Accelerated method for determining chemical oxygen demand with aqueous solutions containing organic compounds in the form of suspended particles

FIELD: analytical methods in industrial sanitation.

SUBSTANCE: method envisages bringing solution to be analyzed into contact with potassium bichromate, sulfuric acid, and mercuric sulfate, ageing resulting mixture and allowing it to cool to ambient temperature, adding ferroin indicator, titration of excess of potassium bichromate with 0.125 n. Moor salt solution, and calculating chemical oxygen demand value from amount of Moor salt consumed in the titration. Method is characterized by that initial solution is preliminarily homogenized until diameter of suspended particles therein become as large as 0.03 mm, after which solution is allowed to stay for 4 min.

EFFECT: reduced determination inaccuracy.

 

The invention relates to industrial hygiene and can be used sanitary-epidemiological stations (SES) and environmental laboratories in the analysis of wastewater from the food industry.

There is a method of determining chemical oxygen demand (COD) of aqueous solutions containing organic compounds, providing for mixing of the test solution with a solution of bichromate of potash, sulphuric acid, sulphate of mercury and catalyst - sulfate of silver, boiling the mixture for 2 hours, cooled, titration unspent in the reaction of potassium bichromate salt solution Mora and the calculation of COD on the amount of salt solution Mora spent for titration [Lurie UU Analytical chemistry of industrial wastewater. M.: Chemistry, 1984, p.74-77].

The disadvantages of this method is a great time and the need to use expensive catalyst - sulphate of silver.

As a prototype adopted expedited method for determining COD solutions containing organic compounds, providing reduction of the test solution in contact with 0.25 N. the solution of bichromate of potash, sulphuric acid and sulphate of mercury, keeping the mixture for 2 minutes, cooling the mixture to room temperature, adding indie is atora of ferraina, titration of excess of potassium bichromate of 0.25 N. salt solution Mora spent for titration [Leite C. Determination of organic contaminants in drinking, natural and waste waters. M.: Chemistry, 1975, p.64-65].

The disadvantage of the prototype is high uncertainty (low-precision) ± 75 mg O2/ l in the analysis of wastewater from the food industry.

The technical result of the invention is to reduce errors in the determination of the chemical oxygen demand aqueous solutions containing organic compounds in the form of suspended particles.

The technical result is achieved in that in the method of determining chemical oxygen aqueous solutions containing organic compounds in the form of suspended particles, providing reduction of the test solution in contact with bichromate of potash, sulphuric acid and sulphate of mercury, exposure, cooling the mixture to room temperature, adding indicator ferroin and titration of excess of potassium bichromate salt solution Mora and calculating the amount of chemical consumption of oxygen by the quantity of salt solution Mora spent for titration, according to the invention, the sample is analyzed liquid pre-homogenized until it reaches the diameter of the suspended particles is not more than 0.03 mm, the mixture was incubated for 4 of the minutes, and the concentration of the salt solution Mora 0.125 N.

The proposed method is as follows.

The analyzed fluid in the number of 100-150 ml is placed in a laboratory homogenizer.

The homogenizer must be a vessel with a multi-blade stirrer having a sharpened edge. The rotational speed of the agitator is not less than 14 000 rpm air Gap between the stirrer and the edges of the vessel is not more than 5 mm.

The investigated liquid is homogenized for at least 3 minutes. This ensures that the grinding suspended particles up to a diameter of 0.005-0.03 mm (size diameters are set by sedimentation in a centrifugal field). Further homogenization does not reduce the diameter of the particles.

In the conical flask from heat-resistant glass with a capacity of 250 ml was placed 1 ml of homogenized sample, 2.5 ml of 0.25 n solution of bichromate of potash and 0.25 g of sulphate of mercury. Then carefully with stirring into the mixture contribute 7.5 ml of concentrated sulfuric acid.

The mixture immediately samorazogreva (due to the exothermic reaction of sulfuric acid with water). The mixture was incubated for 4-5 minutes, and then cooled with water to room temperature. To make a mixture of 100 ml of distilled water and 3-4 drops of the indicator "ferroin". The mixture is then titrated 0,125 N. salt solution Mora to the color transition of the solution from pale yellow to brown.

