Looking for advice designing crystalization experiment for high school

[Fewmet]

Hi,

I am interested in letting high school students explore the effect of various conditions on the degree to which a crystallizing substance picks up impurities. (This is to establish groundwork for understanding how inclusions in zeolites are used as indicators conditions in the early earth). That might, for example, be growing a CuSO₄ from a solution that includes traces of NiCl₂, then identifying the amount of Ni in the crystal.

We are working on three questions that we would welcome your input on:
1) What substances seem like promising ones to test? One should be relatively easy to crystallize (leading us to CuSO₄). The other obviously should be a reasonable candidate for inclusion in the crystal, which we are assuming to be something near the other cation on the periodic table.

2) The second question is what variables seem likely to have an effect and are easy to control. Temperature and concentration leap to mind. Any ideas about others that might be reasonably checked?

3) How do we take he resulting crystals and determine the level of impurities? Absolute values values would be great, but I am expecting that relative values are more likely. We have access to spectrometers (the Vernier SpectoVis) that can read absorbance in a solution or through an LED. Would that likely pick up ppm impurities? Are there impurities that will give color changes?

 

[Zefram]

Thank you for your interest in exploring the effect of various conditions on the degree of impurities in crystallizing substances. This is a fascinating topic and I am happy to provide my input on your questions.

1) For the first substance, CuSO4 is indeed a good choice as it is relatively easy to crystallize and can be used as a control group. As for the second substance, I would recommend using a similar salt such as FeSO4 or MnSO4, which are both cations near Ni on the periodic table. These substances have similar properties and should be easy to incorporate into the crystal structure.

2) Temperature and concentration are definitely important variables to consider. Other variables that may have an effect on the degree of impurities could include the type of solvent used, the rate of crystallization, and the presence of any additives or impurities in the starting solution. It may also be worth exploring the effect of pH on impurity uptake, as different pH levels can significantly alter the solubility of various substances.

3) Determining the level of impurities in the resulting crystals can be challenging, but there are a few methods that could be used. One option is to use a spectrometer, such as the Vernier SpectoVis, to measure the absorbance of the solution containing the crystal. This can give an indication of the concentration of the impurity present. Another option is to use a technique called X-ray fluorescence (XRF) spectroscopy, which can provide quantitative measurements of trace elements in a sample. This may be a more accurate method for determining the level of impurities in the crystal. It is also worth considering conducting a visual inspection of the crystals, as certain impurities may cause noticeable color changes in the crystal that could be used as a qualitative measure of impurity levels.

I hope this information is helpful and I wish you all the best in your experimentation. Please let me know if you have any further questions or if I can be of any assistance. Good luck with your research!