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I accidently washed the Fisherbrand weighing paper (cat. 0989812A) with acetonitrile : water (1:8) to remove some sample. The weighing paper appeared to deform a little bit after the wash. I wonder if ACN:H2O mixture dissolved anything from the paper or the paper deformed just because it absorbed some water?

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    $\begingroup$ "Rinse" might be a more appropiate word than "wash" here. $\endgroup$ – Wayne Jan 7 at 22:27
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    $\begingroup$ Why on earth do you care about the paper? Do you want to re-use it? $\endgroup$ – Karl Jan 8 at 7:16
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    $\begingroup$ @Karl, Rinsing or so-called washing the weighing boat, or weighing funnel is a standard procedure for quantitative transfer to the receiving vessel. He must have applied the same logic on the weighing paper. $\endgroup$ – M. Farooq Jan 8 at 15:13
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The response to your concern depends on the application. If it were just an academic exercise, forget about it, since you may have already done the experiments. However, the point to keep in mind is that paper based weighing materials should not be rinsed or washed for standard preparation until and until unless it is specifically allowed to do so. Paper can have leachable materials which can contaminate the standard- this applies very well to trace analysis. If you note, Fisherbrand specifically mentions "low nitrogen" content.

The second reason why the paper based weighing paper should not be rinsed is that the bottom of the paper has been in contact with the balance pan (which are often horribly dirty in academic labs due to misuse). Since that paper is thin and permeable to solvents, rinsing the paper is akin to washing the dirty pan into your standard solution rather indirectly. Of course, balances in a professional or research settings are better maintained.

Sometimes it appears that some solids particles are still sticking to the weighing paper after transfer process. In such cases, one uses the "weighing by difference" method. You know the weight of the paper+solid, after transferring, you would weigh the paper again, and record the actual weight transferred to the flask or other container. No washing is required.

Analytical work, where extreme care is required, paper is not used at all for standard preparation. One should use "weighing funnels" which are commercially available and there is no concern in washing or leaching or soluble materials.

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