Determination Of Tungstic Acid Content

Determination of WO3 in tungstic acid (cinchon gravimetric method)
1. Method summary
In a hot solution of pH 3.00 ~ 4.4, tungstic acid is precipitated with cinchonine and separated from the impurities, which is tungsten trioxide after burning, weighed and colorimetric.

2. Reagents
Methyl violet indicator 0.1%.
Hydrochloric acid (1 + 1).
Hydrochloric acid solution 10% (formulated in 1 + 1 hydrochloric acid).
Cinnabar hydrochloride lotion 0.2%.
Solid NaOH.
Extensive test strips.

3. Operation steps
Weigh the sample 0.5 grams in 250ml beaker, dissolved with a little water, add 60 ~ 80 ℃ hot water 100ml, 0.1% methyl orange finger 2 drops (if indicator brown added) with hydrochloric acid (1+ 1) Neutralize to red, add 10ml of cinchonine 10ml with constant stirring, continue stirring to make the precipitate coagulate, the supernatant is more clear and let stand until the sediment sinks, with fast quantitative filter paper, the filtrate is taken in a 250ml volumetric flask , The precipitate was washed five times with 0.2% cinnoline lotion, the precipitate was washed into the porcelain crucible together with the filter paper, dried in an electric furnace charring, and then charred at 750 ~ 800 ℃ muffle furnace burning for 30 to 45 minutes, Cooling, weighing.
The filtrate by tannin, methyl violet colorimetric method.

4. Analysis of the results of the calculation
WO3 (%) = (W / G) × 100 + C
Where: W - tungsten trioxide weight (g)
G - Weigh the sample weight (g)
C - measured in the filtrate WO3 content.

5. Precautions
① precipitation must clean sodium ions, or blackened black, the result is high.
② indicator available 0.1% methyl orange, then the filtrate can be thiocyanate colorimetric method.
Here's how:
The filtrate in the flask was poured back into the beaker of the original sample, the solid NaOH was added to the beaker, added to the filtrate PH> 12, and then the filtrate back to 25Oml volumetric flask, dilute to the mark, dry filtration, the following According to thiocyanate absorption spectrophotometry.

Calculation formula:

Where: C-- W03 filtrate content (g)
r - WO3 measured from the working curve (μg)
G - Weigh the sample (g)
V1 - points for the volume
V-- total volume.