RU2257569C1 - Electrochemical analysis method - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: invention relates to quantitative determination of substances, which are components of reversible redox systems and consists in titration of a solution to be analyzed including measurement of consumed titrant volume by measuring current intensity using two platinum electrodes with their surface areas being at 1:2 ratio. When measuring current intensity, electrode with greater surface is first used as anode, after which polarity of electrodes is reversed and current intensity is remeasured using as anode electrode with lesser surface. Volume of titrant in end point (V_e.p.) is found from following formula: V_e.p = V(Sc-Sa)(1+i'/i")/(Sc-Sa*i'/i"), where V is volume of titrant solution added, Sa anode surface area, Sc cathode surface area, i' current intensity with greater electrode as anode, and i" current intensity with lesser electrode as anode.

EFFECT: simplified analytical procedure and improved measurement accuracy.

2 dwg, 3 tbl

Description

The invention relates to the field of analytical chemistry and can be used for the quantitative determination of substances that are components of reversible redox systems.

A known method of electrochemical analysis of multicomponent solutions in a wide concentration range, including potentiostatic concentration of the analyzed elements on the working electrode, subsequent dissolution of the precipitate in the galvanostatic mode, repetition of the dissolution and precipitation cycles, change the direction of the current between the dissolution and precipitation operations, control the time deposition intervals, calculate the value charge and the concentration of the determined component in the sample, with the second and subsequent e precipitation is carried out in galvanostatic mode to potential values corresponding to the beginning of dissolution detecting component [1].

The disadvantages include the complexity of the analysis.

A known method for the determination of boric acid by the electrochemical titration method, including titration of the analyzed solution with the addition of titrant, determination of the titration end point using two electrodes of the indicator electrode and the comparative electrode [2].

A known method of determining technetium by the electrochemical titration method, which consists in titration of pre-oxidized technetium in an electrolyte solution and determination of the equivalence point by the amperometric method with two indicator electrodes [3].

The disadvantages include the limited application of the method, namely, only for the study of a solution containing technetium.

The closest in technical essence is the method of electrochemical analysis, including titration of a solution containing a defined ion, by immersion in a solution of two identical platinum electrodes and applying a constant voltage on them, the value of which lies in the range of 0.01-0.5 V. The current passing through the cell, measure with a galvanometer; the solution is evenly mixed during titration. A titrant solution is gradually added from the burette, fixing the current strength in the circuit after adding each portion of the titrant. The dependence of the current strength on the volume of the added titrant is built (titration curve) [4].

The titration curve for the examples of table 1 is as follows (figure 1).

In this case (see figure 1) near the end point of the titration there is a rapid decrease in current. The titration endpoint is determined graphically from the constructed titration curve. Tangents to relatively linear sections of the curve are drawn. From the point of their intersection, the perpendicular to the axis of the volumes is _lowered , the titration end point V is _found , the unknown concentration of the analyte is calculated. The current flowing through the cell, according to the Delahey equation, is equal to:

- степень оттитрованности), С⁰ - исходная концентрация определяемого титруемого вещества, U_C - приложенное к электродам напряжение.where n is the number of electrons participating in the reversible redox reaction, T is the absolute temperature, V is the volume of the added titrant solution, V_i.e. - titrant volume at the end point of titration (

- degree of titration), C⁰ - initial concentration of the titratable substance to be determined, U_C - voltage applied to the electrodes.

The equation is derived under the assumption that:

1) the value of U _{c is} quite small;

2) the titratable substance and the product resulting from it during the reaction are a reversible pair;

3) the proportionality coefficient P for the anodic and cathodic processes are equal.

It follows from the equation that, ceteris paribus, the current strength is determined by the amount of titrant added.

The disadvantages of this method include the fact that the repeated addition of equal portions of the titrant solution and measuring the magnitude of the current, as well as plotting to determine the end point of the titration requires a lot of time, and also does not exclude a graphic error in determining the end point of the titration associated with the construction of the curve titration. Also, a fairly large amount of titrant is spent on the titration process and special equipment for titration, for example, a burette, is required.

The invention solves the problem of expanding the arsenal of analytical methods for electrochemical analysis, which reduces the duration of titration and reduces the time to determine the end point of titration.

The technical result is to simplify the method and increase the accuracy of electrochemical analysis.

