Method of Standard Additions for Unknown sample of surface water
You will be given a sample of water from a body of water in Oregon. Measure out 90 mL
of it, and add 10 mL of TISAB, noting the actual final volume. Determine the fluoride
concentration in a manner similar to that used in determining the fluoride in the QC solution. Use
a linear regression to report the concentration of fluoride in your water sample, as described below
in "Data Linearization", and think about uncertainty contributions for your determination.
Data Linearization
Potentiometry-4
Harvey, pp. 164-167, gives a very general demonstration of how to use the Method of
Standard Additions to determine the concentration of analyte in an initial solution that works well
for many analytical techniques. For Potentiometry, we introduce a slight variation that is needed
due to the exponential part of the Nernst equation, and you can view a more full explanation in the
separate document "Potentiometry Help Sheet". In any case, plot (Vs + Vadd)e-EnF/RT vs. total
volume of fluoride added for when 0, 10 mL of KNOWN ADD was mixed with the QC
solution. Here E is the cell potential, Vs is the QC solution volume, and Vadd is the total volume of
the standard added to the sample when the voltage reading is taken. If the slope of the trend line
through your data is negative, change the sign of the potential; a line through your data should
have positive slope.
[Question 2: Why does the equation above include both Vs and Vadd? What will be the result
of the calculation if the term Vadd is missing? Be specific about issues that can occur with
data if this quantity is not accounted for.]
