CN106525740A - Method for determination of hexavalent chromium in basic chromium sulfate - Google Patents

Abstract

The invention relates to a method for determining hexavalent chromium in basic chromium sulfate, belonging to the technical field of chemical analysis. The method of the present invention comprises the following steps: after the sample to be tested is dissolved, MgCl ₂ and FeCl ₃ are added immediately, and the pH is adjusted to about 10, so that the chromium hydroxide is completely precipitated, filtered, and the precipitate is washed, so that the hexavalent chromium and trivalent chromium are completely separate. Then, the hexavalent chromium in the filtrate is detected by diphenylcarbazide spectrophotometry to obtain the content of hexavalent chromium. The present invention solves the following problems, one is the elimination of interfering elements, and completely separates trivalent chromium and hexavalent chromium; the other is to avoid the oxidation of trivalent chromium to hexavalent chromium in the separation process, resulting in high hexavalent chromium content; the third is High accuracy and good precision.

Description

Determination method of hexavalent chromium in basic chromium sulfate

technical field

The invention relates to a method for determining hexavalent chromium in basic chromium sulfate, belonging to the technical field of chemical analysis.

Background technique

Basic chromium sulfate is an important inorganic chemical product, widely used in printing and dyeing, ceramics, green ink and tanning process. Vanadium-making enterprises use vanadium-chromium waste residue recovered from vanadium-containing wastewater as raw materials to prepare basic chromium sulfate, which not only improves enterprise efficiency, but also is an effective way to solve chromium-containing waste. The valence state of chromium in basic chromium sulfate is mainly trivalent chromium and trace amount of hexavalent chromium. The industry standard HG/T2678-2015 "Industrial Basic Chromium Sulfate" has strict regulations on the content of hexavalent chromium in basic chromium sulfate. The determination methods of hexavalent chromium mainly include spectrophotometry, fluorescence quenching method, oscillographic polarography, graphite furnace atomic absorption method, inductively coupled plasma spectroscopy, mass spectrometry and ion chromatography. The methods are all based on the form of chromium in the sample. For a single hexavalent chromium.

The currently published literature includes "Determination of Trace Cr (6-valent) in Basic Chromium Sulfate" and HG/T2678-2015 "Industrial Basic Chromium Sulfate", the former uses tributyl phosphate to extract and separate hexavalent chromium in basic chromium sulfate And trivalent chromium, the latter uses isoamyl alcohol to separate hexavalent chromium and trivalent chromium, which are all directly colored after extraction with organic solvents, and analyzed by spectrophotometry. Because organic reagents are highly volatile and extremely toxic, they are easily absorbed by the human body during extraction and endanger health, and waste liquid treatment is difficult, and direct discharge is extremely harmful to the environment.

The author believes that the determination of hexavalent chromium in basic chromium sulfate needs to solve the following problems. First, the elimination of interfering elements. Basic chromium sulfate contains a large amount of trivalent chromium. The solution is dark green and cannot be directly determined by color development. The second is to completely separate trivalent chromium; the third is to avoid the oxidation of trivalent chromium to hexavalent chromium during the separation process, resulting in high hexavalent chromium content, and the fourth is trivalent chromium and hexavalent chromium The reagents used in the separation process are green and friendly to the environment and people.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for determining hexavalent chromium in basic chromium sulfate.

The assay method of hexavalent chromium in basic chromium sulfate comprises the following steps:

A. Weigh the sample to be tested, add water to dissolve, and obtain the dissolved test solution; the water is preferably hot water at 70-80°C;

B. Immediately add solid MgCl ₂ and solid FeCl ₃ to the dissolved test solution, mix well, adjust the pH value of the test solution to 9-10, filter, wash the precipitate, and obtain the filtrate; wherein, the added MgCl ₂ and the sample to be tested The mass ratio is 1:4～1:1, and the mass ratio of FeCl ₃ to the sample to be tested is 1:10～1:2;

C, test the content of hexavalent chromium in the obtained filtrate.

Preferably, in step A, the mass of the sample to be tested is ≥1 g, preferably 1-2 g.

Preferably, in step B, the pH value is adjusted to 10.

