WO₃ Content Determination of Na₂WO₄

1.Method
In hot solution at a pH of 3.00 to 4.4, tungsten acid is precipitated with hydrochloric acid cinchonine so as to separate from the impurities, after ignition goes to tungsten trioxide, and then weighing and do the color comparisons.

2.Reagents
Methyl violet indicator 0.1%
Hydrochloric acid (1+1)
Cinchonine hydrochloric acid solution 10% (prepared by 1+1 hydrochloric acid)
Cinchonine hydrochloric acid lotion 0.2%
NaOH(s)
Extensive pH indicator paper

3. Procedure
0.5 g sample weighed in 250ml beaker, dissolved with a little water, added 60 ~ 80 ℃ hot water 100ml, 0. 1% 2 drops of methyl orange agent (add more if the indicator is brown) , neutralize with hydrochloric acid (1+1) to red, stirring continuously, 10% cinchonine 10ml, continue stirring to conglomerate precipitate, the supernatant was clear stand until the precipitation sinks, then filter with rapid quantitative filter paper, the filtrate undertake in 250ml volumetric flask precipitated with 0.2 percent cinchonine lotion washed five times or more, after the precipitate was washed together with the filter paper into a porcelain crucible, drying charring on the stove, then at 750 ~ 800 ℃ muffle furnace burning charring 30 to 45 minutes, cooled, and weighed.
The filtrate by tannin, methyl violet colorimetric method.

4. Analysis calculation
WO₃（%）＝（W/G）×100+C

W——weight of WO₃（g）
G——weight of sample（g）
C——WO₃ content of filtrate.

5. Precautions
1. Sodium ions included in precipitate must be washed up, otherwise the burning will go black and results will be too high.
2. The indicator 0.1% of methyl orange is available, then filtrate proceeds by thiocyanate colorimetric method..
As follows:
The filtrate flask containing the sample back into the original beaker, the beaker was added to the solid NaOH, was added to the filtrate PH> 12, and then fell back filtrate 250ml volumetric flask, dilute to the mark, dry filter, the following press thiocyanate photometric method performed.