What is the NH3 nitrogen concentration in the water?

An ammonia-selective compound electrode is used to measure ammonia in a fish tank. The procedure is to mix 100.0 mL of unknown or standard with 1.0 mL of 10M NaOH and then to measure NH3 with an electrode. The purpose of NaOH is to raise the pH above 11 so that ammonia is in the form of NH3 and not NH4+.

(a) A series of standards game the following readings. Prepare a calibration curve of potential (mV) vs. log (nitrogen concentration in ppm) and determine the appropriate least squares equation for the line.

NH3 nitrogen Electrode potential
concentration (ppm) (mV vs SCE)
0.100 72
0.500 42
1.000 25

(b) Two students measured NH3 in the aquarium and observed values (duplicate samples) of 106 and 115 mV. What is the NH3 nitrogen concentration in the water?