Metrohm NH3 User Manual

NH3-selective electrodes

Manual

8.109.8031EN

Metrohm AG
CH-9100 Herisau
Switzerland
Phone [phone]
Fax [phone]
[e-mail]
www.metrohm.com

NH3-selective electrodes

8.109.8031EN

Manual

11.2011 ebe

Teachware
Metrohm AG
CH-9100 Herisau
[e-mail]

This documentation is protected by copyright. All rights reserved.

Although all the information given in this documentation has been
checked with great care, errors cannot be entirely excluded. Should you
notice any mistakes please send us your comments using the address
given above.

Documentation in additional languages can be found on

http://documents.metrohm.com.

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Table of contents

1 Introduction 1

2 Mode of operation 2

3 Selecting the electrode 4

4 Assembling the electrode 5

5 Storing the electrode 8

6 Performing measurements 9

6.1 Measured quantities ............................................................. 9

6.2 General notes ........................................................................ 9

6.3 Performing direct measurements ..................................... 10

6.4 Performing standard addition ........................................... 11

Table of contents

6.5 Performing measurements with very low concentra-

tions ..................................................................................... 12

6.6 Influence of dissolved particles on the measurement ..... 13

7 Troubleshooting 14

7.1 Problems and their solutions ............................................. 14

8 Technical specifications 16

9 Accessories 17

9.1 Scope of delivery ................................................................ 17

9.2 Optional accessories ........................................................... 18

Index 19

NH3-selective electrodes

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III

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1 Introduction

The NH3-selective gas membrane electrodes are combined electrodes, i.e.
they contain one measuring electrode and one reference electrode.

They enable the rapid, simple, inexpensive and precise determination of
dissolved ammonia (NH3) in aqueous systems, e.g. in natural water, sew­age, boiler feed water, beer, etc.

In addition, the NH3-selective gas membrane electrodes can detect the fol­lowing substances:

￭ Ammonium ions (NH
￭ Organic nitrogen (N) after a Kjeldahl digestion

Discolored or turbid samples do not compromise the measurement, which
means that, generally speaking, no pre-distillation is necessary.

The membrane of the NH3-selective electrode is gas-permeable and
water-repellent, i.e. water cannot dampen the membrane and cannot per­meate the pores. In the case of samples that contain surface-active sub­stances, or in the case of nonaqueous systems, fluid makes its way into
the membrane. This leads to difficulties, e.g. in the case of sewage sam­ples (detergents) or with nylon or dye samples (nonaqueous systems). An
ammonia determination of such samples should therefore be carried out
by suspending the electrode above the solution instead of immersing it.

+

) after their conversion into ammonia

4

1 Introduction

NH3-selective electrodes

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1

2 Mode of operation

Ammonia (NH3) reacts in water with the formation of ammonium ions

+

(NH

):

4

The equilibrium of this reaction, i.e. the relative proportion of ammonia
and ammonium ions, is determined by the pH value of the solution. In
acidic solutions, ammonia reacts almost completely to ammonium ions. At
a pH value of 9.2, the ratio of ammonia to ammonium ions is approxi­mately 1 : 1. If the pH value of the solution is known, then the ratio of
ammonia to ammonium ions can also be calculated directly. The following
applies:

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(1)

(2)

The value of the equilibrium constant K is also dependent on the tempera­ture.

Table 1

Temperature dependency of the ammonia – ammonium ions
equilibrium

Temperature in °C – log(K)

5 4.13

25 4.78

35 5.08

The addition of an excessive amount of a strong base, e.g. concentrated
caustic soda (NaOH), causes the ammonium ions to be transformed into
ammonia in their entirety. Determination of the ammonium concentration
is thus indirectly possible by means of the determination of ammonia. In
the case of the NH3-selective electrode, a gas-permeable, hydrophobic
membrane separates the measuring solution from the measuring electro­lyte. Ammonia now diffuses through the membrane for as long as the
partial pressure on the two sides is identical in size. The partial pressure of
ammonia is always proportional to its concentration. The measuring elec­trolyte is comprised of a sufficiently concentrated ammonium chloride
solution, which means that the ammonium concentration can be assumed
to be constant.

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2

The potential U of the measuring electrode is proportional to the hydrox­ide concentration of the measuring electrolyte:

NH3-selective electrodes