Click here to enlarge imageThe ammonia electrode can also be used to measure the ammonium ion (NH4+) after converting to ammonia (NH3) or organic nitrogen after Kjeldahl digestion of the sample. Nitrate samples can be measured using an ammonia electrode. The use of the Thermo Scientific Orion Nitrate Test Kit makes this possible. After samples and standards are made basic, nitrate is converted to ammonia by addition of a special reagent. Ions, such as chloride, that could cause interference with the nitrate electrode at high levels have no effect when performing this test with the ammonia electrode. This technique can be performed on samples containing 0.1 to 20 ppm of nitrate-nitrogen.
Ammonia measurements by ion selective electrodes offer many benefits for the user. A key benefit is cost savings. Measurement of ammonia by ISE is inexpensive per test as compared to other techniques which require more costly equipment. In addition, only a small sample size is required and minimal sample preparation is needed. For example, when testing according to Standard Methods 4500 - NH3 D, sample distillation is unnecessary. Unlike the Nesslerization technique for ammonia, toxic metals are not used in the ISE method.
The ammonia electrode can be used over a wide concentration range, from 0.01 to 17,000 ppm as NH3 or 0.01 to 14,000 ppm ammonia reported as Nitrogen (N). Ammonia ISE analysis provides a quick measurement, five minutes (at the lowest levels) or less, while providing accuracy comparable to other accepted methods. The electrode can be used in samples in the temperature range of 0 to 50ºC. The measurement is not affected by sample color or turbidity.
Performing measurements with reproducible, accurate results when measuring low level ammonia samples can be difficult at times. Previously, proper installation of membranes could be challenging as the user needed to stretch the membrane just right over the electrode body. This is not the case with the new ammonia electrode, as no stretching of the membrane is needed. It is gently placed across the electrode body. In addition to this, pre-assembled bodies with membranes are available. These provide much more accurate results than previously designed bonded membrane caps that many manufacturers use.
Ammonia samples must be aqueous and cannot contain organic solvents. Ionic strength adjustor (ISA) must be added to all samples and standards just prior to measurement. This will adjust the pH above 11. Solutions will be blue when using the Orion 951211 ISA. In this basic solution, all ammonium ions will be converted to ammonia.
There are different techniques available to the analyst when measuring ammonia with an ISE. Direct measurement is a popular procedure used when measuring a large number of samples. The system is calibrated using a series of standards. The concentration of the samples is determined by a single reading that is compared to the standards. As an additional aid to easier low level measurements, the Orion Star series ISE meters all feature a special calibration mode to improve the measurement accuracy at low levels. Known addition is an incremental technique that is used to measure ammonia concentration without calibration. Known addition measures dilute samples by placing the electrode in a sample and adding an aliquot of a standard solution to determine the concentration.
About the Author:
Bob Caporiccio is the product line manager for the Thermo Fisher Scientific Orion products electrode, solution and titration lines. He has worked at Thermo Fisher on the Orion products for 28 years. Bob holds a BS in Natural Sciences from Boston State College and a MBA from Bentley College.
