Determination of Oxygen in Anhydrous Ammonia

GBH Enterprises, Ltd.

Plant Analytical Techniques
ANHYDROUS AMMONIA: DETERMINATION OF OXYGEN

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ANHYDROUS AMMONIA:

1

DETERMINATION OF OXYGEN

SCOPE AND FIELD OF APPLICATION
This method is suitable for the determination of trace amounts of oxygen
in Liquefied anhydrous ammonia.
The trace oxygen analyzer provides for trace oxygen analysis in decade
steps ranging from 0 - 10 to 0 - 10,000 ppm v/v (full scale).

2

PRINCIPLE
Liquid ammonia is vaporized by passing it through a stainless steel coil
immersed in warm water. Oxygen diffusing into the fuel cell of the analyzer
reacts chemically to produce an electric current which is proportional to
the oxygen concentration in the gas phase immediately adjacent to the
sensing surface of the fuel cell.
The signal produced by the fuel cell from the trace oxygen is amplified by
a two-stage solid state amplifier.

3

APPARATUS

3.1

Trace oxygen analyzer: a Teledyne Analytical Instruments, model 311-1,
or similar, is suitable. This instrument is marketed in the UK by Analysis
Automation Limited, and adapted for use with anhydrous ammonia.

3.2

Stainless steel coil: a minimum of 20' of 1/8" O.D. coiled to approximately
6” diameter is suitable. This coil is to be fitted at one end with Swagelok
quick connect QC4-S-400 (1/4”) single end shut-off). The other connection
to the coil should be a stainless steel compression fitting suitable for
connection to the sample line.

3.3

Sample line: stainless steel reduced to 3/8” outside diameter, with a
purge line and double isolation valves.


4

CALIBRATION

4.1

Stand the analyzer (3.1) upright on a level surface, and with the range
switch in the "OFF" position, check the alignment of the meter pointer with
the zero mark on the scale. If necessary, make any adjustment using the
screw on the face of the meter.

4.2

Set the range switch to the "CAL" position

4.3

Install the plastic tube equipped male disconnect fitting to either of
the analyzer's sample ports and a blank disconnect fitting in the other port.

4.4

Gently suck air through the plastic tube using a hand aspirator and
observe the meter reading. Continue to suck air through the tube until the
reading is stable.

4.5

Unlock and adjust the span control until the meter pointer is in
coincidence with the "CAL" mark on the meter scale.

4.6

Immediately after stage 4.5 has been accomplished, connect a source of
inert gas, containing only a trace amount of oxygen, and allow to purge at
5L/min until the reading obtained is at a very low level (~10 ppm v/v). An
overnight purge may be required to obtain this low level.

5

Procedure

5.1 Purging
5.1.1 Before proceeding with a determination of oxygen in ammonia, purge with
an inert gas down to at least 1.0 ppm v/v, using a purge rate of 5L/min
5.1.2 When a satisfactorily low oxygen level is achieved, withdraw the
disconnect fittings rapidly, ensuring that the purge gas supply line is the
first to be removed. The fuel cell should not be subject to excessive
pressure since leaks in the cell manifold may result.


5.2

Analysis

5.2.1 Connect the coil (3.2) to the sample point with a suitable stainless
steel compression fitting, and immerse in a container of warm water.
5.2.2 Carefully open the isolation valve, and purge the sample line (3.3) and
coil (3.2) free of air.
5.2.3 Establish a flow rate of approximately 5L/min, using a suitable
flow-meter such as a rotameter, and continue the purging for a few
minutes to ensure that the system is delivering a representative sample of
vaporized ammonia.
5.2.4 Connect the disconnect fittings to the analyzer, and ensure that the
vent line is fitted before the ammonia line, but with a minimum time
elapsing between the two connections.
5.2.5 A satisfactory result can be expected within 15 minutes of starting to
pass ammonia, if all connections have been made correctly. Periodically
change the warm water in which the coil (3.2) is immersed, such that an
effective temperature for the vaporization of ammonia is maintained. The
response of the fuel cell to oxygen is likely to be very slow at these low
levels of oxygen A direct reading for oxygen in ppm v/v will ultimately be
obtained.
5.2.6 When the determination is complete, ammonia should be purged from the
instrument using an inert gas.

6

Expression of Results
The oxygen in anhydrous ammonia, expressed as ppm m/m is given by
the expression


Where
C is the reading for oxygen concentration, obtained directly from the analyzer
(in ppm v/v);
32

is the relative molecular mass of oxygen;

17

is the relative molecular mass of ammonia.

7

Notes

7.1

An 3/8” Ermeto coupling is suitable for the connection from the coil to
The sample line.

7.2

Nitrogen is generally used as the inert gas for purging purposes.

7.3

Carbon dioxide should never be used for purging the analyzer, since the
fuel cell life would be drastically reduced.

7.4

The rapid withdrawal of the disconnect fittings may be assisted if the
operator wears a pair of rubber gloves, which ensures a better grip in the
confined space around the connections.

7.5

During the ammonia analysis, it is advisable to ensure that a supply of
warm water is situated close to the sample point. Longer periods of
effective sampling may then be employed.


Determination of Oxygen in Anhydrous Ammonia

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