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# LMP91200: reference to build Ion-selective electrode measuring circuits

Part Number: LMP91200

Dear Fellow members,

I would like to build a circuit to measure Ammonia in water using Ammonia ion-selective electrode. Can you help me with a reference model/design for such a circuit? Can LMP91200 be proposed for the same?

Thanks

Gopi Krishna

• Gopi.

Technically the ammonia ISE probe is similar to a PH probe in measurement. Then you resolve it in math for the ammonia content. See below. This is a standard circuit and is shown in the data sheet.

TI does not have demo boards for the sensor. However look on Mouser.com as they do have some reference circuits that use the LMP9x parts for PH probes.

The ammonia electrode is a complete potentiometric cell that contains both a silver/silver chloride (Ag/AgCl) reference and a pH measurement element. These elements are housed within a thermoplastic body in a chloride ion-containing electrolyte, and are isolated from the sample by a gas permeable membrane made of polytetrafluoroethylene (PTFE). Dissolved gas in the sample solution diffuses into the membrane and changes the pH in the thin film of electrolyte on the surface of the pH glass. Diffusion continues until the partial pressures of the gas in the sample and thin film are equal. The change in pH is proportional to the concentration of dissolved gas in the sample solution. The Nernst expression for an ammonia sensor is expressed in the equation below. Note that the potential is a function of the ammonia gas, which in turn is related to the hydroxyl ion concentration. The glass internal, Ag/AgCl reference, and Henry’s law constant are rolled into the E’ and Eo constants. The Nernst equation for the sensor becomes the equation noted below:

E = E’–2.3RT/nF log [A]= Eo–0.059 log [OH- ]

E = observed potential

E’ = Reference and fixed internal voltages

R = gas constant (8.314 J/K Mol)

n= Charge on ion (equivalents/mol)

A ion = ion activity in sample

T = absolute temperature in K

F = Faraday constant (9.648 x 104 C/equivalent)

The mV should decrease in a Nernstian manner as the ammonia partial pressure increases in the sample.

• Dear Gordon,

Thank you so much for the detailed explanation. This helps us to get started. Thank you.