The influence of the distinctive features on echnosti definition was studied in metrological certification of the proposed method.

Homogenization of the analyzed fluid reduces the measurement error for the following reason.

Wastewater enterprises of the food industry have a high chemical oxygen demand (COD) from 500 to 4000 mg O2/ l and the presence of a large number of suspended particles. These wastewaters are not the solution, and the suspension of a number of organic substances (proteins, fats, polysaccharides) in the solution of organic (proteins and polysaccharides) and inorganic substances.

The size of suspended particles reach 0.3 mm, such particles are difficult to oxidize, even after prolonged boiling with acid. Indeed, on the surface of particles of a dense layer (cortex), consisting of products of incomplete oxidation. This layer prevents further reaction that lowers the result of the analysis. Because the kinetics of destruction of this layer depends on many uncontrollable factors, the process of destruction can be considered purely random. Therefore, the underestimation of the results is random, and therefore, to consider precisely its value is impossible.

In other words, in the way the prototype is purely random error, to compensate for which any calculation is impossible. This error is a major component of the errors in the determination of the prototype.

Homogenization reduces the size of suspended shall ASTIC not less than an order of magnitude and, accordingly, in the same time reduces the probability of formation of a solid crust on the surface of suspended particles. This reduces the random component of the error of determination ±31 mg O2/ l to ±16 mg O2/ HP

The increase in time of incubation of the mixture with 2 minutes on the prototype to 4 minutes can reduce the random component of the error with ± 56 mg O2/ l to ±44 mg O2/ l due to more complete destruction of the suspended particles. A further increase in time-keeping does not reduce the error.

Use during the titration of a solution of salt Mora concentration 0,125 N. can improve the accuracy by increasing the accuracy of the titration. The random component of error determination decreases with ±40 mg O2/ l to ±29 mg O2/ HP

Thus, according to the rule of addition errors [Schenk R. Theory of engineering experiment. M.: Mir, 1972, p.76-79], the total random error in the determination decreases with ± 75,4 mg O2/ l prototype:

to ±55,1 mg O2/ l for the proposed method:

that is 1.4 times.

Example. A sample of the wastewater in the amount of 150 ml, selected in accordance with the applicable rules of the sampling of wastewater, placed in a laboratory homogen the congestion MPW - 302 manufactured by " Mechanika Precizijna" (Poland).

The sample is homogenized for 5 minutes. In the conical flask from heat-resistant glass with a capacity of 250 ml was placed 1 ml of homogenized sample, 2.5 ml of 0.25 n solution of bichromate of potash and 0.25 g of sulphate of mercury. Then make a mixture of 7.5 ml of concentrated sulfuric acid.

The mixture was incubated for 4 minutes, and then cooled with water to room temperature. To make a mixture of 100 ml of distilled water and 4 drops of indicator solution "ferroin". The mixture is then titrated 0,125 N. salt solution Mora to the color transition of the solution from pale yellow to brown.

The COD value is calculated by the formula

where V0- volume of sample, ml

V1- quantity of a solution of bichromate of potash, ml

V2- quantity of salt solution Mora spent for titration, ml

With the concentration of the salt solution Mora, g-EQ/ l

8 - chemical equivalent of oxygen

1000 is the conversion factor.

Accelerated method for determining the chemical oxygen demand aqueous solutions containing organic compounds in the form of suspended particles, providing reduction of the test solution in contact with bichromate of potash, sulphuric acid and sulphate of mercury, exposure, cooling the mixture to room temperature, adding indicator ferroin, titration of excess of potassium bichromate solution Sol is Mora and calculating the amount of chemical consumption of oxygen by the quantity of salt solution Mora, spent for titration, characterized in that the analyzed solution pre-homogenized until it reaches the diameter of the suspended particles is not more than 0.03 mm, the mixture was incubated for 4 min, and the concentration of the salt solution Mora 0.125 N.



 

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