To get a better idea of what potentiometric methods of analysis are, why they exist, and what their purpose is, it is necessary to begin to deal with potentiometry itself. This is what we will do now.
Potentiometry
The explanation begins with what potentiometry is. This is not a separate field of science or something like that. This subsection includes the totality of all physico-chemical research methods that exist, but with one small condition. This includes only those of them that are based on measuring the electromotive force of the chains.
These chains are made up of two main elements:
- indicator electrode;
- reference electrode.
In order to use the potentiometric analysis method, it is necessary to find the dependence of the electrochemical electrode, or rather, its potential, on the concentration of ions or on their activity. There is another point, and it suggests that potentiometry is a section that includes electrochemical methods of analysis based on the excitation of a substance by an electric field and the need to measure the equilibrium electrode.
Basics of Potentiometry
The founder of this section of the analysis as potentiometry is V. Nernst. The potentiometric method of analysis is based on the equation that was introduced by this person in 1889. The equation obtained by Nernst served for equilibrium electrode potentials, for their finding. But soon after that, potentiometry began to gain wider distribution and began to be used in such a section as analytical chemistry.
In 1893, the scientist R. Berend conducted the first potentiometric titration in human history. And at present, such a section as potentiometry is quite widely used in analytical and physical chemistry. In addition, there are several other areas of application of the potentiometric analysis method.
Potential use areas
Analytical and physical chemistry are, as we have said, not the only areas where potentiometry has found its application. This analysis method is also widely used in redoscometry. This section combines several methods that are based on the measurement of redox potentials in the test solution.
Potentiometric methods of analysis are also used in ionometry. This is the name for the branch of direct potentiometry, the essence of which is to determine the concentration or activity of ions in their various phases. For this, an ion-selective electrode is used. In addition, ionometry combines all the methods of analysis related to the determination of concentration. It is also worth adding that pH-metry, cation- and anionometry also belong to this section.
Another use of the potentiometric analysis method is potentiometric titration. The essence of titration is the need to monitor the progress of the titration reaction and the subsequent determination of equivalence points.
The essence of the potentiometric analysis method
The essence of potentiometry, as well as methods for analyzing this area, is to measure the difference in the electromotive force of two electrodes. One of them is measuring, and the other is auxiliary. In addition, both of these electrodes should be placed in the test solution.
It is very important to note that the magnitude of the electromotive force is highly dependent on temperature. For this reason, all instruments that are used for analysis, in particular pH meters and ionometers, are additionally equipped with temperature compensators, which can be controlled both in manual mode and in automatic mode.
In short, the potentiometric analysis method is based on measuring the EMF of galvanic cells. By these elements we understand the connection of two electrodes, one of which, as mentioned earlier, is an indicator, and the other is necessary for comparison.
Direct potentiometry
The classification of potentiometric analysis methods is quite simple and has only 2 points. And the first subgroup is direct potentiometry. The essence of this method is that it is necessary to measure the exact value of the electrode potential, and then find the activity of the potential-determining ion in solution using the Nernst equation. This method is used to find the pH of aqueous and non-aqueous solutions. In addition, this method can also be used to find the pH for production solutions of oligomers.
In addition, potentiometric methods for the analysis of direct potentiometry are used to analyze acidic and basic impurities in dimethylformamide. Also, this method can be used to determine chloride ions and acid components in various industrial solutions.
The question of what the potentiometric method of analysis of this group is based on can be answered this way - by measuring the exact values. And from this it becomes clear why this method of analysis has found its wide application in such fields as thermodynamics, electrochemical, and also chemical reactions.
Titration
The essence of the potentiometric analysis method in this area of potentiometric titration is based on the need to determine the equivalence point in the neutralization reaction, as well as in the precipitation and complexation reactions , which will be known from the results of potentiometric measurements. Once as a result of the analysis this exact equivalence point, which is also the end point of the reaction, becomes known exactly, it will be possible to find an unknown amount of the substance in the solution that is being monitored. This can be determined by the consumption of titrant, the concentration of which is known in exact numbers.
Titration Principles
The principle of operation of the potentiometric analysis method - titration - is as follows. Close to the equivalence point, a strong change or even a jump in potential from the indicator electrode usually occurs. The strength of this jump will depend on the strength of the titratable acid solution or the base, as well as on the concentration of the solution itself. If titration of mixtures of various acids is carried out, several jumps can be observed in one reaction.
The use of this method is carried out for purely applied purposes, when it is necessary to detect the end point of titration, with the help of which it is possible to determine whether an analyte in a solution belongs to any group. However, despite this, this method is quite widespread, and also has its advantages, which distinguish it from other analysis methods.
Potentiometry Benefits
Among the advantages of potentiometry, in general, the following points can be distinguished:
- High accuracy of analyzes.
- High measurement sensitivity.
- If we talk about titration, then the method can be used in turbid or multi-colored solutions.
- Using the direct potentiometric method of analysis - the speed and simplicity of the measurements.
- One of the big advantages is the ability to automate the titration process. Some industrial facilities are engaged in the production of automatic autotitrators, which are equipped with potentiometers. The result is a small autonomous complex for analysis.
It also distinguishes this method from others in that it is possible to determine several substances in the same solution without prior separation.
Features and disadvantages of potentiometry
If we talk about potentiometry, as well as its methods of analysis, we can distinguish several features. For example, potentiometric titration greatly expands the possibility of practical application of this field of analysis among non-aqueous solvents. These methods allow people to find the content of various components that cannot be titrated individually, and it is also possible to analyze substances that do not dissolve or, on the contrary, decompose in water.
Another important feature of potentiometric methods is that they are classified as a nondestructive group. In other words, the analysis by one of the potentiometric methods does not affect the composition of the analyzed solution and does not destroy it, as a result of which the mixture can be used in the future without any problems. The measurement error when using the direct potentiometric method is only from 2 to 10%. And if you use potentiometric titration, then the error is generally from 0.5 to 1%.
The disadvantage of this method is that it is impossible to use it for continuous monitoring, and in some cases it is impossible to determine too low concentrations in solution.
Advantages of the potentiometric analysis method
Scientists highlight the main advantage of this method is its speed, as well as the simplicity with which the analysis is carried out. It is these two qualities that distinguish potentiometry and methods of its analysis from others.
The time required to establish the equilibrium potentials of the indicator electrode is rather small, which, in turn, makes it possible to conveniently study the kinetic reactions and the reactions of automatic control of various technological processes.
If microelectrodes are used as electrodes, then measurements in samples become available, the volume of which is equal to tenths of a cm³. In addition, there are several different solutions in which measurements can be made using potentiometry.
There is a potentiometric method for analyzing water, various turbid and colored solutions, as well as various viscous pastes. Its huge advantage is that when using the named method there is no need for distillation or filtration of the solution. And this in some cases saves a lot of time.
Benefits of Potentiometric Titration
The use of the potentiometric method of titration analysis is distinguished by such advantages:
- This method allows you to completely eliminate subjective errors, which are often associated with visual observation of the solution.
- This method is considered more sensitive. In other words, with the same manipulations, titration allows the determination of less concentrated substances.
- In addition, the method allows titration in clouded or colored solutions without problems. This is very convenient, since it is impossible to use color indicators in such solutions, say, at all.
- Also, this analysis method allows a person to consistently determine a mixture of substances from the same portion of a solution.
These advantages distinguish titration compared to other potentiometric methods, and therefore it is used quite widely.