence of the ionic strength of the substance aqueous solution needs to be considered.

It is necessary to pay attention to data processing techniques when measuring the dissociation constant of substances by potentiometric titration, because choosing an appropriate data processing method can bring great convenience to the processing of experimental data, save a lot of time, and improve the accuracy of the data.

The biggest disadvantage of potentiometric titration is that it is difficult to determine the dissociation constant of low-solubility substances, but its operation is simple. Some experiments only need a few sets of data to obtain the results. Scientific mathematical methods and images can be used to process the pH titration data. Results accurate. Therefore, potentiometric titration is the most widely used method for measuring the dissociation constant of a substance. ^[3]

Dissociation constant spectrophotometry

Spectrophotometry is another commonly used method for determining the dissociation constant of a substance. It is an analytical method based on the principle that the molecular state and the ionic state of a substance have different absorptions of light of a certain wavelength. When a substance reaches the dissociation equilibrium in a solution, both the molecular and ionic states of the substance are present in the solution, and the two states have different absorptions of light of the same wavelength. Therefore, it is measured with a spectrophotometer The absorbance of a solution is a comprehensive expression of the absorbance of molecules and ions in the solution. If the test object is a monovalent weakly acidic substance, its dissociation formula is:

Put the above

And Ka are expressed by their paralog pH and pKa, respectively, and the substance concentration is expressed by their absorbance:

Where

100% ionic absorbance,

100% molecular absorbance,

Is the measured absorbance of the substance solution. After trying to obtain and measure the absorbance of the solution, the dissociation constant of the substance can be obtained.

Spectrophotometry has a good application for determining the dissociation constant of substances with strong absorption in the ultraviolet-visible light region, and it can also be used to determine the solution of substances with weak (or no absorption) absorption in the ultraviolet and visible light regions. Separation constant, but you need to add an indicator on the basis of the original. Fang Haihong et al. ^[4] used nitrophenol as an indicator to determine the dissociation constant of fumarate B by spectrophotometry, and introduced the method of selecting the analysis wavelength and measuring the absorbance of pure substance molecules and ions.

Spectrophotometry has a wide range of applications, simple operation, accurate and reliable results, and is a good method for determining the dissociation constant of a substance. It is not only suitable for compounds with significantly different UV-visible absorption spectra in dissociated or non-dissociated form, but also in the case of low-solubility compounds and phenanthrene dissociation. However, the spectrophotometric method must measure the absorbance of the acidic dissociation, basic dissociation, and weak acid and weak alkali of each substance one by one. For the measurement of a large number of biological alkaline dissociation constants, the workload is large. In addition, the spectrophotometric method has strict requirements on the selection of the working wavelength when it is used to determine the pK _a value of the acid-base indicator and the organic acid-base of the developer.

Dissociation constant capillary electrophoresis

Capillary electrophoresis is a new analytical technique that has developed rapidly in recent years. It is driven by a high-voltage electric field. A new technique for liquid phase separation analysis using a capillary tube as a separation channel and based on the difference in electrophoretic mobility or distribution behavior between various components in a sample. Zhang Lan et al. ^[5] introduced a capillary electrophoresis-electrochemical detection experiment based on the knowledge of the ionization balance of weak acids, the distribution coefficient of ions, the basic principle of mobility, and the ionic product constant of water, ignoring the ionic strength. principle.

Capillary electrophoresis can be used to determine the dissociation constant of compounds that have no absorption or are not sensitive to absorption in the ultraviolet-visible light region. In order to verify the accuracy of the dissociation constant of the substance obtained by capillary electrophoresis, some people compared the value of the dissociation constant of the substance obtained by capillary electrophoresis with the value obtained by other traditional methods. C. Foulon et al. Compared the dissociation constants of eight aromatic inhibitors obtained by capillary electrophoresis with the values obtained by potentiometric titration, spectrophotometry, and calculation methods. Methods.

When capillary electrophoresis is used, buffer solutions of different pHs need to be prepared. The preparation process is complicated, and several hours of electric cleaning are required before the capillary is applied. The viscosity of the sample solution often has a large impact on the results. However, this method is simple and fast, has low running cost, high separation efficiency, and high sensitivity. Therefore, capillary electrophoresis is a development direction of dissociation constant detection methods.

Other methods of dissociation constants

Fluorescence has certain applications in determining the dissociation constant of a substance. The basis of this method is: a solution of a substance with fluorescence will have different fluorescence intensities at different concentrations. Therefore, if the concentration of the substance in the solution is changed, the fluorescence intensity of the solution will change accordingly. The dissociation constant of the substance can be obtained according to the change of the substance concentration corresponding to the change in the fluorescence intensity of the solution.

Thin layer chromatography pH method is an analytical method based on the relationship between the pH of the chromatographic system and the Rf (function of partition coefficient) value of dissociative substances. The essence is: the same amount of the test substance is spotted and adsorbed on a thin layer of color plate treated with a buffer solution of different pH value, and then developed in the same solvent system, so that a series of test substance can be measured. Rf value. The Rf value is plotted against the corresponding pH value. The characteristic curve of the pH-Rf of the substance can be obtained, and according to this characteristic curve, the dissociation constant can be known. Thin-layer chromatography pH method requires less sample and simple experimental equipment. However, it is necessary to choose a suitable solvent system during the experiment, strictly control the pH of the thin-layer plate and configure the buffer solution with different pH, which is not widely used.