Nico2000 Ltd - Test Report CCR/20/11/07
( www.nico2000.net )

Ion Selective Electrode measurements of NH4 and K in natural waters.

Introduction:

Previous measurements of NH4 and K at Nico2000 have all been made on pure standard solutions.

This investigation uses natural waters to test the behaviour of the electrodes in real samples and the validity of a novel measurement of, and correction for, the interference from K.  It also addresses the problems of coping with samples with high ionic strength and samples with pH above 6.5 where the NH4+ ion progressively converts to free NH3.

Method:

1)  The sample was first measured using a 4channel analyser to find the initial concentration of K, NH4, pH and temperature simultaneously.  NB: pH was measured in a separate beaker to avoid K contamination from pH electrode.

2)  A 2-channel analyser was calibrated with standards which span the range of the initial NH4 and K values.

3)  Ammonium value (NH4a) was re-measured (by Direct Potentiometry) in a measured volume of sample (e.g. 50mls), then a measured volume of K standard was added to give a significant increase in the NH4 value (assuming “normal” Selectivity Coefficient of about 0.1).  For example, adding 2 mls of 1000ppm to 50mls gives increase in K of 38ppm and thus should increase NH4 by about 3.8 ppm. 

NH4 ppm then re-measured (NH4b).

4)  Selectivity Coefficent was calculated from increase in NH4 caused by K addition:

                        SC= (NH4b – NH4a) / 38

5)  Sample was then measured by Standard Addition method for NH4 and K and the first mV reading (in pure sample) was used to calculate/read the concentrations from the Direct Potentiometry calibration graphs.  (If both SA concentration are significantly higher than DP then this indicates that the sample has a high ionic strength and simple DP results should not be used.).

6) True value of NH4 calculated by subtracting the K contribution (Kppm x SC) from measured value.

Results: (all measurements at 17±1 °C)

KEY:  ppmK, ppm NH4  = measured values. 

SC(K) = Selectivity Coefficient  for K calculated from addition of K to actual sample.

NH4(c) = ppm NH4 corrected for K contribution.

Rain = water taken from garden rain butt draining garage roof.

Pond = water from garden pond – planted, but NO fish & NO additives, fed by rain butt water.

River = water taken from river Lea at Batford (Harpenden), down-stream from Luton sewage works.

DP4 = Direct Potentiometery measurement with 4-channel analyser

DP2 =      “             “                      “                  2-    “           “

SA = Measured by Standard Addition method

Sample                        ppmK              ppmNH4         SC(K)              NH4(c)            pH

Rain (DP4)                   3.0                   2.5                   n/a                    n/a                    6.4

Rain (DP2)                   2.98                 2.42                 0.057               2.25

Duplicate (DP2)           3.11                 2.47                 0.057               2.25

Rain (SA)                     2.87                 2.60                 0.095               2.33

Pond(DP4)                   2.1                   0.5                   n/a                    n/a                    6.3

Pond (DP2)                  2.06                 0.50                 0.078               0.34

Duplicate(DP2)            2.38                 0.49                 0.078               0.30

Pond (SA)                    2.12                 0.59                 0.091               0.40

River (DP2)                  12.4                 1.38                 0.069               0.52                 8.0

River (SA)                    15.8                 1.82                 0.093               0.35

pH correction for River Water
Using table below (copied from unknown source on the internet - reliability not guaranteed)
NH4/NH3 equilibrium is about 3.2 % un-ionized at pH8 and 17°C.

Thus NH4 in river water corrected for pH = 0.35x1.032 = 0.36ppm , and free NH3 = 0.01ppm.

1)  All three water types have a significant interference on the NH4 measurement from K.
 

2)  The method for determining SC and correcting for interference would appear to work reasonably well.

3)  River water has significantly high ionic strength and thus simple direct potentiometry cannot be used, but it also has a high pH so that some of the NH4 is present as NH3.  Thus ISAB cannot be added because this would disturb the equilibrium. Therefore high pH samples must be analysed by Standard Addition method.

4)  Once the effect of K contamination, pH,  and Ionic strength have been evaluated in a typical sample then all similar samples can be analysed by the same method.  It should not be necessary to measure the SC in every sample.

5)  In order to fully evaluate the validity of these methods it will be necessary to make several repeat measurements to estimate the precision, and obtain separate analyses by a different method to assess the accuracy.

                                                                                                                      HTML version:  ccr/22/11/2007