Quote Originally Posted by Photo Engineer View Post
Patrick;

I'm going to take an extreme case here but bear with me.

Lets assume that you have Borax that is 10% impure, or onlly 90% pure! Lets also assume that the impurity is Sodium Chloride.

A saturated Borax solution will have less Borax in it than expected due to the common ion effect. This causes the less soluable of two sodium salts to precipitate more than predicted.

The 10% Sodium Chloride would become MORE saturated in solution and therefore the impurity level would be increased.

So, your saturated solution is less pure than the starting example.

This hypothetical example shows what you can do to yourself if you don't know the basic guidelines of what you are purifying or how you are purifying it.

Now, if there were a less soluable sodium salt, such as Sodium Aluminate (IIRC), it would concentrate in the solids that remained behind. So the precipitated Borax would end up with more of the alumiate salt.

Either way, you can make things worse.

PE
The basic guidelines of Technical grade borax powder are well known, and the worst case I have seen is guaranteed to have less than 700 ppm of chloride. Under what circumstances could that amount of chloride NOT be dissolved in the first solution? I am depending on the total chloride, sulphate and iron which total 1330 ppm max in the original charge of borax to be dissolved in the first charge of water, 95% of which will be removed by decanting it. That solution certainly would yield a more contaminated borate powder than the original if it were evaporated. I would in fact be happy if it had only the soluble impurities and no borax. Instead, it will be discarded or used for drain cleaner or one of several other household uses. How can what remains in the undissolved borax not have less chloride than it had before? What if it does have more insoluble impurity? I am not using the insoluble impurity that remains. I am making a second saturated solution from it, leaving insoluble impurities behind. The second saturated solution removes doubt about hydration of the powder and MUST have smaller percentages of chloride, sulphate and iron. Why, in the name of good science, don't you read thoroughly what I wrote?