pFI Method for Ammonia
When ammonium containing sample is merged with a basic (pH > 11, reagent #1) solution, NH3 is formed. The NH3 passes across a gas-permeable membrane into a acceptor stream (reagent # 2) containing bromothymol blue in acid (yellow) form. Collection of NH3 in acceptor solution raises the pH, and the color change (to blue) is proportional to the initial ammonium content
in the sample.
Because the diffusion is the rate determining step. the flow rate is programmed to slowdown, while sample zone is in contact with the diffusion membrane, Thus with increasing dwell time at a 2% flow rate, (A) the amount of ammonia transported through the membrane increases, and so does the intensity of the color of the indicator. By changing from cFI (0 Dwell) to pFI (120s Dwell), the sensitivity of measurement increses 20 times. (B) The nonlinearity of calibration is due to response of indicator at this pH range (C).
Sample volume 300mcrL, flow rate 2% at dwell time 120 sec, flow rate 50% trough flow cell,
10 cm light path, 2 cm diffusion path. 0.0005% BTB indicator.
1.2.36.
