RU2290637C2 - Method of performing visual, drop-sized tablet rapid colorimetric analysis - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: method consists in rapid visual determination of substances from small volumes of test media providing easiness of analysis, readiness to be used over a long storage time, possibility of performing all required reaction stages in the same volume, in a little changing medium and in a short time.

EFFECT: simplified and accelerated analytical procedure.

Description

The invention relates to the field of analytical chemistry, in particular to “drip colorimetry” proposed by N. A. Tananaev [1]. It is intended for use in industrial and domestic conditions in order to control product quality or technology parameters; to determine the ecological state of soils, waters, plants and other environmental objects; for testing food products, medicines, etc.

The closest prototypes of the proposed method are drip colorimetry on special plates [1, 2], as well as tablets or powder mixtures, ready to use, that are powders containing all the necessary analytical preparations of this color reaction [3-6], ready to use.

The second prototype, tablet colorimetry, is currently used as a means of mass testing, but this is achieved at the cost of using significant volumes of the analyzed samples, i.e. the advantage of the first prototype, namely very small sample volumes, are not realized in it.

Analogues of the proposed method are plastic sticks (Stäbchen) by Merck [7] and reactive indicator papers (RIB) or plates (RIP) [8]. These two methods are not prototypes, since they use reflection spectra, and not transmission spectra, as in the prototypes of the proposed method.

The essence of the proposed method is to combine drop colorimetry on the plates using tablets or placers, which until now has not been realized primarily because of the need to develop special tablets or placers that can give the desired colorimetric result in very small volumes of the analyzed sample and without external intervention from the moment the sample begins to interact with the substances of the tablet (placer).

The principle of the proposed method is as follows. The tablets or dosages of the placer are stored in sealed cells in blister packs until use, like conventional medicinal tablets. Immediately before testing, the sealing foil is removed from the blister cell and a few drops of the test liquid are applied to a tablet or placer in the blister cell. Drops have a volume of ≈0.05 ml, their number (1, 2, 3 ...) is determined by a specific technique, tablet weight (placer), cell volume. From the moment the sample drops are applied, a color reaction begins. After a few minutes, the intensity and hue of the color of the sample in the blister cell is stabilized, after which the result is visually compared with a color scale obtained using solutions of the analyte of various concentrations. A direct comparison is also possible with stains formed in other blister cells by solutions with known concentrations. In this case, the blister pack is used as a comparator.

Special tablets or placers for use in the proposed method are developed on the basis of three groups of requirements. The first group is the usual requirements for colorimetric color reactions (the equilibrium of the reaction should be shifted towards the formation of colored products, the specificity and selectivity of the color reaction is ensured, the possibility of eliminating interfering reactions, ease of carrying out, etc.). The second group consists of requirements aimed at the ability of tablets and placers to remain ready for use over long periods of storage.

The third group - special requirements of the proposed method. The most important here are:

the ability to carry out all the necessary stages of the reaction in one volume, in a slightly changing environment and in a short time;

the high intensity and differentiability of the color of the product with a very small thickness of the observed liquid layer (2-4 mm) suggests large values of light absorption and fairly high concentrations of reagents and products;

in this method there is no possibility of mixing a small volume by external means and this requires special techniques;

compatibility of reagents and reaction products with high concentrations of matrix substances of tablets and placers.

Example. Determination of nitrate ion in aqueous solution, cucumber and tomato juice

To determine the content of nitrate ions, two drops (volume 0.10 ± 0.01 ml) of an aqueous solution, cucumber or tomato juice are applied to a tablet weighing 50 mg in an open cell of a blister pack using an eye pipette with a drawn nose and wait 5-7 minutes during which coloring develops. The color varies in intensity and hue, and with increasing concentrations of nitrate ion changes from yellow with a greenish tint to dark cherry. The color in the cell is compared with a scale obtained using solutions with known concentrations.

The method and procedure for its implementation were investigated with the involvement of students of the Moscow Agricultural Academy. K.A. Timiryazev, who received a short oral briefing. The results are shown in table 1. When processing the results, the so-called emissions were not excluded. The distribution of errors is well consistent with the law of normal distribution.

Table 1 Test object The range of concentrations, mg NO ₃ ^- / ml Number of measurements The number of performers Standard relative error,% aqueous solutions of KNO ₃ 37-2000 939 96 10.6 cucumber juice 44-2000 650 47 11.3 tomato juice 44-2000 465 66 11.2

Thus, “naive”, that is, without experience, the performer with a confidence probability of 0.955 receives results with deviations not exceeding 23%.

Tablets and portions of placers have a mass of at least 50 mg, which is determined by the ease of use and the requirements of the Pharmacopoeia, since the standards and techniques of this industry meet the capabilities of industrial production of tablets (placers) for this method of rapid analysis.

