Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 17)

It is possible to calculate the ratio of ammonia to ammonium ion, theoretically, if the pH is known.
A. Martell and R. Smith in Critical Stability Constants, Plenum Press, New York, 1974, state:
[NH
4
+] [NH
4
+]
⎯⎯⎯⎯ = ⎯⎯⎯⎯ K ~ 10-9.3
[H
3
O+] [NH
3
] 10-
pH
[NH
3
]
at 25oC, X = 0.1 where pH ~ 9.3
The ratio of ammonium to ammonia is given by:
[NH4+]
⎯⎯⎯⎯ = K-pH = 10 9.3-pH
[NH3]
Both temperature and ionic strength will cause the exact value of K to vary. If, for example, while
the pH at 25
o
C and X = 0.1 is 9.3, an increase in ionic strength to X = 1.0 yields a pH of 9.4, at
25
o
C.
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the glassware the electrodes, the standard and reagents, the sample,
and the technique.
Meter
The meter is the easiest component to eliminate as a possible cause of error. Most meters are
provided with an instrument check-out procedure in the instruction manual and a shorting strap for
convenience in troubleshooting. Consult the manual for complete instructions and verify that the
instrument operates as indicated and is stable in all steps.