The instructions on how to use Product A2792 to make 100 mL of a 40 percent solution are provided on the Product Detail Page in the DESCRIPTION section under the heading of Reconstitution:
"To make up 100 ml of A2792, slowly add 63 ml distilled or deionized water to the container and dissolve contents. Do not divide the solid. This makes 100 ml of a 40% stock solution."
To use this 100 mL of the 40% solution to in turn make a 30% solution, add additional distilled or deionized water to bring the total volume to 133 mL; the total volume can be measured in a graduated cylinder. It should take roughly 30 mL of additional water, but the total volume (133 mL) needs to be measured accurately.
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Alternatively, it be more convenient to order the Acrylamide/Bis-acrylamide, 30% solution, BioReagent, suitable for electrophoresis, 29:1, which we list as Product A3574. Please see the link below to review this product:
https://www.sigmaaldrich.com/product/sigma/a3574.