Thermo Fisher Scientific Ion Selective Electrodes Ammonia User Manual

Page 14

Complexation

Metal complexes are formed with ammonia and a number of metal ions. Complexes of mercury,
copper, gold, silver, nickel, zinc, cobalt, and cadmium are removed in the form of hydroxide
complexes or precipitates in basic solution. When ammonia concentration is below 10-

M and

hydroxide is present at the 10-

M level, the only appreciable complex formed will be between

mercury and ammonia. Since the total ammonia level of a sample will be measured if the mercury is
preferentially bound to another species, addition of iodide is recommended for this purpose. Use of
the recommended ISA (10M NaOH solution) inhibits the formation of metal complexes in the
sample, since it contains a high concentration of hydroxide ions.

Temperature Influences

Table 4 gives the variation of theoretical response with temperature. The electrode response will
shift and change slope with change in temperature. Standards and samples should be at the same
temperature. A 2% error results with a 1oC temperature change for a 10-

M solution. Gases are

expelled from a solution at a faster rate as the temperature increases.

TABLE 4: Temperature vs. Values for Electrode Slope

Temp.(oC) "S"
0 54.20
5 55.20
10 56.18
15 57.17
20 58.16
25 59.16
30 60.15
35 61.14
40 62.13

Electrode Response

Plotting the electrode mV potential against the ammonia concentration on semi-logarithmic paper
results in a straight line with a slope of about 56 mV per decade. (See Figure 1.)

For ammonia concentrations above 4X10-

M (0.07 ppm NH3 or 0.06 ppm N), the electrode exhibits

good time response (95% of total mV reading in one minute or less). Response times are longer
below this value and ammonia loss to air may become a source of error. Figure 2 indicates the time
response of ammonia electrode to changes in the ammonia concentration.