Determination of chromic anhydride 

1.Principle

The principle is that hexavalent chromium is reduced to trivalent chromium by ferrous iron in sulfuric acid solution:2H2CrO4+6H2SO4+6FeSO4=Cr2(SO4)313Fe2(SO4)318H2O indicates the end point of the reaction with phenylo-aminobenzoic acid.

2.Reagents

(1) 1:1 Sulfuric acid:Take 50ml to analyze pure sulfuric acid (specific gravity (1) 84), slowly pour into 50ml water, shake well.

(2)Phenylo-aminobenzoic acid indicator:0.27g phenylo-aminobenzoic acid is measured and dissolved in 5ml 5% sodium carbonate solution, diluted to 250ml with water.

(3)Standard Ferrous Sulfate Hinge Solution:Measure that 40g Ferrous Sulfate Hinge is dissolved in 500 ml 5:95 dilute sulfuric acid, after dissolution, if turbid, it should be filtered.Dilute to 1 liter with 5:95 sulfuric acid and calibrate before use.Calibration method:Use a pipette to absorb 25 ml of standard 0.lmol potassium dichromate solution in a 500 ml conical bottle, add 175 ml of water and 10 ml of concentrated sulfuric acid, and cool down.Add 8 drops of phenylo-aminobenzoic acid indicator, the solution is purple red, titrated with 0.lmol ferrous sulfate hinge solution until purple to green as the end point.

M=25×0.1000/V

Formula:M-Molar concentration of standard ferrous sulfate hinge solution V-Milliles of standard ferrous sulfate hinge solution consumed

3. Analysis method

Use pipette to suck plating solution 5ml in 100ml capacity bottle, add water to dilute to scale and shake well, use pipette to suck this solution 5ml in 250ml conical bottle, add water 75ml 1:1ml sulfuric acid 10ml, add phenylo-aminobenzoic acid indicator 3 drops, titrate to magenta green with standard 0.lmol ammonium ferrous sulfate solution as end point.

4. Calculation:

Ccro3=(M)×V×0.0333×1000)/0.25 (g/L)

Formula:M-Molar Concentration of Standard Ammonium Ferrite Sulfate Solution V-Milliles of Standard Ammonium Ferrite Sulfate Solution Consumed

Determination of chromium trivalent

1. Principles

Trivalent chromium is oxidized to hexavalent chromium with hinges of persulfate in acidic solution under the contact of silver nitrate:Cr2(SO4)3103(NH4)2S2O8H2=2H2CrO4103(NH4)2SO4106H2SO(4)The total chromium is measured and the amount of hexavalent lead is subtracted from the total chromium.Excess hinges of supersulfuric acid are completely decomposed after boiling.2(NH4)2S2O8+2H2===2(NH4)2SO4+2H2SO4+O2_

2. Reagents

(1) 1:1 Sulfuric acid:The preparation method is the same as before.

(2)Phenylo-aminobenzoic acid indicator:The preparation method is the same as before.

(3)Standard 0.lmol Ferrous Sulfate as Solution:The preparation method is the same as the standard method.

(4)1% silver nitrate solution:weigh 1g silver nitrate and dissolve in 100ml distilled water.

(5)Persulfuric acid hinge:solid.

3. Analysis method

Use pipette to suck up plating solution 5ml in 100ml capacity bottle, add water and dilute to scale, shake well.Drain 5ml of this diluent into 250ml conical flask with a pipette, add 75ml of water, 10ml of 1:1sulfuric acid, 10ml of 1% silver nitrate and 2G of persulfuric acid as solids, and boil until bubbling takes about two minutes to cool.Three drops of phenylo-aminobenzoic acid indicator were added and titrated with standard 0.lmol ferrous sulfate hinge solution until the end point was from purple to green.

4. Calculation:

Ccr3+=[(V2-V1)×M×0.0173×1000]/0.25

Formula:Vl-milliliter V2 of standard ferrous sulfate solution used for chromium matching analysis-milliliter M of standard ferrous sulfate solution used in this experiment-molar concentration of standard ferrous sulfate hinge solution

Determination of_Sulfuric Acid

1.  Principles

This method uses zinc powder to reduce chromium to trivalent chromium, separates the SO42-precipitation from the plating solution with barium chloride, dissolves the precipitation with excess EDTA, and calculates the content of SO42-by titrating the remaining EDTA with standard zinc solution.

