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Ammonia detector

 

Measurement principle

The alkaline buffer (NaOH+Na-citrate) converts ammonium to NH3 which then diffuses through the PTFE membrane. On the other side of the membrane there is an hydrochloric acid solution which converts NH3 back to ammonium. Ammonium causes a change in conductivity of the HCl solution. Sulfide does not interfere.

Instruments needed

  • Peristaltic pump (Ismatec IPC)
  • Pump tubes white-white or white-black (approx. 1mm / 0.04” ID, tygon3609)
  • Injection valve (Rheodyne 7725) with 100 µL loop and injection syringe
  • Reactor (custom made)
  • PTFE plummer sealing tape
  • Conductivity meter with flow-through-cell (Amber science model 1056, cell 529)
  • Strip chart recorder
 

Solutions

0.2M Na-citrate-2hydrate in 10mM NaOH
1mM HCl
dilute both solutions 1:1 if peaks are too small
Ammonium standard (1000 ppm NH4+)
Calibration solutions:
Calibration solutions have to be prepared freshly (stable for 3 days in the fridge).
Prepare 0.1 / 0.25 / 0.5 / 1 / 2.5 / 5 / 10 and 20 ppm standards

Pre-dilution: prepare a 100 ppm standard (5mL 1000 ppm standard in 50 mL pure water)

The following table gives amounts of 100 ppm standard in 50 mL to obtain calibration standards. For seawater samples dilution solution should be artificial seawater, yet the differences to using pure water for dilution are negligible.

Concentration 100 ppm standard
0.1 0.05
0.25 0.125
0.5 0.25
1 0.5
2.5 1.25
5 2.5
10 5
20 10

 

Important: store standards refrigerated. Warm up to room temperature prior to measurement.
Flush both tube systems with the appropriate solutions (injection side – alkaline, detection side – acidic). Watch the strip chart recorder for straight baseline.
Linear range for this method is 0.25 to 20 ppm. If the sample concentration is higher they have to be diluted.
 

Peristaltic pump

Release pump tubes after use
Pressure handles should be on equal settings
Turn main switch on. Press RUN. Set flow direction. Speed 10.
 

Conductivity meter

Turn main switch on. Range C. ATC (temperature compensation) ON.
 

Strip chart recorder

Connection to conductivity meter red-red black-blue
Speed 6 mm min-1
Switch right below points down
Sensitivity 0.1 Volts

 

Injection

While still in INJECT setting, insert needle all the way
Set valve to LOAD(dont move the valve handle too slow)
Push sample into sample loop (sample loop is 100 µL + flushing + dead volume – approx. 500 µL to 1 mL is enough)
Set valve back to INJECT to start analysis

 

Inside the NH4 detector box

1 – tube to injector .(Ra1 – Valve port 3) 2 –reactor out (waste Ra2 – A3) 3 – tube to detector (Ra3 – D3) 4- HCl input (Ra4 – Pump) 5 – detector in (D3 –Ra3) 6 - detector cell out (waste D2 – A2)
rear panel: R1 – injection side (alkaline solution) supply, R2 – detection side (acidic solution) supply, A1, A2 – waste,

1 - 100 µL sample loop, 2 – Supply alkaline solution, 3 – tube connection to reactor, 4 – 100 µL sample loop, 5 – unused, 6 – waste sample loop