A sample of naphthalene, which should be pure white, was found to have a grayish

Question

A sample of naphthalene, which should be pure white, was found to have a grayish color after the usual purification procedure. The melting point was correct, and the melting point range was small. What is the most likely reason for the gray color? When should a mixture of solvents be used to carry out a recrystallization? What type of filtration is better for isolating the final product from the recrystallization process, gravity filtration or suction filtration? Why? Why should you never use a beaker for recrystallization?

Explanation / Answer

1) To understand it in the better way let us consider the possibilities for the change in colour to gray,

First, the compound that you purified contains soluble impurities that have same melting point or range or both , the compound is not napthalene,

But this possibillity is not possible , since any impurity would change the melting point , even if the impurity has the same melting point as napthalene.

If the procedure involves the removal of coloured impurities via charcoal , then it is likely that is colour is caused by residual charcoal. Keep in mind, Charcoal doesnot form a solution with napthelene, So the melting point would not be affected , even if the charcoal is present as an impurity.

2) If your impurities are soluble in polar solvent , it can be a good idea to first dissolve your impurities and compounds and crash out your product by adding a non polar solvent . Then you can wash out both polar and non polar residues . It would dissolve to a broader range instead of using just one solvent.

For Example, A solvent system of diethyl ether/CH3OH.

3) Suction filtration ,

because when you are doing a filtration , the speed of liquid phase moves through the filter depends on the difference of pressure on the two sides of filter.

During gravity filtration, the pressure below the filter is equal to the atmospheric, while above it is equal to the atmospheric in addition with that generateds because of solvent's weight . As the solvent moves through the filter , the weight of the solution decreases , the difference in the presure decreases, and thus the rate of filtration also decreases . At some point you will stil have some residual solvent but the difference in the prerssure is going to be too small to encourage the solvent to pass through the filter at least with in a reasonable time period.

When you are using a suction pump, you decrease the pressure from the lower side of the filter , keeping a great pressure difference which makes the solvent to pass through the filter quickly, regardless the amount of solution above the filter. In this way , you minimize the residual amount of solvent in you solid product.

4) It is not ideal because it allows too much solvent to boil off when you are heating , so it is hard to keep track of the volume you have in there . Erlenmeyer glass are a better bet and you can loosly sit a watch glass on the top which wil allow some of the vapour to condense and fall back in . I've never seen done whole reflex setups for things that required long boiling to dissolve. Still I've used beakers before , out of laziness. They're also good sometimes to get a second crop of crystals . After you funnel your best batch , you boil off some more solvent and some more crystals will come out, though usually not as clean as the first.

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