How to calculate sample addition. Standard Addition Method

IN one standard solution method measure the value of the analytical signal (y st) for a solution with a known concentration of the substance (C st). Then the magnitude of the analytical signal (y x) is measured for a solution with an unknown concentration of the substance (C x).

This calculation method can be used if the dependence of the analytical signal on concentration is described by a linear equation without a free term. The concentration of the substance in the standard solution must be such that the values ​​of the analytical signals obtained when using the standard solution and a solution with an unknown concentration of the substance are as close as possible to each other.

IN method of two standard solutions measure the values ​​of analytical signals for standard solutions with two different concentrations of a substance, one of which (C 1) is less than the expected unknown concentration (C x), and the second (C 2) is greater.

or

The method of two standard solutions is used if the dependence of the analytical signal on concentration is described by a linear equation that does not pass through the origin.

Example 10.2.To determine the unknown concentration of a substance, two standard solutions were used: the concentration of the substance in the first of them is 0.50 mg/l, and in the second - 1.50 mg/l. The optical densities of these solutions were 0.200 and 0.400, respectively. What is the concentration of the substance in a solution whose optical density is 0.280?

Additive Method

The additive method is usually used in the analysis of complex matrices, when the matrix components affect the magnitude of the analytical signal and it is impossible to accurately copy the matrix composition of the sample. This method can only be used if the calibration graph is linear and passes through the origin.

When using calculation method of additives First, the magnitude of the analytical signal is measured for a sample with an unknown concentration of the substance (y x). Then a certain exact amount of the analyte is added to this sample and the value of the analytical signal (y ext) is measured again.

If it is necessary to take into account dilution of the solution

Example 10.3. The initial solution with an unknown concentration of the substance had an optical density of 0.200. After 5.0 ml of a solution with a concentration of the same substance of 2.0 mg/l was added to 10.0 ml of this solution, the optical density of the solution became equal to 0.400. Determine the concentration of the substance in the original solution.

= 0.50 mg/l

Rice. 10.2. Graphical method of additives

IN graphical method of additives take several portions (aliquots) of the analyzed sample, add no additive to one of them, and add various exact amounts of the component being determined to the rest. For each aliquot, the magnitude of the analytical signal is measured. Then a linear dependence of the magnitude of the received signal on the concentration of the additive is obtained and extrapolated until it intersects with the x-axis (Fig. 10.2). The segment cut off by this straight line on the abscissa axis will be equal to the unknown concentration of the substance being determined.

Method for comparing the optical densities of standard and test stains

solutions

To determine the concentration of a substance, take part of the test solution, prepare a colored solution from it for photometry, and measure its optical density. Then two or three standard colored solutions of the analyte of known concentration are prepared in the same way and their optical densities are measured at the same layer thickness (in the same cuvettes).

The optical densities of the compared solutions will be equal to:

for the test solution

for standard solution

Dividing one expression by the other, we get:

Because 1 X = l ST, E l= const, then

The comparison method is used for single determinations.

Graduated graph method

To determine the content of a substance using the calibration graph method, prepare a series of 5-8 standard solutions of different concentrations (at least 3 parallel solutions for each point).

When choosing the concentration range of standard solutions, the following principles are used:

It should cover the range of possible changes in the concentrations of the test solution; it is desirable that the optical density of the test solution corresponds approximately to the middle of the calibration curve;

It is desirable that in this concentration range at the selected cuvette thickness I and analytical wavelength l the basic law of light absorption was observed, i.e. the schedule D= /(C) was linear;

Operating range D, corresponding to the range of standard solutions, should ensure maximum reproducibility of measurement results.

Under the combination of the above conditions, the optical densities of standard solutions relative to the solvent are measured and a graph of the dependence D = /(C) is plotted.

The resulting curve is called a calibration curve (calibration graph).

Having determined the optical density of the solution D x, find its values ​​on the ordinate axis, and then on the abscissa axis - the corresponding concentration value C x. This method is used when performing serial photometric analyses.

