Experimental determination of chromium in soil by colorimetric method of dibenzoyl dihydrazide

Summary

For food raw materials, soil pollution is an important factor affecting the yield, quality and even food safety of raw materials, especially the heavy metal content in food. The main purpose of this experiment is to let students understand the regional soil pollution situation and the correlation between water pollution and soil pollution, and master the basic determination steps and methods of heavy metal chromium, to further understand the heavy metal pollution pathway and the hazards to food safety.

Operation method

Experimental determination of chromium in soil by colorimetric method of dibenzoyl dihydrazide

Principle

Soil samples are digested with sulfuric and phosphoric acid, and the chromium compounds become soluble. After separation by centrifugation or filtration, trivalent chromium is oxidized to hexavalent chromium by the addition of a slight excess of potassium permanganate, and excess potassium permanganate is removed by decomposition with sodium azide. The chromium reacts with dibenzoyl dihydrazide under acidic conditions to form a purplish-red compound, the absorbance of which is measured at a wavelength of 540 nm.

Materials and Instruments

Concentrated sulfuric acid Concentrated phosphoric acid Concentrated nitric acid Potassium permanganate solution Sodium azide solution Dibenzoyl dihydrazide acetone solution Phosphoric acid solution Sulfuric acid-phosphoric acid mixture Chromium standard storage solution Chromium standard use solution
Hot plate Electric furnace Centrifuge Water bath Spectrophotometer Triangular flask Beaker Volumetric flask Colorimetric tube

Move

I. Experimental apparatus and reagents

(i) Main apparatus used:

1, electric hot plate or electric furnace; 2, centrifuge

3, Water bath; 4, Spectrophotometer;

5, 50 ml triangular flask; 6, 50 ml beaker;

7, 100 ml volumetric flask; 8, 25 ml colorimeter.

(ii) Reagents used:

1, Concentrated sulfuric acid, concentrated phosphoric acid and concentrated nitric acid;

2. 0.5% potassium permanganate solution;

3, 0.5% sodium azide solution;

4, 0.25% diphenylcarbonyldihydrazide acetone solution; weigh 0.25 grams of diphenylthymine into steel dihydrazide, dissolved in acetone, and diluted to 100 ml.

5, 1: 1 phosphoric acid solution: heating to boiling, and dropwise addition of dilute potassium permanganate until slightly red.

6, 5% sulfuric acid - phosphoric acid mixture: take sulfuric acid, phosphoric acid 5 ml each slowly poured into water, diluted to 100 ml, heated to boiling, and add dilute potassium permanganate solution to a slight red.

7, chromium standard storage solution: accurately weigh 0.2829 grams of potassium chromate (excellent pure, baked at 105-110 ℃ for 2 hours), dissolved in water, transferred to 1 liter volumetric flasks and diluted to the standard line, this solution contains 100 micrograms of chromium per milliliter.

8, chromium standard use solution: accurately absorb the above storage solution 10.00 ml in a 1 liter volumetric flask, with the second distilled water diluted to the standard line, this solution contains 1.0 micrograms of chromium per milliliter.

Experimental steps:

1、Standard curve drawing

(1) Accurately aspirate chromium standard solution 0.00, 1.00, 2.00, 4.00, 6.00, 8.00, 10.00 ml in a colorimetric tube. Add 2.5 ml of 5% sulphuric acid-phosphoric acid mixture. Dilute to the mark with water. Prepare standards containing 0.00, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.00 micrograms of chromium.

(2) Add 1 ml of 1:1 phosphoric acid, shake well, add 1 ml of acetone diphenyl dihydrazine carbonate, shake well quickly, and measure the absorbance value at 540 nm with a 3 cm cuvette after 10 minutes.

(3) Plot the standard curve of chromium content by absorbance.

2、Sample analysis:

(1) Pretreatment

Accurately weigh 0.500 grams of soil samples in 50 ml triangular flask, add a little water to moisten, and then add concentrated phosphoric acid, concentrated sulfuric acid 1.5 ml each, milliliters on the small funnel, placed on the furnace heating to white smoke, removed slightly cooled, repeated drops of concentrated nitric acid 2-3, and then placed on the furnace heating to the emergence of a large number of white smoke until the soil samples become white, the abatement solution is a light green until.

Remove the triangular flask and cool, rinse the small funnel and the wall of the flask with distilled water, weigh the digestion solution (about 50 ml) together with the residue into a 50 ml centrifuge tube and centrifuge. Transfer the supernatant into a 100 ml volumetric flask, rinse the walls of the tube with distilled water and stir the residue in the lower part of the tube with a glass rod, then centrifuge and combine the supernatant into a 100 ml volumetric flask and dilute to the mark.

(2) Determination

Accurately suck 10.00 ml (the amount of sample solution can be changed depending on the content of different) after centrifugation of the clear solution in a 50 ml beaker, add 0.5% potassium permanganate drops to the purple-red color, placed in a water bath and boiled for 15 minutes, (if the purple-red color fades in the process of boiling, you should add potassium permanganate to the purple-red color is not faded) Zhao hot drops of sodium azide, and shaking, to the red color is just faded. Cool quickly in cold water. Then transfer to a 25 mg cuvette and dilute to the mark with cold distilled water.

Add 1 ml of 1:1 phosphoric acid to the above cuvette, shake well, then add 1 ml of colorant, shake well quickly, and then measure the color with a 3 cm cuvette at a wavelength of 540 nm after 10 minutes.

(3) Calculation

Caveat

1, soil samples party with the pretreatment method of alkali dissolution method, hydrofluoric acid dissolution method, sulfuric acid - nitric acid digestion method, aqua regia digestion method, perchloric acid digestion method, sulfuric acid - phosphoric acid digestion method and so on. With this method of soil digestion, the time is not too long, the temperature is not too high, not steam dry, in order to prevent pyrophosphate production, affecting the results, and residue clumps bonded in the glass is not easy to wash down, easy to damage the triangular bottle.

2， this method with phosphoric acid masking iron, so that the formation of colorless chromium compounds, but also with other metal ions and chromium, to avoid the precipitation of some salts and turbidity. In the presence of phosphoric acid can be excluded NO3-, Cl- effect. If turbidity occurs during oxidation or color developmentIf there is turbidity in oxidation or color development, consider increasing the dosage of phosphoric acid.

3、Usable to replace sodium azide with sodium nitrite of 10% urine quality, attention should be paid to prevent sodium nitrite from reducing hexavalent chromium. Until the solution should be pre-added urea, so that the sodium nitrite reduction of potassium permanganate, that is, with urea reaction. The amount of sodium nitrite must be controlled, and must be fully shaken after joining.

4、After adding diphenylprecipitate into steel dihydrazide acetone solution, it should be shaken immediately to prevent the partial organic solvent from excess and make the hexavalent chromium partially reduced to trivalent, so that the determination result is low.

5、Use potassium permanganate to oxidize low-valent chromium, in the process of oxidation, heptavalent manganese may be reduced to brown manganese dioxide, which interferes with the observation of violet-red color and affects the oxidation of low-valent chromium completely. Therefore, the acidity of the solution and the dosage of potassium permanganate should be controlled.

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