Wherein, in step B, a 0.05mol/L-0.2mol/L NaOH solution is used to adjust the pH value of the test solution, preferably a 0.1mol/L NaOH solution.

Preferably, in step B, the precipitate is washed with hot water at a temperature of 70-80°C for 8-10 times.

Preferably, in step B, after adjusting the pH value, keep the temperature at 70-80° C. for 40-60 minutes, and filter while it is hot.

Preferably, in step C, the method for testing the content of hexavalent chromium in the obtained filtrate is spectrophotometry.

Among them, the spectrophotometric method uses diphenylcarbazide as a chromogen.

Among them, the specific steps of spectrophotometry are: take the obtained filtrate and dilute it with water in a volumetric flask, add 0.1mol/L～0.2mol/L sulfuric acid to adjust the pH to 6～7, add 4.0mL of 2.57mol/L sulfuric acid, add di Phenylcarbazide chromogen, add water to make up volume, place for 5-10min, use spectrophotometer at wavelength 540nm, measure absorbance; by making hexavalent chromium standard working curve, abscissa is Cr ⁶⁺ concentration, ordinate Is the absorbance value, according to the absorbance value to determine the Cr concentration to be measured; wherein the concentration of diphenylcarbazide is 0.25%; the amount of diphenylcarbazide added is ^3.0mL .

Beneficial effects of the present invention:

The mensuration of hexavalent chromium in basic chromium sulfate of the present invention has solved following difficult problem, 1, eliminated interfering element, separated trivalent chromium and hexavalent chromium; Resulting in high hexavalent chromium content.

The method for determining hexavalent chromium in basic chromium sulfate of the present invention has high accuracy, good precision, is environmentally friendly to the environment and people, and has wide popularization and application prospects.

detailed description

The technical problem to be solved by the present invention is to provide a method for determining hexavalent chromium in basic chromium sulfate.

The assay method of hexavalent chromium in basic chromium sulfate comprises the following steps:

A. Weigh the sample to be tested, add water to dissolve, and obtain the dissolved test solution; because basic chromium sulfate has the highest solubility in hot water at 70-80°C, the water is preferably hot water at 70-80°C.

B. Immediately add solid MgCl ₂ and solid FeCl ₃ to the dissolved test solution, mix well, adjust the pH value of the test solution to 9-10, filter, wash the precipitate, and obtain the filtrate; wherein, the added MgCl ₂ and the sample to be tested The mass ratio is 1:4 to 1:1, and the mass ratio of FeCl ₃ to the sample to be tested is 1:10 to 1:2. Among them, adding solid _MgCl immediately is in order to suppress the _trivalent chromium in the solution from being oxidized to hexavalent chromium, and adding solid FeCl is in order to form co-precipitation with chromium hydroxide, avoiding the precipitation of chromium hydroxide to filter through during filtration; adjust the pH value of the test solution The pH value is 9-10, which not only ensures the complete precipitation of chromium hydroxide at this pH, but also prevents trivalent chromium from being oxidized under this alkaline condition.

C, test the content of hexavalent chromium in the obtained filtrate.

Preferably, in order to ensure that the detection limit of the method meets the product quality standard, in step A, the mass of the sample to be tested is ≥ 1 g.

Preferably, in step B, the pH value is adjusted to 10.

Wherein, in step B, in order to ensure the complete precipitation of chromium hydroxide at this pH and avoid the oxidation of trivalent chromium under this alkaline condition, use 0.05mol/L～0.2mol/L NaOH solution to adjust the pH value of the test solution, preferably 0.1mol /L NaOH solution.

Preferably, in step B, the precipitate is washed with hot water at a temperature of 70-80°C for 8-10 times.

Preferably, in order to ensure the complete precipitation of chromium hydroxide, in step B, after adjusting the pH value, keep the temperature at 70-80° C. for 40-60 minutes, and filter while it is hot.

Preferably, in step C, the method for testing the content of hexavalent chromium in the obtained filtrate is spectrophotometry. Compared with the volumetric method, the spectrophotometric method has the characteristics of high sensitivity, simple and fast analysis procedures, and is often used for the determination of trace components in substances. The content of hexavalent chromium in basic chromium sulfate is low, and the spectrophotometric method is used to meet the analysis requirements.

