Quote Originally Posted by Kirk Keyes View Post
It's interesting that NF grade only is concerned with heavy metals (as quoted my Pat), and not any of the other substances that the other grades are spec'ed for, like chloride or insoluble matter. It really does point out the differences in the grading system for chemicals and how one should consider these things when chosing a particular grade for a particular use.
I don't know that heavy metal, expressed as lead, is the only impurity they are concerned with. It is the only one they listed. There is also a grade SQ for special quality that is granular and has very low allowable maximums of chloride, sulphate and iron. I imagine that the analysis for insoluble material is like the one you described a while back. If you can't see it in the solution and all the solution passes through the filter, whatever is insoluble must be negligible. I figure that if the Powdered Technical grade showed observable insoluble stuff, there would be a comment like < whatever the filter is specified to retain. The powdered and granular have the same specs in the soluble impurities.

I still don't know the ANSI Specifications for Photo Grade, but have applied for a copy. I don't expect to hear from them until after the holiday season.

I want to make sure everyone knows I am not expecting to produce chemically pure borax by my purification. It is a fractionation method, and since each fraction is less than 1, the final product cannot reach absolute purity until the amount left is 0.

I do not see, given the nature of borax, how a large amount could be stored in a bin so that any random sample would have the same hydration as any other. The manufacturer says that when humidity is lowered some of the borax that started as 10 mol will become 5 mol or even anhydrous. If the humidity becomes higher, the hydration moves back to 10 mol causing some clumping. It is not like NaOH which will form a puddle if left open to high humidity, so I suppose that if your borax were stored at high enough humidity it would all settle out at 10 mol and the percentage of borax decahydrate would simply be 100 - impurities. But if there is not enough humidity in the atmosphere to maintain the decahydrate, what you weigh to be 100 grams of borax decahydrate could be equal in sodium tetraborate content to more than 100 grams. If you purchase it as AR grade, and accuracy of measurement accuracy is critical to your purpose, then it would be well to make sure the the humidity in your storage container stays at the proper value. I don't know what that value is because most uses in developer are not very critical as to weight of borax, however critical they might be in ppm of impurities.