Combined determination of Cr, P and Mn in high carbon ferrochrome

Production of fusion with sodium peroxide is generally used in carbon ferrochrome acidification specimen at high temperature, and then analyzed. Thus, when the respective elements in the high carbon ferrochrome are separately measured, a large amount of time is wasted in the melt acidification pretreatment of the sample. To this end, it is proposed to determine the content of chromium, phosphorus and manganese in high carbon ferrochrome by systematic analysis.
After alkali-melting acidification of the sample, the determination of the amount of chromium by the ammonium ferrous sulfate titration method and the determination of the amount of phosphorus by the molybdenum sulfate-reduced molybdenum blue method have been reported. This law has been improved on this basis:
(1) After hot water leaching, the heating and boiling decomposition H ₂ O ₂ process is omitted, and after direct neutralization with sulfuric acid, a small amount of trivalent which may be reduced by H ₂ O ₂ is oxidized with ammonium persulfate in the presence of the catalyst silver nitrate. Chromium is hexavalent.
(2) After caustic acidification, directly add perchloric acid and hydrochloric acid to chrome, and then develop color with ammonium molybdate barium sulfate, and measure the amount of phosphorus by molybdenum blue light method. The above two improvements make the determination procedure of chromium and phosphorus simple, fast, accurate, and the analysis period is greatly shortened. The remaining test solution can also be used to determine the amount of manganese by ammonium persulfate silver salt oxidative spectrophotometry.
First, the instrument and reagent 721 spectrophotometer.
Sodium peroxide: solid;
Sulfuric acid: 1+1;
Phosphoric acid: ρ about 1.70g / mL;
Manganese sulfate solution: 4%;
Silver nitrate solution: 1%;
Ammonium persulfate solution: 15%;
Sodium chloride solution: 5%;
N ^- benzoo-aminobenzoic acid indicator: 0.2%;
Ammonium ferrous sulfate standard solution: 0.08 mol / L;
Perchloric acid: ρ about 1.67g / mL;
Hydrochloric acid: ρ about 1.19g / mL;
Sodium bisulfite solution: 10%;
Ammonium molybdate solution: Weigh 20g of ammonium molybdate dissolved in 200mL of warm water, add 700mL of sulfuric acid (1+1) while stirring, then dilute to 1000mL with water, shake well;
Barium sulfate solution: Weigh 1.5g of barium sulfate dissolved in 200mL of water, dilute to 1000mL with water, shake;
Color developer solution: when used, take 25mL ammonium molybdate solution, 10mL barium sulfate solution and 65mL water, mix;
EDTA solution: 5%.
Second, the experimental part
(1) Preparation of test solution
0.5000 g of the sample was weighed into an iron crucible containing 5 g of sodium peroxide in advance, stirred and mixed, and then covered with 1 g of sodium peroxide, and the crucible was placed on a low-temperature electric furnace and baked to a burnt yellow color. Cover the lid, transfer it to a high temperature furnace and heat it to 700 ° C for 5 min. Remove it and cool it slightly. Place it in a 400 mL beaker containing 100 mL of hot water. Heat it to dissolve the melt in the crucible, rinse it with water and cover it. . Add sulfuric acid (1+1) to pH test paper until it is acidic, and excess 10mL sulfuric acid (1+1), heat and boil until the residue dissolves. If it is not completely dissolved, add sulfuric acid until the solution is clear (no residue on the bottom of the cup) If there is residue, the melt treatment is not complete. After cooling, transfer to a 250mL volumetric flask, dilute to the mark with water, and shake to a test solution.
