You have a 5.0 mg/L phosphate standard that exceeds the upper test
limit for the low-range phosphate analytical procedure. For it to be
considered adequate for a QA/QC known-concentration sample youíll need
to dilute it to a 0.2 mg/L concentration. Can you easily and
accurately make this dilution?
Or your 6 N sodium hydroxide solution is too strong to properly adjust
the pH of BOD samples. Instead, a 1 N solution would give greater
control. How much of the 6 N solution would you need to dilute to get
a 1 N solution?
There are also many other instances in a wastewater laboratory where
the dilution of an acid, a base, or a laboratory standard is required
so that when the resulting solution is used in an analytical
procedure, it will help you attain the most accurate result.
The formula for diluting these types of solutions is simple:
(volumeA)(concentrationA) = (volumeB)(concentrationB)
Here are two examples using this formula:
#1) You want to make 250 mL of a 0.20 mg/L phosphate solution from a
stock solution of 5.0 mg/L. How much of the 5.0 mg/L phosphate stock
solution will you need to dilute to 250 mL so that the resulting
concentration is 0.20 mg/L phosphate?
Put the information into the formula: (250 mL)(0.20 mg/L) = (X mL
Then solve for X: (X mL needed) = (250 mL)(0.20 mg/L)/(5.0 mg/L) = 10
Take 10 mL of the 5.0 mg/L phosphate solution and dilute it to 250 mL
with distilled water (10 mL of solution with 240 mL water). The
resulting solution will be 250 mL with a concentration of 0.20 mg/L
#2) You have a 6.0 N sodium hydroxide solution from which you want to
make only 75 mL of a 1.0 N solution.
Put the information into the formula: (75 mL)(1.0 N) = (X mL
Then solve for X: (X mL needed) = (75 mL)(1.0 N)/(6.0 N) = 13 mL
Take 13 mL of the 6.0 N sodium hydroxide solution and dilute it to 75
mL with distilled water (13 mL of solution with 62 mL water). The
resulting solution will be 75 mL with a concentration of 1 N sodium
This formula is good for almost any dilution needed in the laboratory,
from phosphate and metal standards, metal salt concentrations for jar
testing, and acids and bases for sample pH adjustment.
There are some exceptions to the accuracy of this type of dilution.
When you use an acid or base for titration in analytical procedures,
then you should standardize the acid or base after dilution to confirm
its chemical strength. Acids and bases tend to lose some of their
strength, especially low concentration ones, when exposed to the
atmosphere or are diluted. However, using this formula to dilute the
acid or base will get you very close to the desired concentration.
Donít forget that a diluted solution cannot be any more accurate than
the stock, or parent, solution. In fact, any form of dilution will
lessen the accuracy of the final solution. So, to maintain the
integrity of a dilution, always be sure of the accuracy of the stock
solution and make your measurements using type A glassware.
The information in this article is very general. As usual, check your
federal, state, and local regulations. You may have additional
regulations or requirements that you must meet.
If you have any questions, suggestions, or comments, contact NEWEA Lab
Practices Committee Chair Tim Loftus at (508) 949-3865 firstname.lastname@example.org.
For more information on the NEWEA Laboratory Practices Committee,
please contact Tim Loftus or Elizabeth Cutone, NEWEA Executive
Director, 100 Tower Office Park, Woburn, MA 01801, (781) 939-0908,
All past articles are posted on our website. Go to www.NEWEA.org and
follow the link to the Committee Pages then to the Laboratory