Among them, the spectrophotometric method uses diphenylcarbazide as a chromogen.

Among them, the specific steps of spectrophotometry are: take the obtained filtrate and dilute it with water in a volumetric flask, add 0.1mol/L～0.2mol/L sulfuric acid to adjust the pH to 6～7, add 4.0mL of 2.57mol/L sulfuric acid, add di Phenylcarbazide chromogen, add water to make up volume, place for 5-10min, use spectrophotometer at wavelength 540nm, measure absorbance; by making hexavalent chromium standard working curve, abscissa is Cr ⁶⁺ concentration, ordinate is the absorbance value, and the concentration of Cr ⁶⁺ to be tested is determined according to the absorbance value; wherein the concentration of diphenylcarbazide is 0.25%; the amount of diphenylcarbazide added is: 3.0 mL.

The present invention specifically can adopt following method to implement:

1. Separation of trivalent chromium and hexavalent chromium and determination of hexavalent chromium in the liquid to be tested (color development and determination are in accordance with 4.3.3-4.3.4 in GB/T223.12-1991):

Weigh 1.0-2.0g of the sample to be tested in a 250mL beaker, add hot water at 70-80°C to dissolve the sample. Immediately after the sample is dissolved, add 0.5-1.0g of solid magnesium chloride and 0.2-0.5g of ferric trichloride to the test solution, stir evenly, adjust the pH value of the test solution to 10 with 0.1mol/L sodium hydroxide solution, and place it on a low-temperature heating plate Keep warm at 70-80°C for 40-60 minutes, filter while hot, wash the precipitate with hot water for 8-10 times, transfer the filtrate into a volumetric flask, dilute with water to the mark, and mix well. Dispense an appropriate amount of filtrate into a 100mL volumetric flask, adjust the pH to 6~7 with 0.1mol/L~0.2mol/L sulfuric acid, add 4.0mL of 2.57mol/L sulfuric acid, add 0.25% diphenylcarbazide for color development Solution 3.0mL, constant volume with water for color development, place for 5min to 10min, measure the absorbance at the wavelength of 540nm in the spectrophotometer;

2. Preparation of Cr ⁶⁺ standard solution (prepared according to 3.10.2 in GB/T223.12-1991, the concentration of Cr ⁶⁺ standard solution is 2.0ug/mL):

Weigh 0.2829g of potassium dichromate (baseline) which has been dried at 150°C to constant weight, dissolve it in water, transfer it into a 1000mL volumetric flask, dilute to the mark with water, and mix well. 1mL of this solution contains 100μg of hexavalent chromium. Put 20.00mL of the above chromium solution in a 1000mL volumetric flask, dilute to the mark with water, and mix well. 1mL of this solution contains 2μg of hexavalent chromium to prepare hexavalent chromium standard solution.

3. Drawing of working curve (performed in 4.4 of GB/T 223.12-1991):

Pipette 0, 1.00, 2.00, 4.00, 7.00, 10.00mL Cr ⁶⁺ standard solutions into six 100mL volumetric flasks, add 4.0mL sulfuric acid (1+6), dilute to about 90mL with water, add 3mL diphenylamide Dihydrazine solution, mix well, dilute to the mark with water, mix well, transfer part of the solution into a 2cm absorption dish, take the reagent blank as a reference, measure its absorbance on a spectrophotometer at a wavelength of 540nm, take the absorbance as the ordinate, The corresponding chromium amount is the abscissa, and the working curve is drawn.

4. Calculation of analysis results (conducted in 5 of GB/T 223.12-1991):

Cr ⁶⁺ (%)=(m ₁ ·V)/(m·V ₁ )×100

In the formula: V ₁ ——the volume of sample to be taken, mL;

V - the total volume of the test solution, mL;

m ₁ ——the amount of chromium obtained from the working curve, g;

m——weighing sample weight, g.

