That is because you do 2 things which need to be highlighted.In fairness I've not had a Metastannic problem for twelve years
One, you return it to the solution before de NOx
Two, you never return it to the solution after filtering and beginning your drop.
A problem some refiners have is they think if it’s OK to do it on a small scale, it’s OK to over do it as well. You surely are talking a drop of stannous per test. I have seen refiners dropping gold and pick up a squeeze bottle full of stannous and squirt it into the solution to see if they are done. Then they wonder why their gold isn’t pure!
Personally I’m a drop of solution and a drop of stannous on a strip of filter paper guy. A refinery always generates a collection of waste for burning and recovery and the test papers go that route.
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