I'm not a chemist and does not know much about the theory behind the zinc titration method. I have a sample and wold like to test for the zinc content. Following is the protocol that I have performed:
1. Weight 2g of zinc sample and dissolve it in 50mL of distilled water.
2. Add about 50mg xylenol orange tetrasodium salt and 2g of hexamethylene-tetramine into solutions.
3. Color turns to become violet-pink.
4. Titrate with 0.1M ethylenediaminetetraacetic acid tetrasodium salt dihydrate until the color change from violet pink to yellow.
I have repeated the same experiment few times but the color of xylenol orange still remain in purple without any changes.
Can anyone give me some idea on it?
The zinc that you weight comes from metallic zinc or from a salt, like ZnCl2?
For starters to verify your reagents and method are working properly, try a known quantity of zinc as a positive control so to speak. This would provide good feedback for the color change process, some titrants are very faint and color change can be hard to observe under some lighting conditions. ZnCl2 is highly soluble in water and would be a good choice. The end result should quantify well with your know quantity of dissolved zinc.
As Jordi alluded to, typically this titration is for Zn++. Therefore, if your zinc is in an unknown state such as Zn+ or metallic. This titration would fail to give you a result. If it is in another form you may need to do some additional chemistry on your unknown sample to put it into the required form for titration. As indicated by Prof V.S. Muralidharan, Vogel is an excellent source for analytical chemistry. Although, a number of analytical chemistry textbooks have this type of titration in good detail.
Shi Yiing Tang,
'The laboratory where I worked during college had an English translation, so it exists but may be harder to find now. I believe there is a chinese translation as well. I am not certain what year but a former coworker had it, not certain whether Mandarin or Cantonese.
I am familiar with 3,3'-Dimethylnaphthidine. We used it in analytical chemistry for quantification using the method Georg indicated but for iron content in breakfast cereal. We had to use an acid digest because some of the iron content in some breakfast cereals was in the form of iron oxide as reddish brown solid and had to be dissolved before titration could occur.
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