Hanna Instruments HI 4101 - Manual (Page 16)

16
XIV XIV XIV XIV XIV. . . . . Other Measurement T Other Measurement T Other Measurement T Other Measurement T Other Measurement Techniques echniques echniques echniques echniques
Known addition
An unknown concentration of ammonia can be determined
by adding a known amount (volume and concentration) of
ammonia standard to a known volume of the sample. This
technique is extremely useful for ammonia as the sensor
may drift from calibrated values over time, however the
slopes remain constant. With known addition, the stan-
dard and sample are measured within minutes of one
another. The technique can use an ideal sensor slope, but
actual slopes at the temperature of measurement should
be determined and used if possible. This will improve ac-
curacy. Known addition is Method 4500-NH
3
E. from
Standard Methods for the Examination of Water and Waste-
water.
1) The volume of the unknown sample (V
Sample
) is mea-
sured accurately and placed into the closed sample
vessel. The sensor is secured in the vessel and then the
vessel is placed on a stirrer.
2) ISA is added at 1 part per 50 parts sample.
3) When the measurement is stable the mV value is
noted.
4) A known amount, volume (V
Standard
) and concentration
(C
Standard
), of NH
3
standard is then added to the sample.
mV values are again noted when the measurement is
stable.
5) The mV change is then calculated (
∆
E).
6) Using the measured and calculated values, the
sample concentration (C
Sample
) can be determined.
C
sample
=
(V
T
)10 ∆E/S - (V
S’
)
C
standard
V
standard
V
sample
V
S’
(V
sample
+V
standard
+V
ISA
)= V
T
(V
sample
+V
ISA
)= V
S’