Method principle
Using hydroxylamine hydrochloride to reduce iron trivalent in the test solution to iron divalent under PH2-9 conditions, iron divalent ion can react with phenanthrene to produce orange-red complex at the maximum absorption wavelength of spectrophotometer 510nm to measure its absorbance.
Reagent and solution
Hydrochloric acid solution: 1+3;
Hydrochloric acid solution: 2mol/L; Ammonia solution: 1+3;
Acetic acid-sodium acetate solution with pH = 4.5: weigh 164g (CH3COONa ·3H2O), dissolve in water and add 84mL of ice acetic acid to dilute to 1000mL volume bottle and shake well;
Hydroxylamine hydrochloride: 100g/L;
Phenanthroline: 2g/L The solution should be kept away from light only colorless solution can be used;
Ammonium ferric sulfate: NH4Fe (SO4) 2 · 12H2O; Standard reserve liquid for iron: 0.100mg/mL.
Preparation 1: Weigh 0.863g ammonium ferric sulfate to 0.001g, put it in 200mL beaker, add 100mL water, 10mL sulfuric acid, dissolve it, transfer all of it to 1000mL volumetric bottle, dilute it with water, and shake well.
Preparation 2: Weigh 0.100g pure iron wire (99.9%) and weigh to 0.001g. Put it in 100mL beaker and add 10mL concentrated hydrochloric acid and heat slowly until completely dissolved and cooled. Transfer all of it to 1000mL volumetric bottle and dilute it with water to scale and shake well.
Iron standard solution (2 μg/mL) : Remove 10.0mL reserve solution and place it in a 500mL volumetric bottle and dilute it with water to scale and shake well. The solution is ready to use and mix.
Hydrochloric acid solution: 2mol/L; Ammonia solution: 1+3;
Acetic acid-sodium acetate solution with pH = 4.5: weigh 164g (CH3COONa ·3H2O), dissolve in water and add 84mL of ice acetic acid to dilute to 1000mL volume bottle and shake well;
Hydroxylamine hydrochloride: 100g/L;
Phenanthroline: 2g/L The solution should be kept away from light only colorless solution can be used;
Ammonium ferric sulfate: NH4Fe (SO4) 2 · 12H2O; Standard reserve liquid for iron: 0.100mg/mL.
Preparation 1: Weigh 0.863g ammonium ferric sulfate to 0.001g, put it in 200mL beaker, add 100mL water, 10mL sulfuric acid, dissolve it, transfer all of it to 1000mL volumetric bottle, dilute it with water, and shake well.
Preparation 2: Weigh 0.100g pure iron wire (99.9%) and weigh to 0.001g. Put it in 100mL beaker and add 10mL concentrated hydrochloric acid and heat slowly until completely dissolved and cooled. Transfer all of it to 1000mL volumetric bottle and dilute it with water to scale and shake well.
Iron standard solution (2 μg/mL) : Remove 10.0mL reserve solution and place it in a 500mL volumetric bottle and dilute it with water to scale and shake well. The solution is ready to use and mix.
Instrument and equipment
General laboratory instruments, spectrophotometers (Shanghai Meishe Instrument Co., LTD.) with a 3cm thickness of the cupola.
Drawing of working curve
Add iron standard solution 0.00, 0.50, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.00mL each with water to about 20mL, add p-nitrophenol one drop, adjust p-nitrophenol to yellow with ammonia solution, immediately adjust with 2mol/L hydrochloric acid to colorless and excess 2 drops add 5mL hydroxylamine hydrochloride Solution 5mL acetic acid-sodium acetate buffer solution 5mL phenanthroline solution diluted with water to scale shake well at the same time do blank test for color development for 10min at the wavelength of 510nm to determine the absorbance and draw the working curve.
Determination of the sample
Accurately remove a certain amount of methanol sample is generally 15-25ml or so placed in a 50mL volumetric bottle with a drop of p-nitrophenol with ammonia or hydrochloric acid solution to adjust the pH value of the solution to about 5 and then add 5mL hydroxylamine hydrochloride solution 5mL acetic acid-sodium acetate buffer solution 5mL o-phenolline solution (after adding a reagent to fully shake) diluted with water to the scale. And at the same time do blank experiment color development 10min at the wavelength of 510nm for determination.
Several problems that should be noted in the determination:
Several problems that should be noted in the determination:
1. Methanol synthesis has high pressure method, medium pressure method and low pressure method. The range of iron content in refined and crude methanol of different production processes is different. The sampling quantity should be adjusted according to the actual situation to make the absorbance of the complex in the linear range of the curve.
2. If there is no densitometer in order to reduce the error of analysis, the sample can be directly weighed by the syringe decrement method to measure the sample quality.
3. Due to the differences in the alkalinity of refined and crude methanol of various enterprises, the pH of the solution should be strictly controlled before each measurement. The pH value of Congo red test paper solution is generally controlled at about 5, which can be adjusted by acidity meter.
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