In the method for measuring hexavalent chromium content of the present invention, the suitable sample size of the sample is 1.0g to 2.0g; magnesium chloride is solid, analytically pure and above, and the addition is 0.5g to 1.0g; ferric chloride is Solid, analytical pure and above, the dosage is 0.2g~0.5g; sodium hydroxide is analytical pure and above, the concentration is 0.1mol/L. The quality of water used in the analysis complies with GB/T6682-1992 grade 3 distilled water and above; sulfuric acid is analytically pure and above, with a concentration of 2.57mol/L; the preparation method of the chromogenic agent diphenylamide dihydrazine solution (0.25%) To weigh 0.25g of diphenylamide dihydrazine, dissolve it in 94mL of absolute ethanol and 6mL of glacial acetic acid (ρ1.05g/mL), and store it in a brown bottle.

The constant volume and aliquoted volume of the test solution of the present invention can be determined according to the sample size and detection limit requirements of the sample.

The specific implementation of the present invention will be further described below in conjunction with the examples, and the present invention is not limited to the scope of the examples.

The mensuration of hexavalent chromium content in the basic chromium sulfate of embodiment 1

1. Separation of hexavalent chromium: Weigh 5 samples of basic chromium sulfate to be tested, put 1.0000g of each sample in a 250mL beaker, add hot water at 75°C to dissolve the sample. Immediately after the sample was dissolved, 1.0 g of solid magnesium chloride and 0.3 g of ferric chloride were added to the test solution, stirred evenly, and the pH value of the test solution was adjusted to 10 with 0.1 mol/L sodium hydroxide solution. Incubate on a low-temperature heating plate at 70°C to 80°C for 40 minutes, filter while hot with medium-speed filter paper, wash the precipitate with hot water 8 times, transfer the filtrate into a 100.0mL volumetric flask, dilute to the mark with water, and mix well.

2. Color development: Separate 50.0mL of the filtrate into a 100mL volumetric flask, adjust the pH to 6~7 with 0.1mol/L~0.2mol/L sulfuric acid, add 4.0mL of 2.57mol/L sulfuric acid, and dilute with water to about 90mL, add 3.0mL diphenylamide dihydrazine solution, mix well, dilute with water to the mark, mix well.

3. Measure its absorbance: move part of the solution into a 2cm absorption dish, use water as a reference, measure its absorbance on a spectrophotometer at a wavelength of 540nm, subtract the absorbance of the blank solution accompanying the sample, and find out the obvious from the working curve The corresponding amount of chromium in the color solution.

4. To draw the working curve, pipette 0, 1.00, 2.00, 4.00, 7.00, 10.00mL chromium standard solution into six 100mL volumetric flasks, add 2.57mol/L sulfuric acid 4.0mL, dilute with water to about 90mL, add 3.0mL diphenylamide dihydrazine solution, mix well, dilute to the mark with water, mix well, transfer part of the solution into a 2cm absorption dish, take the reagent blank as a reference, measure its absorbance on a spectrophotometer at a wavelength of 540nm, and The absorbance is the ordinate, the corresponding chromium amount is the abscissa, and the working curve is drawn.

5. The analysis results are shown in Table 1.

Table 1 Determination results of hexavalent chromium content

The determination recovery rate of hexavalent chromium content in embodiment 2 basic chromium sulfate chromium

Take by weighing 4 parts of the basic chromium sulfate sample in Example 1, each part of 1.0000g is placed in a 250mL beaker, and 0.0, 2.0, 4.0, ^6.0mL of Cr standard solution is added respectively (according to 3.10.2 in GB/T 223.12-1991 Preparation, the concentration of Cr ⁶⁺ standard solution is 2.0ug/mL) plus 75°C hot water to dissolve the sample. Immediately after the sample is dissolved, add 1.0 g of solid magnesium chloride and 0.3 g of ferric chloride to the test solution, stir evenly, and adjust the pH of the test solution to about 1.0 with 0.1 mol/L sodium hydroxide solution. Incubate on a low-temperature heating plate at 80°C for 40 minutes, filter while hot with medium-speed filter paper, wash the precipitate with hot water 8 times, transfer the filtrate into a 100mL volumetric flask, dilute to the mark with water, and mix well. Separate 50.0mL of the filtrate into a 100mL volumetric flask, adjust the pH to 6~7 with 0.1mol/L~0.2mol/L sulfuric acid, add 4.0mL of 2.57mol/L sulfuric acid, dilute to about 90mL with water, add 3mL di Benzamide dihydrazine solution, mix well, dilute to the mark with water, mix well, transfer part of the solution into a 2cm absorption dish, take water as a reference, measure its absorbance on a spectrophotometer at a wavelength of 540nm, subtract the accompanying sample After the absorbance of the blank solution was measured, the corresponding amount of chromium in the chromogenic solution was found from the working curve, and the results are shown in Table 2.