2. Reagents

(1) 10% barium chloride solution.

(2)Ethanol mixture:A mixture of one volume of ethanol, one volume of hydrochloric acid (specific gravity (1) 19) and one volume of glacial acetic acid.

(3)Buffer solution (PH=10), same preparation method as before

(4)Chrome black T indicator:0.5g chrome black T plus 4G hydroxylamine hydrochloride, dissolved in 100ml ethanol.

(5)Standard 0.05 molEDTA lotion:same preparation method as before

(6)Standard 0.05 mol zinc solution:same preparation method as before

3. Analysis methods

Use pipette to suck 5 ml plating solution in 400 ml beaker, add 100 mL water and 30 mL ethanol mixture, boil for 10 minutes, slowly add 10 mL barium chloride while stirring under heat, boil lmin and place in warm place for 1 hour.Filter with compact filter paper, wash the precipitation with hot water several times, add 100 mL of water and 10 mL of ammonia to the original beaker, add 0.05 molEDTA25mL to the pipette, heat to 60-70C, stir to dissolve the precipitation completely.After cooling, add a little buffer solution smL and chrome black T and titrate with 0.05mol zinc solution until the end point changes from blue to red.

4. Calculation

CH2SO4=[(M1V1-M2V2)×98]/n (g/L)

Formula:Ml-molar concentration of standard EDTA solution Vl-milliliter of standard EDTA solution consumed M2-molar concentration of standard zinc solution V2-milliliter of plating solution taken

Determination of_Iron

1. Principles

The iron titanium indicator itself is colorless, but it forms a dark green complex with the iron platelet, which is less stable than the iron-EDTA complex at pH=(2)0-(2)5, so EDTA can capture the iron in the complex and free the colorless indicator from green to yellow at the end point.

2. Reagents

(1) 1:10 ammonia solution:1 volume ammonia water (specific gravity 0.89) and 10 volume distilled water are mixed evenly.

(2)1:10 sulfuric acid solution:1 volume sulfuric acid (specific gravity (1) 84) is slowly added to 10 volume distilled water and mixed evenly.

(3)Titanium indicator:10g Titanium indicator is dissolved in 100ml water.

(4)The buffer solution (PH=10) dissolves 54 grams of ammonium chloride in water, adds 350 ml of ammonia water, and dilutes to 1 liter.

(5)Standard 0.05mo1EDTA solution:20 g of pure ethylenediaminetetraacetic acid disodium is weighed and analyzed, dissolved in water, cooled and diluted to 1 liter.Calibration:Accurate weighing and analysis of pure metal zinc (3)2690g (accurate to four significant digits) in 250ml conical flask, dissolved with a small amount of 1:1 hydrochloric acid, heated to dissolve completely, cooled, moved into 1000ml capacity flask, diluted with water to scale, shake well.Drain 20ml in a conical bottle with a pipette, add 50ml water, adjust to microammonia with ammonia water, add 10ml buffer solution of PH-10 and a few drops of chrome black T indicator, shake well, titrate the prepared 0.05molEDTA standard solution to a white-red to a blue end point.

M=[W×20/100]/(V×0.06538)

Formula:Molar concentration of M-standard EDTA solution V-consumption of milliliters of standard EDTA solution W equals weight of zinc extraction

3. Analysis methods

Use pipette to suck 2 ml plating solution in 250ml conical bottle, add 100ml water, adjust PH to (2)5 with 1:10 ammonia water or 1:10 sulfuric acid (checked with a small amount of precise PH paper), add 6 drops of iron titanium indicator, titrate the standard 0.05 molEDTA solution to the dark green recession (yellow) of the solution as the end point.

4. Calculation

CFe3+=(M)×V×0.05585×1000)/n.

Formula:M-Molar concentration of standard EDTA solution.V-The number of milliliters of standard EDTA solution consumed.N-Number of milliliters of plating solution Note:Titrate immediately after adding indicator to prevent it from being oxidized and discolored by chromium acid.

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