Tungsten Content Testing Method for Sodium Tungstate
The most commonly used testing method of tungsten content in the sodium tungstate is cinchonine gravimetric method, which is using cinchonine hydrochloride to precipitate tungstic acid at pH value of 3.0~4.4 in a hot solution to achieve the purpose of impurities separation, then burning tungstic acid to get tungsten trioxide, weigh, contrast color and calculate tungsten content.
The reagents used in the experiment are: 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
Steps are:
1. Weigh 0.5 g sample and place in 250ml beaker, dissolve with a little water, add 100ml of 60~80℃ hot water, 2 drops of 0.1% indicator of methyl orange, and neutralize with hydrochloric acid until it turns to red;
2. Add 10ml of 10% concentration cinchonine with stirring constantly, continually stir to come out precipitate and precipitate;
3. The filtrate is undertaken in 250ml volumetric flask, and the precipitation is washed with 0.2% cinchonine for more than five times, after washing, transfer precipitate together with the filter paper into a porcelain crucible to dry and carry out carbonization on the electric stove, then get on carbonization burning at 750~800℃ muffle furnace for 30~45 minutes, cool and weigh;
4. Use methyl violet colorimetric method to calculate and analyze the results: WO3(%)＝(W/G)×100＋C. In the equation: W refers to tungsten trioxide weight (g), G refers to sample weight (g), C refers to measured WO3 content in the filtrate.
Precautions:
①The precipitation should be washed until no sodium ions exists, otherwise the cautery will be darken and result high.
②Indicator uses 0.1% methyl orange, and then filtrate can calculate by thiocyanate colorimetry method.
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