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 x 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 x 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 X 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 X 0.55 g = 2.2 g is needed to make 400 mL of the 0.55% solution.