Table 2 Cr ⁶⁺ amount recovery experiment

project 1 2 3 4 Added Cr ⁶⁺ amount, ug 0.0 4.0 8.0 12.0 Measured Cr ⁶⁺ amount, ug 10.8 14.63 18.85 22.68 Recovered Cr ⁶⁺ amount, ug / 3.83 8.05 11.88 Recovery rate,% / 95.75 100.62 99.00

It can be seen from the above Examples 1-2 that the determination of hexavalent chromium in basic chromium sulfate according to the present invention has high accuracy and good precision, and has a wide application prospect.

Although this invention has specifically described the method for determining the content of hexavalent chromium according to the present invention, without departing from the teaching of the present invention, various modifications, substitutions or changes can be made to the above embodiments.

Claims (9)

1. the assay method of hexavalent chromium in basic chromium sulfate, is characterized in that, may further comprise the steps: A. Weigh the sample to be tested, add water to dissolve, and obtain the dissolved test solution; the water is preferably hot water at 70-80°C; B. Immediately add solid MgCl ₂ and solid FeCl ₃ to the dissolved test solution, mix well, adjust the pH value of the test solution to 9-10, filter, wash the precipitate, and obtain the filtrate; wherein, the added MgCl ₂ and the mass of the sample to be tested The ratio is 1:4～1:1, and the mass ratio of FeCl ₃ to the sample to be tested is 1:10～1:2; C, test the content of hexavalent chromium in the obtained filtrate. 2. The assay method of hexavalent chromium in basic chromium sulfate according to claim 1, characterized in that, in step A, the quality of the sample to be tested is ≥ 1g, preferably 1-2g. 3. The assay method of hexavalent chromium in basic chromium sulfate according to claims 1 to 2, characterized in that, in step B, the pH value is adjusted to be 10. 4. according to the assay method of hexavalent chromium in basic chromium sulfate described in claim 1～3, it is characterized in that, in step B, use 0.05mol/L～0.2mol/L NaOH solution to adjust test solution pH value, preferably 0.1mol/L NaOH solution. 5. The assay method of hexavalent chromium in basic chromium sulfate according to claims 1 to 4, characterized in that in step B, the precipitate is washed with hot water at a temperature of 70 to 80°C for 8 to 10 times. 6. according to the assay method of hexavalent chromium in basic chromium sulfate described in claim 1～5, it is characterized in that, in step B, after adjusting pH value, be incubated at 70～80 ℃ of temperature 40～60min, while hot filter. 7. according to the assay method of hexavalent chromium in basic chromium sulfate described in claim 1～6, it is characterized in that, in step C, the method for testing the content of hexavalent chromium in the gained filtrate is spectrophotometry. 8. the assay method of hexavalent chromium in basic chromium sulfate according to claim 7 is characterized in that, spectrophotometry uses diphenylcarbazide as chromogen. 9. according to the assay method of hexavalent chromium in basic chromium sulfate described in claim 1～7, it is characterized in that, described spectrophotometry concrete steps are, get gained filtrate and add water dilution in volumetric flask, add 0.1mol/ L～0.2mol/L sulfuric acid to adjust the pH to 6～7, add 4.0mL of 2.57mol/L sulfuric acid, add diphenylcarbazide color reagent, add water to make up the volume, let stand for 5～10min, use a spectrophotometer Measure the absorbance at a wavelength of 540nm; by making a hexavalent chromium standard working curve, the abscissa is the ^Cr concentration, and the ordinate is the absorbance value, and determines the ^Cr concentration to be measured according to the absorbance value; wherein the diphenylcarbonyl The concentration of dihydrazine is 0.25%; the amount of diphenylcarbazide added is 3.0 mL.