Additive Method

The additive method is a variation of the comparison method. Determining the concentration of a solution by this method is based on comparing the optical density of the test solution and the same solution with the addition of a known amount of the substance being determined. The additive method is usually used to simplify work, to eliminate the interfering influence of foreign impurities, and in some cases to assess the correctness of the photometric determination method. The additive method requires mandatory compliance with the basic law of light absorption.

The unknown concentration is found by calculation or graphical methods.

Subject to the basic law of light absorption and a constant layer thickness, the ratio of the optical planes of the test solution and the test solution with the additive will be equal to the ratio of their concentrations:

Where Dx- optical density of the test solution;

D x + a- optical density of the test solution with the additive;

C x- unknown concentration of the test substance in the test colored solution;

S a- concentration of the additive in the test solution.

Determine the analytical signal of the sample ( y x) and the signal of the same sample with the addition of some additive of the determined component of known content ( y x + ext), then the unknown concentration of the component being determined is:

where V add, V sample are the volumes of additive and sample, respectively.

Another goal of analytical chemistry is to lower the detection limit. This is due to the continuously growing requirements for the purity of materials used in the space and military industries.

Under detection limit understand the minimum concentration of a substance that can be determined by the chosen method with a certain acceptable error. Quite often, analytical chemists use the term « sensitivity» , which characterizes the change in the analytical signal with a change in the concentration of the component being determined, i.e. above the detection limit the method is sensitive to the component being determined, below the detection limit it is insensitive,

Exists some ways increasing the sensitivity of reactions , For example:

1) concentration (increase in sample signal):

2) increasing the purity of reagents (reducing the background signal).

Reaction sensitivity is reduced the following factors:

1) heating. As a rule, it leads to an increase in solubility, and, consequently, to a decrease in the magnitude of the analytical signal;

2) excess reagent. May lead to the formation of by-products, for example:

Hg 2+ + 2 I - ® HgI 2 ¯ (red precipitate);

HgI 2 + 2 I - ® 2- (colorless solution);

3) discrepancy between the acidity of the environment. May lead to lack of analytical response. Thus, the oxidation reactions of halides with potassium permanganate in acidic media significantly depend on the pH of the medium (Table 5.1);

4) interfering components. May lead to the formation of by-products.

Table 5.1

Optimal acidity of the medium during the oxidation of halides with potassium permanganate

Oxidation reaction	Optimal acidity of the environment
2 I - ® I 2 + 2 e
2 Br - ® Br 2 + 2 e
2 Cl - ® Cl 2 + 2 e

The first three factors that reduce the sensitivity of the reaction can be dealt with by careful execution of analytical procedures.

The influence of foreign (interfering) ions is suppressed by the use of complexing substances, oxidizing agents or reducing agents. These substances are called masking agents, and the procedure itself is called masking of interfering ions.

Thus, when detecting Co(II) using a reaction with potassium thiocyanate, the analytical signal is the appearance of a blue color of the solution due to the formation of tetrarodancoboltate(II) ion:

Co 2+ + 4 SCN - = 2- (blue solution).

If Fe(III) ions are present in the solution, the solution will acquire a blood-red color, since the stability constant of complex 3- is much greater than the stability constant of the cobalt(II) thiocyanate complex:

Fe 3+ + 6 SCN - = 3- (dark red solution).

Those. the iron(III) ions present are interfering with cobalt(II) ions. Thus, in order to determine Co(II), it is necessary to first (before adding the KSCN solution) mask Fe(III). For example, “bind” iron(III) ions into a complex that is more stable than 3-. Thus, complexes 3-, 3-, 3- are more stable with respect to 3-. Therefore, solutions of KF, K 2 HPO 4 or (NH 4) 2 C 2 O 4 can be used as masking agents.

The method is applicable in linear regions of the calibration curve.

