Glacial acetic acid for industrial use (3)

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Determination of total reducing substances: Excessive sodium hypobromite solution oxidizes the formic acid and other reducing substances in the sample, and then the remaining sodium hypobromite is measured by the method

Determination of other reducing substances except formic acid: In an acidic medium, excess potassium bromide and potassium bromate oxidize other reducing substances except formic acid, and the remaining potassium bromide-potassium bromate is measured by iodometry

The content of formic acid is obtained from the difference between the measured values ​​of the two steps

Reaction formula:

HCOOH+NaBrO→NaBr+CO ₂ ↑+H2O

NaBrO+2KI+2HC1→2KC1+NaBr+H ₂ O+I ₂

2Na ₂ S ₂ O ₃ +I ₂ →Na ₂ S ₄ O ₆ +2NaI

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Pipette 2.

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Weigh 10 g of potassium bromide and 2.

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1 Erlenmeyer flask: capacity 500mL, resistant to vacuum
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2 Dropping funnel: 100mL capacity, vacuum resistant
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3 Vacuum pump or water flow pump: Witte vacuum degree is below 1×10 ⁴ Pa
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The assembly diagram of the measuring instrument for formic acid content is shown in Figure 1:

Figure 1 Diagram of the measuring instrument for formic acid content

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4 Analysis steps

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1 Determination of total reduction

Place the dropping funnel 2 on the Erlenmeyer flask 3 containing 80 mL of water as shown in Figure 1, open the dropping funnel piston, and use the pump to extract the vacuum that can suck 200 mL of liquid (reference vacuum: 7.
5×10 ⁴ Pa or less), Close the plunger of the dropping funnel and pull out the plunger connected to the pump
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Suction the 25mL sodium hypobromite solution sucked with the pipette through the dropping funnel, rinse the dropping funnel with 5mL water each time: rinse twice, and then suck the 10mL sample sucked with the pipette through the dropping funnel, each time Still rinse the dropping funnel with 5 mL of water, rinse twice, mix well, let stand at room temperature for 10 minutes, then suck in 5 mL of potassium iodide solution and 20 mL of hydrochloric acid solution through the dropping funnel , shake vigorously for 30 seconds, open the dropping funnel piston, and remove the drop.
Liquid funnel, add 50mL water to the Erlenmeyer flask, titrate with sodium thiosulfate standard titration solution until the solution is light yellow, add about 2mL starch indicator solution, and continue titration until the blue color just disappears as the end point
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2 Determination of other reducing substances except formic acid

Pipette 25mL of potassium bromide-potassium bromate solution into an Erlenmeyer flask 3 containing 90mL of water, place the dropping funnel on the Erlenmeyer flask as shown in Figure 1, open the piston, and use the pump to draw a vacuum that can suck 200mL of liquid (Refer to the vacuum degree: 7.
5×10 ⁴ Pa or less), close the plunger of the dropping funnel, pull out the plunger connected to the spring, and suck in the 10mL sample drawn with the pipette through the dropping funnel, and rinse the dropping funnel with 5mL water each time , Rinse twice, and then inhale 10mL hydrochloric acid solution
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Mix well, let stand at room temperature for 10 minutes, then suck 5mL potassium iodide solution and 50mL water through the dropping funnel and mix well, open the dropping funnel piston, remove the dropping funnel, and titrate with sodium thiosulfate standard titration solution until the solution is early When it is light yellow, add about 2mL starch indicator solution, and continue titrating the umbrella blue just to disappear as the end point
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3 At the same time of the measurement, perform a blank test with the same amount of reagent solution without adding the sample (10mL of water instead of the sample) according to the same procedure as the measurement
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Related links: Glacial acetic acid for industrial use (2)