Here FDM to introduce the content of potassium dichromate volumetric method tests iron. The basic principle is: In acidic solution, using stannous chloride reduction of the ferric iron is bivalent, High mercury chloride was added to remove excess stannous chloride, sodium diphenylamine as indicator, titrate with standard potassium dichromate solution to purple. The advantages of this approach are: Easy to remove excess stannous chloride, potassium dichromate solution is relatively stable, significant changes in the titration end point, by the effect of temperature (30 ℃ below) is small, a more accurate measurement result.
一、Sulfur - phosphorus mixed acid soluble samples
1. Drugs and reagents
1）（2+3）Parathion mixed acid
2）Potassium dichromate standard solution: 1.00mL solution is equivalent to 0.0020g iron.
Weigh 1.7559g pre-dried at 150 ℃ for 1h potassium dichromate (reference reagents) in 250mL beaker, dissolved in a small amount of water moved into 1L volumetric flask, water volume.
3）Stannous chloride solution：10%
Weigh 10g of stannous chloride dissolved in 20mL of hydrochloric acid, dilute with water to 100mL.
4）High mercury chloride saturated solution：5%
5）Diphenylamine sulfonate indicator：0.5%
Weigh 0.2g sample in 250mL Erlenmeyer flask, with a little water, wet, shake. Join 10mL parathion mixed acid and 0.5 g of sodium fluoride, shake. On the high temperature heating furnace completely dissolved, remove cooling, 15mL of hydrochloric acid, heated to near boiling, and a low temperature to maintain 3-5min, dark solution became clear, a solution of stannous chloride was added dropwise while still hot to remove iron (III) ions yellow disappears and two drops of excess water rinse the cup wall. Cooled in a water bath, was added 10mL of saturated solution of high mercury chloride, placed after shaking 2-3min, add water to about 120mL, was added 5 drops of 0.5% sodium diphenylamine indicator After cooling, titration with a standard solution of potassium dichromate to purple. Simultaneously with the blank test sample analysis.
1）When you need to use a high temperature furnace sample dissolution, and continue to shake the flask to accelerate decomposition, or in the bottom precipitated pyrophosphate or metaphosphate, make the results unstable.
2）Fusion and temperature must be strictly controlled. Iron ore is typically heated at 250 ~ 300 ℃ 3-5 minutes decomposition. Temperature is too low, the sample is not easy to break down; humidity is too high, too long, will be converted into insoluble phosphate, pyrophosphate, cemented lumps at above 350 ℃, the impact of titration end discrimination, low and to the results.
3）Good control of tin dichloride reduced iron (III), dropping amount. Excess tin dichloride was oxidized mercury dichloride, should produce a white filamentous precipitate. If too much excess of the reducing tin dichloride, tin dichloride is further reduced to metallic mercury, a gray or black precipitate. Dichromate oxidation of metallic mercury is easy to make the results of the high iron. When this happens, you should re-determination of said sample.
4）Mercuric chloride solution should be added in a small volume, silky luster white precipitate. This mercury precipitates produced relatively slow. So after adding mercury dichloride should shake and text placed 2 ~ 3min, time is too short, the result is high.
5）Indicator must be freshly prepared, should be replaced once a week.
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