GB/T 223.18-1994Methods for chemical analysis of iron, steel and alloy--The sodium thiosulfate separation iodimetric method for the determination of copper content (English PDF)

3.1 Potassium pyrosulfate

3.2 Ammonium fluoride

3.3 Potassium iodide

3.4 Nitric acid (rho 1.42 g/mL)

3.5 Sulfuric acid (rho 1.84 g/mL)

3.6 Sulfuric acid (1+1)

3.7 Phosphoric acid (rho 1.69 g/mL)

3.8 Glacial acetic acid (rho 1.05 g/mL)

3.9 Sulfuric acid-phosphoric acid mixed acid

in 400 mL of water, slowly add

100 mL of sulfuric acid (3.5) while stirring; cool for a little while; add into 200 mL of phosphoric acid (3.7); mix well.

3.10 Aqua regia

mix hydrochloric acid (rho 1.19 g/mL) and nitric acid (3.4) according to (3+1).

3.11

Sodium thiosulfate (Na 2 S2 O 3 · 5H2 O) solution (50%). If the solution is turbid, use it after filtration.

3.12 Ammonia (1+1)

3.13 Starch solution (1%)

weigh 1 g of soluble starch and water to make a paste; pour into 80 mL of boiled water; add 2 drops of hydrochloric acid (1+1);

boil till the starch is completely dissolved; use water to dilute to 100 mL; well mix; prepare when needed.

3.14

Ammonium thiocyanate solution (20%).

3.15 Copper standard solution

weigh 1.0000 g of pure copper (copper content of 99.9% or more) in a 250 mL beaker, add 20 mL of nitric acid (1+1), cover the watch glass, heat to dissolve. Add 10 mL of sulfuric acid (3.6), heat and evaporate till sulfuric acid smokes for 1 min. Cool it. Use water to dissolve the salt. Move into a 1000 mL flask. Use water to dilute to scale. Well mix.

This solution shall contain 1.000 mg of copper per 1 mL.

3.16 Sodium thiosulfate standard solution

3.16.1 Preparation

Weigh 2.48 g crystalline sodium thiosulfate (Na 2 S2 O 3 · 5H2 O) to dissolve in boiled and cooled water. Add 0.2 g of anhydrous sodium carbonate. After completely dissolved, dilute to 1000 mL with boiled and cooled water. Well mix. Store the solution in a brown bottle. Use it after placing it in a dark place for two days of calibration.

3.16.2 Calibration

Pipette three 20. 00 mL of copper standard solution (3.15) and place in 300 mL beaker, respectively. Add a small amount of water. Titrate according to

4.3.4. The standard deviation of sodium thiosulfate standard solution consumed by the three copper standard solutions does not exceed 0.05 mL.

Take its average value.

Calculate the titer of sodium thiosulfate standard solution to copper by the following formula:

4.3.2

Cool. Add 250 ~ 300 mL of water. Add 50 ~ 70 mL of sodium

thiosulfate solution (3.11) while stirring. Boil it till boiling for 10 min to make precipitation condense, solution clear.

4.3.3

Use medium-speed filter paper to filter, transfer the precipitation to the filter paper with boiled water, wash the precipitation and filter paper 5 to 7 times. Place the filter paper together with the precipitate in a porcelain crucible, drying, ashing, burning at 550 ~ 600°C for 10 ~ 15 min. Cool. Add 4

~ 5 g of potassium pyrophosphate (3.1), melt at 600°C till it is transparent.

Cool. Place the porcelain crucible in a 300 mL of beaker. Add about 20 mL of water. Add a few drops of sulfuric acid (3.6). Heating and leaching till the melt are dissolved. Wash them out of the crucible. After cooling, add 0.5 g of ammonium fluoride (3.2).

4.3.4

Add aqueous ammonia (3.12) in drops till the solution shows a blue color of stabilized cuprammonium complex. Add drops of glacial acetic acid (3.8) till the solution blue facades and exceeds 1 mL. Add 3 g of potassium iodide (3.3), well mix, cover the watch glass, place the beaker in a dark place for 2 min. Titrate with sodium thiosulfate standard solution (3.16) to a light yellow color. Add 5 mL of starch solution (3.13) and 5 mL of ammonium thiocyanate solution (3.14). Continue to titrate with sodium thiosulfate standard solution (3.16) till the solution turns from light blue to milky white.

Write down the number of milliliters of sodium thiosulfate standard solution consumed.