2.1. Multiple addition method

Several (at least three) portions of volume Vst are introduced into the test solution, prepared as specified in the private pharmacopoeial monograph. solution with a known concentration of the ion being determined, observing the condition of constant ionic strength in the solution. Measure the potential before and after each addition and calculate the difference ∆E between the measured

potential and potential of the test solution. The resulting value is related to the concentration of the ion being determined by the equation:

where: V – volume of the test solution;

C is the molar concentration of the ion being determined in the test solution;

Build a graph depending on the volume of additive Vst. and extrapolate the resulting straight line until it intersects with the X axis. At the intersection point, the concentration of the test solution of the ion being determined is expressed by the equation:

2.2. Single addition method
To the volume V of the test solution, prepared as described in the private pharmacopoeial monograph, add the volume Vst. standard solution of known concentration Cst. Prepare a blank solution under the same conditions. Measure the potentials of the test solution and the blank solution before and after adding the standard solution. Calculate the concentration C of the analyte using the following equation and making the necessary corrections for the blank solution:

where: V is the volume of the test or blank solution;

C is the concentration of the ion being determined in the test solution;

Vst. – added volume of standard solution;

Cst. – concentration of the ion being determined in the standard solution;

∆E – potential difference measured before and after the addition;

S is the slope of the electrode function, determined experimentally at a constant temperature by measuring the potential difference of two standard solutions, the concentrations of which differ by a factor of 10 and correspond to the linear region of the calibration curve.

The standard additive method is based on the fact that an exact portion of the analyte present in the control mixture is added to a sample of the control mixture, and chromatograms of the original control mixture and the control mixture with the standard additive added to it are taken.

Method of analysis. About 2 cm 3 of the control mixture (800 mg) is pipetted into a pre-weighed flask with a ground stopper and weighed, and then one of the substances (100 mg) present in the control mixture is added (as directed by the teacher) and weighed again.

Next, chromatograms of the initial control mixture and the control mixture with a standard additive of the component being determined are taken. The area under the peak of the analyzed component is measured on chromatograms and the analysis result is calculated using the formula

, (1.6)

Where S X– area under the peak of the analyzed component in the sample;

S x+st– area under the peak of the analyzed component in the sample after introducing its standard additive into the sample WITH st ;

WITH(X) – concentration of the analyzed component in the sample;

WITH st– concentration of standard additive of the analyzed component, %:

Where m ext– mass of additive, g;

m samples – mass of the chromatographed sample, g.

Absolute calibration method (external standardization)

The absolute calibration method consists of constructing a calibration graph of the dependence of the chromatographic peak area ( S) on the content of the substance in the chromatographic sample ( m). A necessary condition is the accuracy and reproducibility of sample dosing, and strict adherence to the operating mode of the chromatograph. The method is used when it is necessary to determine the content of only individual components of the analyzed mixture and therefore it is necessary to ensure complete separation of only the peaks of the substances being determined from neighboring peaks in the chromatogram.

Several standard solutions of the component being determined are prepared, equal quantities are introduced into the chromatograph, and the peak areas are determined ( S 1 , S 2 , S 3). The results are presented graphically (Figure 1.3).

Figure 1.3 – Calibration graph

Concentration i th component in the sample (%) is calculated using the formula

Where m samples– mass of the chromatographed sample, g;

m i- content i th component, found from the calibration graph (see Figure 1.3), g.

1.2.3 Block diagram of a gas chromatograph

The block diagram of a gas chromatograph is shown in Figure 1.4.

Figure 1.4 – Block diagram of a gas chromatograph:

1 – cylinder with carrier gas; 2 – drying, cleaning system and unit for regulating and measuring the rate of supply of carrier gas; 3 – device for sample introduction (dispenser); 4 – evaporator; 5 – chromatographic column; 6 – detector; 7 – thermostatic zones ( T And– evaporator temperature, T To – column temperature, T d – detector temperature); 8 – chromatogram

A chromatographic column, usually steel, is filled with a solid carrier (silica gel, activated carbon, red brick, etc.) with an applied stationary phase (polyethylene glycol 4000 or another modification, vaseline, silicone oil).

The temperature of the evaporator thermostat is 150 °C, the column temperature is 120 °C, and the detector thermostat is 120 °C.

Carrier gas – inert gas (nitrogen, helium, etc.).