Solution Making, Tris, and adjusting pH - before/after? - (Oct/02/2007 )

This is driving us nuts!

Now if you have a recipe for a solution and it says 20mM NaCl, 0.3M NaOH, 20mM Tris (pH 7.4), 0.4% SDS...

So do you make a stock solution of say 1M Tris and adjust the pH, then add the 7.4 pH Tris to your recipe
OR
do you mix it all up and adjust the pH to 7.4?

I know you shouldn't use some kinds of pH probes with SDS containing solutions, I also know the solution is for alkaline hydrolysis so we may want the pH above 7.4? But who knows, I have had people write recipes for solutions and assume you know they want you to adjust the pH at the end, I have had people tell me outright to adjust pH at the end (that's easiest), and I have had people assume you know they mean use the pH adjusted stock solution-don't adjust at the end.

What do you think the SOP is on this?

This means to make a 1 M Tris-HCl solution at pH 7.4 and use 20 ml of it per liter of the solution you are mixing. It would have said something like "adjust pH to 7.4" if it had meant to adjust the pH of the final solution. Note the placement of the pH indication within the parentheses in your recipe.

i agree. the stock Tris-HCl solution is at pH7.4

"20mM Tris (pH 7.4)' means Tris of pH 7.4 so you should make a seperate soln for Tris and adjust pH 7.4 and then add this in the solution you desire to make

Surely this is good advice in general, but in this instance the pH of the final solution is uncontrolled. 300 mM NaOH and 20 mM Tris does not create a buffer. One wonders a bit about the rationality of the person writing the recipe. This does not change the meaning of the recipe, but simply questions its design.