(II) Determination of chromium by ammonium persulfate oxidation capacity method
Pipette 50 mL of the test solution into a 500 mL Erlenmeyer flask, add 12 mL of sulfuric acid (1+1), 8 mL of phosphoric acid, and dilute with water to a volume of about 200 mL of the test solution. Add 3 drops of manganese sulfate solution, 10mL silver nitrate solution, 15mL ammonium persulfate solution, and heat to boil until the chromium is completely oxidized to high-priced chromium (the test solution is magenta-colored magenta, indicating that the chromium has been oxidized to hexavalent). Continue to boil for 4-6 minutes to completely decompose the excess ammonium persulfate. Add 10mL of sodium chloride solution, heat to boil until the purple red disappears, if the residual purple does not disappear, add 3 ~ 5mL of sodium chloride solution, continue to boil until the silver chloride precipitates and sinks, the solution becomes clear. Remove a little cooling (avoid sudden cooling with running water, broken conical flask), cool to room temperature with running water, add 20mL sulfuric acid (1+1), titrate the solution with ammonium ferrous sulfate standard solution when yellowing green, add N ^- benzene 3 drops of the o-aminobenzoic acid indicator, continue to titrate until the solution rose disappears and turns to bright green as the end point. Calculate the chromium content as follows:
Cr%=T _Cr ×V
Wherein T _Cr - the standard solution of ferrous ammonium sulfate per milliliter corresponds to the percentage of chromium;
V - The number of milliliters of ammonium ferrous sulfate standard solution consumed when titrating the sample. [next]
(III) Determination of phosphorus by molybdenum blue light method
Pipette 50mL of test solution into a 200mL high-type beaker, add 20mL of perchloric acid, heat and evaporate to white smoke (when the cup bottom test solution is high-priced chrome yellow), add hydrochloric acid to volatilize the chromium (brown chromic chloride gas) After heating, the reduced chromium is oxidized, and then hydrochloric acid is repeatedly added dropwise to remove chromium, until the chromic acid brown vapor is no longer present, and the white smoke is further heated for 30 seconds to remove chlorine.
After taking off a little cooling, add 50mL of hot water to dissolve soluble salts (silicic acid insoluble in the test solution), boil, remove, filter with a cotton wool in a 250mL volumetric flask, wash the precipitate (silicic acid) and filter paper 4~ with warm water. 6 times, the precipitate was discarded, the filtrate was cooled to room temperature with running water, diluted with water to the mark, and shaken.
Pipette 25mL of the filtrate into a 100mL volumetric flask, add 10mL of sodium bisulfite solution, heat in a boiling water bath until the solution is colorless, remove, immediately add 25mL of color developer solution, then heat in boiling water bath for 10min, remove, Cool slightly, cool to room temperature with water, dilute to the mark with water, and shake well. The color developing solution was transferred to a 3 cm cuvette, and water was used as a reference solution, and the absorbance value was measured at a wavelength of 700 nm of a spectrophotometer.
Weigh 3 to 5 high-carbon ferrochrome standards with different phosphorus contents, measure the absorbance value by the same analytical method as the sample, and plot the working curve with the phosphorus content as the abscissa and the absorbance value as the ordinate.
(4) Determination of Manganese by Oxidation Spectrophotometry of Ammonium Persulfate Silver Salt
Pipette 50mL of test solution into a 200mL high-type beaker, add 20mL of perchloric acid, heat and evaporate to white smoke (when the cup bottom test solution is high-priced chrome yellow), add hydrochloric acid to volatilize the chromium (brown chromic chloride gas) Continue heating to oxidize the reduced chromium, then add hydrochloric acid repeatedly to remove chromium, until the brown gas no longer appears, continue to heat and boil white smoke to remove chlorine.
Remove a little cooling, add 30mL of hot water, heat near boiling to dissolve soluble salts. Filter with a cotton wool in a 150 mL Erlenmeyer flask, and wash the precipitate (silicic acid) and filter paper 4 to 6 times with hot water. Add 5mL sulfuric acid (1+1) and 3mL phosphoric acid to the filtrate. At this time, the volume of the test solution should be about 60mL (if the volume of the test solution is too large, it should be heated and evaporated). Add 5mL silver nitrate solution, 10mL ammonium persulfate solution, heat. Boil for 1 min, remove and cool slightly, then cool to room temperature with running water, transfer to a 100 mL volumetric flask, dilute to the mark with water, and shake well.
Part of the color developing solution was transferred to two 3 cm cuvettes, one of which was added with 2 drops of EDTA solution, and the purple color was removed as a blank solution. The absorbance value was measured at a wavelength of 530 nm of the spectrophotometer. The percentage of manganese was found from the working curve.
3 to 5 high-carbon ferrochrome standards with different manganese contents were weighed, and the absorbance values ​​were measured by the same analytical method as the sample, and the working curve was plotted with the manganese content as the abscissa and the absorbance value as the ordinate.
Third, the results and discussion
Chromium, phosphorus and manganese were tested by high carbon ferrochrome standard in this method. The comparison of the results is shown in Tables 1-3.