Comparison with prototypes and analogues

The performance characteristics of prototypes, analogues, and the proposed method itself vary significantly depending on the substance being determined. Therefore, the comparison is best done using a specific example illustrating the method in a representative, reliable and with some "margin of safety". The example given earlier meets these criteria, since the color reaction used here is complex in itself and in terms of the conditions. The characteristics of the compared methods are collected in table 2. However, in the case of droplet colorimetry on the plate, only the method of opening the nitrate ion is known ([2]. T.1, p.147) and therefore we characterize droplet colorimetry by typical indicators [1, 2]. A comparison of the data in Table 2 leads to the following conclusions.

table 2
Comparative characteristics of methods Way drip colorimetry [1, 2] Colorimetry with tablets and placers [3-6, 9, 10] Drip tablet colorimetry Merck Test Sticks [7] Reactive Indicator Strips, RIP [8] The reason for considering the method prototype prototype the proposed method analog analog General characteristics: the nature of the spectrum used passing passing passing reflection reflection device in which the observed color is localized a drop colorimetric cell blister cell multilayer test stick RIP with three layers of reagents tools solutions, mini-droppers, mini-capillaries, etc. colorimeter, cuvettes, tablets (placers), simple dishes blister pack with tablets, dropper, scale mini container with chopsticks mini container with rip, pipette, scale fulfillment conditions laboratory covered room “Field conditions” “Field conditions” “Field conditions” compactness and transportability low low high high high contractor qualifications experienced professional inexperienced performer inexperienced performer inexperienced performer inexperienced performer Metrological characteristics interval or gradation of measured concentrations, their upper limit, mg / l 10-1000 1000 (upper limit) 20-2000 0-10-30-60-100-250-500 0-10-20-50-100-200-500 relative error of visual colorimetry,% of the measured concentration ± 10-20 ± 5-10 ± 20 ± 40-70 (see top line) ± 40-50 (see top line) Test Procedure Characteristics the volume of the analyzed sample. ml ~ 0.05 5-10 0.05-0.20 ~ 0.10 0.05-0.10 number of operations (without preparatory) 4-10 4-9 2 2 2 lead time, min 15-60 15-30 5-7 2 8

The proposed method differs from its prototypes and analogues in a specific combination of the used tools and techniques. Specific means for it are tablets or placers of a special composition that ensures their functional suitability, as well as a blister cell, which serves as a sealed container when storing tablets and placers and turns into a container cell in which a color reaction is carried out and observed. A specific technique is that one or more drops of the analyzed solution is applied to a tablet or placer, the mass and volume of which are comparable with the mass and volume of the applied sample and the capacity of the blister cell.

The proposed method is superior to its direct prototypes in a number of features of the group "General characteristics". It is almost toolless, efficient in the field, compact and portable, does not require a qualified performer. It surpasses its prototypes in the simplicity of the execution procedure and expressness.

The proposed method differs from analogue methods not only in the nature of the phenomenon used (transmission spectrum), but also surpasses them in accuracy and structural simplicity of the device in which the observed color of the color reaction is localized.

The last statement needs clarification. Merck test sticks have a complicated design. They have small test zones, for the area of which reagents for various purposes are impregnated into several layers. In nitrate test sticks, this is a reducing agent, a buffer mixture, color reaction reagents, ensuring its three-stage course in the mode of diffusion mixing of active substances; the layers of impregnated powdery reagents are quite thin.

A similar three-stage color reaction for testing the nitrate ion in the RIP is provided by three layers of reagents deposited on two paper layers, between which a layer of reducing agent is placed. There are two windows in the outer coating of the RIP. The top is used for applying 1-2 drops of the analyzed solution, the bottom - when observing the result. When testing other substances in this way, simpler, single-layer reactive indicator papers (RIB) are used [8]. Tests using RIBs are performed by pumping the analyzed solution through a layer of indicator paper in a certain volume and at a certain speed, for which a complex mechanical device is used. As a result, the qualification requirements, the instrumental level of the test, the required volumes of the solution, and the execution time increase [8].

The complexity of devices for carrying out color reactions and localizing their visual effects in methods [7] and [8] is a consequence of the fact that all stages must be carried out in one volume and without any intervention in the course of these stages and the reaction as a whole. In the methods of [7, 8], the solution is achieved by diffusion, since all processes are carried out in very thin layers of solutions. In the proposed method, rapid interaction in a compact reaction volume by diffusion cannot be ensured. Dissolution of the tablet or placer in a sample volume of comparable mass, mixing, homogenization and creating a uniform color of the entire observed volume should not be achieved through diffusion, but also without external interference.

The requirements for the composition of tablets and placers, by virtue of the above, are different from the requirements for the composition of tablets and placers of tablet colorimetry, where the usual methods of mixing by the artist are used.

The tablet and the placer in the proposed method for these reasons is a product of special development and therefore may be subject to dependent patenting “for composition”.

Information sources

1. Tananaev N.A. Drip method. M.-L., 1954.

2. Feigl F., Anger V. Drip analysis of inorganic substances, trans. from English M., 1976, T. 1, 2.

3. Palintest Water Tests. Palintest Web Site. http./www.palintest. com / dpd_test. htm.

4. LaMotte Company. LaMotte Company website http./www.lamotte/test/updates.htm.

5. Aquarium Pharmaceuticals, Inc. Water Test Kits. http./www.aqualinc.com/direct/testkit.html.

6. http./www.aquaticlife.ca/pocket.htm.

7. Rapid Test Handbook. Merck E. 1987. p. 191.

8. Ostrovskaya V.M. Reactive indicator means (RIS) for multiple testing of water, M., 1992.

9. Sea Test, Aquarium Systems Inc., 8141. Tyler Baulevard Mentor OH 44060.

10. Soil Tester, Lister Leaf Products Inc., 2220 Techcourt, Woodstock IL 60098.

Claims (1)

The method of visual-drop-tablet colorimetric express analysis, characterized in that the analyzed liquid sample in the amount of 1 or several drops (approximately 0.05-0.20 ml) is applied to a compact form - a portion of a mixture of all functionally necessary reagents and auxiliary substances in in the form of a tablet or a portion of the placer stored in a sealed container — a blister cell, the mass and volume of the tablet or portion of the placer being comparable with the mass and volume of the applied sample and the capacity of the blister cell, which after application It serves to implement and observe the result of a color reaction; after 5-7 minutes, the result is visually compared with a color scale.