Expert: Dr. Henry Boyter, Jr. - 11/24/2008

Question
Hi there,
I work in a lab and am wondering if you could help me.  I am making a 1 mg/mL standard that is not soluble in isooctane at room temp in a volumetric flask.  I mass out the standard powder and bring to volume with solvent.  Then I heat gently to get the powder to dissolve.  This heating causes the isooctane to expand slightly and will rise above the marked line on the volumentric flask.  Would this "dilute" the concentration if I remove 1 mL for evaporation?  (After evaporation of the mL, can I assume 1mg is left behind?)

Answer
A couple of things you have left out.  Is the solute volatile?  If so you can not heat the solvent since you would not know what evaporated.

If not volatile, you should heat the solution to dissolve, cool and return the level to the volumetric line.  Any other way is not acceptable for standards.  Removing any while hot is not allowed since you have no idea what is present.

What you should do is only add 3/4-7/8 of the solvent, heat to dissolve, cool, and then make up to the line with mixing.