To obtain such a technical result in the proposed method of electrochemical analysis, including titration of the analyzed solution with the addition of titrant and determination of the titrant volume at the titration end point, by measuring the current strength of the analyzed solution with two platinum electrodes, according to the invention, the titrant is added once, and the current is measured simultaneously by adding titrant, and to measure the current strength, platinum electrodes are used, the surface areas of which are correlated as 1: 2, and when measuring the current strength, initially, an electrode with a larger surface area is used as the anode, then the polarity is changed and the current is measured a second time, using an electrode with a smaller surface area as the anode, and the titrant volume at the end point is determined by formula (2), where S _a is the area of the anode; S _to - the area of the cathode; i 'is the current strength during the initial measurement, when an electrode with a larger surface area is used as the anode; i '' is the current strength when measuring polarity, when an electrode with a smaller surface area is used as the anode.

The method is as follows: an aliquot (20-25 ml) of the titratable solution containing the ion to be determined is placed in the titration cell. Platinum wire electrodes of the same diameter, but of different lengths, with surfaces S ₁ and S ₂ mm ^{2, are} immersed in the solution. A constant voltage is applied to the electrodes, the value of which lies in the range of 0.01-0.5 V. An electrode with a larger surface area is connected to the positive pole of the voltage source. Using a syringe dispenser, V ml of titrant solution is added. The solution is mixed. Using a galvanometer, the current strength between the electrodes (i ') is measured. The polarity of the electrodes changes (platinum wires are interchanged) and the current strength in the circuit is measured again (i ''). Equation (2) determines the titrant volume at the titration endpoint.

Consider an example (see Fig. 2). With the same volume of added titrant V, the currents measured by changing the polarity of the electrodes will differ.

When the electrodes have different surface areas, the Delahey equation (1) can be written as:

- in the case when the electrode with a larger surface area is the anode:

- in the case when the electrode with a larger surface area is the cathode, and an electrode with a smaller surface area is used as the anode (polarity of the electrodes is changed):

We divide equation (3) by (4) and get:

We solve equation (5) with respect to λ:

где V - объем добавленного раствора титранта.Knowing λ, to find the end point of titration, you can use equation (6):

where V is the volume of the added titrant solution.

In general terms, the equation can be written:

To prove the feasibility of this method, water was determined in different portions of chloroform by amperometric titration, according to GOST 14870-77 - Methods for the determination of water - and the proposed calculation method for determining the end point of titration. The determination results are shown in the table.

In a similar manner, a titration of a solution containing ferrous ion (see Table 1) was carried out with a solution of potassium permanganate. The determination was carried out in the usual way - with the repeated addition of titrant and the construction of a titration curve, and the proposed method. The determination results are shown in the table.

[5] [suggestion] V _i.e. ml

V _i.e. ml

12.00 0.0120 1.20 11.10 0.0111 3.00 12.10 0.0121 6.00 11.90 0.0119

- From the tables it follows that the comparative results obtained by the proposed method, with the results of determination according to GOST (prototype) are identical. But the advantage in the calculation method for determining the titration end point is that the titration operation is excluded, i.e. repeated addition of equal portions of the titrant solution and measurement of the magnitude of the current.

- The graphic error in determining the titration endpoint associated with the construction of the titration curve is eliminated.

- Decreases detection time.

- The flow rate of the titrant solution required for analysis is reduced.

- The burettes are being replaced with easier-to-use dispensing syringes.

Sources of information

1. RF patent No. 2199734, G 01 N 27/48, 02.27.2003.

2. RF patent No. 2035041, G 01 N 31/16, 05/10/1995.

3. RF patent No. 2132552, G 01 N 27/48, 06/27/1999.

4. Delahey P. New devices and methods in electrochemistry. M .: Inostr. literature, 1957.

5. Songina O.A., Zakharov V.A. Amperometric titration. M .: Chemistry, 1979.

Claims (1)

The method of electrochemical analysis, including titration of the analyzed solution with the addition of titrant and determination of the titrant volume at the titration end point, by measuring the current strength with two platinum electrodes, characterized in that the titrant is added once, and the current is measured simultaneously with the addition of titrant, and for measuring the force current use platinum electrodes, the surface areas of which are correlated as 1: 2, and when measuring the current strength, initially use an electrode with a larger with a flange of the surface, after which the polarity is changed and the current is measured a second time using an electrode with a smaller surface area as the anode, and the titrant volume at the end point is determined by formula (I), where V is the volume of the added titrant solution; S _a is the area of the anode; S _to - the area of the cathode; i 'is the current strength when an electrode with a larger surface area is used as an anode; i '' is the current strength when an electrode with a smaller surface area is used as an anode; V _TE - titrant volume at the end point of titration.

Images