CHEM高手請進！（Please answer in english briefly.）

2007-03-18 1:00 am

There two questions about an experiment(*Determination of iron) of gravimetric analysis.
1.Is it necessary to oxidise all the ironⅡions to ironⅢions before precipitation is made？Explain your answer briefly.
2.What is the purpose of washing the precipitate in the beaker by decantation.

更新1:

*The iron(Ⅱ) is oxidise in an acidic medium to form iron(Ⅲ).It is then precipitated as iron(Ⅲ) hydroxide in an alkali medium. The iron (Ⅲ) hydroxide is collected and ignited to form the iron(Ⅲ)oxide. The mass of iron(Ⅲ)oxide is weighed and hence the content of iron in the sample can be determined.

回答 (1)

殿居男

2007-03-20 8:29 pm

✔ 最佳答案

Gravimetric method is a measurment by means of weight determination.
the solid you weight must be pure and stable chemically.
1.If your sample containing both Fe(II) and Fe(III) oxide the solid you take to weight is not pure at all so, you cannot obtain an accurate results.
But Why Fe(III)??It is because the Fe(II) can be oxidized in atmospheric condition(air oxidized) so you need to oxidized all the Fe(II) to Fe(III) and wieght the product as Fe(III)
Besides the molar weight of Fe(III) oxide is higher than Fe(II) oxide, so the weight error caused by Fe(III) oxide is lesser.
2. to remove all the soluble impurities (which cannot be evaporate in high temperature) why decantation?? it is because if you use filtration you will induce some error via the filtration procedures (becuse you may need to wash the ppt for many times)

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