Calculating the amount of solute needed for a specific solution is often required in the histology laboratory. The following ratio can be used:
% desired / 100 mL = g or mL needed / volume desired
Example 1:
Preparation of a 1% acid alcohol solution is needed for the decolorization step of the Ziehl-Nielsen method. A liter of the solution is made routinely with concentrated HCl and 70% alcohol. How many milliliters of concentrated HCl are needed to make 1 liter of the 1% acid alcohol solution?
(Remember, 1 L = 1000 mL.)
1 / 100 mL = X mL / 1000 mL
Cross multiplying, 1 × 1000 = 100X
X = 10 mL concentrated HCl
Example 2:
The histologist needs to prepare 500 mL of a 29% ferric chloride solution. Calculate the amount of solute needed.
29 / 100 = X g / 500 mL
Cross multiplying, 29 × 500 = 100X
X = 145 g ferric chloride
Example 3:
A 0.55% solution of potassium metabisulfite is needed for the periodic acid-Schiff (PAS) stain technique. The histotechnician needs to make 400 mL of the solution. How many grams of potassium metabisulfite will be needed?
0.55 / 100 = X g / 400 mL
Cross multiplying, 0.55 × 400 = 100X
X = 2.2 g potassium metabisulfite
You can also look at this %w/v problem in this manner:
A 0.55% solution of potassium metabisulfite contains, by definition, 0.55 g of solute in 100 mL.
The histotechnician needs to make 400 mL of this percent solution or 4 times the amount of this 0.55% solution of potassium metabisulfite. 4 × 0.55 g = 2.2 g is needed to make 400 mL of the 0.55% solution.
