Electro-Chemical Devices (ECD) HYDRA Ammonium - Manual (Page 33)

HYDRA NH
4
+ -N Page 25
4.2.4 Two point Calibration
The second point of a Two Point Calibration sets the slope of the sensor, the mV per decade. The slope is
calculated by comparing the millivolt values and ppm values in the “1NH4-N 1.00 ppm” line to the
values in the “2NH4-N 100 ppm” line of the Buffer Menu. The concentration of the solution used for
the second point should be at least 10 times higher than the value used in the first point of the
calibration. The recommended calibration standards for both the ammonium ion and potassium ion
calibrations are 1 ppm and 100 ppm. Perform a two point calibration whenever an electrode is replaced.
After completing the single point calibration, see section 4.2.3 above, rinse the sensor with distilled
water and gently dab it dry with a paper towel or soft tissue. Carefully cleaning the sensor prevents
errors in the calibration due to carryover from the first solution. Place the sensor in the second solution,
either the pH 4.01 buffer or one of the 100 ppm solutions and wait for the reading to stabilize before
accepting the value. See section 3.2.5 above for further instruction.
4.2.5 Potassium Ion Compensation
Potassium ions have a positive interference on the Ammonium Ion Electrode, i.e. the ammonium
reading is higher than the actual value. The Potassium Ion Compensation, K+ comp, adjusts the
measured potassium ion concentration using the interference ratio value, the Ks value, and subtracts
the resulting amount from the Ammonium Ion Signal. The Ks value in the NH4-N Set Up screen allows
the interference ratio to be adjusted as needed. The default setting is Ks = .100 which is 10 potassium
ions produce the same signal as 1 ammonium ion, Ks = 0.04 would change the correction to 25:1. The
0.100 correction factor is good for most solutions with low levels of potassium, less than 20 ppm.
Each Potassium Ion Electrode should have similar characteristics but adjustments are necessary when
the potassium electrode is replaced or when high levels of potassium are present in the solution. If the
ammonium ion concentration is less than expected when compared to a grab sample measurement
then adjust the Ks value to a smaller value. Adjusting from 0.100 > 0.082 for example would increase the
potassium to ammonium ion ratio from 10:1 to 12:1 thereby slightly increasing the displayed
ammonium ion concentration by subtracting less in the compensation.