Can an Ammonia ISE System give Accurate and Repeatable Measurements in Online Applications?

Ion Selective Electrodes for measuring Ammonia are known to significantly drift from the Calibration Curve in non-stable ways. The are 3 main causes of Inconsistent Measurement in Ammonia ISE Probes.

·        Electrode output drift.

·        Ionic Interference from unpredictable changes in the Liquid Matrix being measured.

·        Poisoning of the Electrode Junction leading to drift and ultimately sensor failure.

The data in the graphs below are taken from a controlled experiment carried out in Instran Analytica’s laboratory in Barcelona, Spain. Here we present the first 30 days of data although the test will run for 90 days at least. A reservoir of stabilised Ammonia solution at 5ppm is provided and at regular intervals a sample is automatically taken by the Instran ISE Ammonia Analyser. In the first chart below we are seeing the actual raw value data from the installed ISE Ammonia electrode. This is typical of the unstable reading generated by an ISE probe, from all manufacturer’s even though the Ionic Matrix and the Measured Concentration are stable.

Figure 1 – Electrode Raw Value Output.

As all test conditions are held constant, the data above shows both a long-term general change in Electrode output over time and instantaneous instability around the median value. The unstable swings in electrode output, if uncorrected, yield measurements at +/- 30% of Ammonia Full Scale Value of 5ppm. The calculated final output of this sensor if not re-calibrated during the one-month timespan would be as shown below.  (Calculated from the initial calibration curve).

Figure 2 – Calculated Final Value of Figure 1 Raw Data with Once per month calibration.

Before analysing the data further let us examine how Ammonia using ISE probes is measured under certified laboratory conditions.

ASTM D-1426 98 Method B – Ion Selective Electrode.

This test method is specified for measuring Ammonia by ISE probe in Reagent and Effluent Waters in the range 0.5 to 1,000mg/l. A 3-point calibration curve is made at various concentrations around the expected measurement range produced by serial dilution from 1,000ppm stock solution. The sample (and calibration fluid) being tested is converted to Ammonia by pH adjustment to >11 and the ISE mV output is measured whilst the sample is stirred. Some significant points of note.

·        Crucially, this ASTM standard requires checking the calibration of the apparatus every 3-hours if a series of measures is being performed or daily if spot samples are being checked. The accuracy of measurement will not be held if this protocol is not adhered to.

·        1,000ppm stock solution is stable for 3 months storage at Standard Room Temperature.

·        Test inaccuracies can arise from electrode drift and standard solution weighing. Thereafter the main source of error in this methodology is pipetting the sample and NaOH solutions into the measuring chamber. ISO 8655 Part 2 lists this value at ≤± 0.8% - so a 2-10ml pipette will have an accuracy of +/- 0.08ml.

The ASTM documentation provides data on Precision and Bias if this method is correctly followed.

It would appear therefore extremely challenging to consistently and accurately measure Ammonia by ISE in online applications – particularly where only once per month calibration is planned. However, there is a solution which allows the Reliable, Consistent and Accurate use of ISE probes to Measure Ammonia in these applications.

Online Automated Instrumentation based on ASTM D1426 98 Method B

There is on the market a TRL 9 - ISE Ammonia Analyser which solves these well-known issues of ISE drift & instability, its operation is based on the ASTM D1426 98 Method B methodology and therefore allows the Reliable, Consistent and Accurate use of ISE probes to Measure Ammonia in Reagent, Effluent, River and Surface Water applications. The solution provided by this Analyser automatically uses a Standard Known Addition (SKA) methodology at the end of each measurement and adjusts the calibration curve based on the change in concentration obtained following the SKA injection. The Instrument is manufactured by Instran in Spain and Distributed in the UK by Dooley Wastewater & Environmental Consulting Ltd.

Please see Appendix 1 for larger print version of the Instran Analyser.

The Instran Analytica procedure is as follows. Initially the ISE probe is Auto-Calibrated using a Standard 2-point calibration (during which 10ml of the Standard calibration fluid is injected into the test chamber and stirred while the Ammonium is converted to Ammonia by dosing 0.5ml of NaOH into the test chamber, which adjusts pH to >11).

During sample analysis – The sample chamber and ISE probe are automatically cleaned using both DI water for soft cleaning and chemical addition for Hard Cleaning should the application require it. The sample being measured and NaOH are dosed with very high accuracy into the measuring chamber using a Stepper Motor Driven Syringe. The mixture is stirred for a pre-set time and the initial electrode output is measured. Then the sample is further dosed with a fixed known volume of Higher Strength Ammonia and stirred for a pre-set time. The electrode output is now remeasured. The Analyser software can then adjust the calibration curve instantly for the initial measurement of Ammonia. This methodology simultaneously adjusts, under stable and known conditions the analysis for both Electrode Drift and any Interfering Ionic Concentrations in the Sample Matrix.  Essentially then the ISE probe is continuously recalibrated every 15 minutes. The key to the accuracy and repeatability of the Instran Analytica system is the 2,000 Step Stepper Motor Driven Volumetric Syringe which doses all ingredients into the test process. The syringe arrangement allows accurate dosing to 0.015ml for each component.

The following graph shows the final value that was measured by the Instran ISE Ammonia Analyser when the Electrode Output in Figure 1 was corrected by the Standard Known Addition Methodology.

Figure 3 – Calculated Final Value of Figure 1 Raw Data with Automated SKA correction.

Applying this technique in Real Life Application on a Sewage Treatment works in Barcelona yielded the following results.

Figure 4 – Barcelona WWTP data.

Comparing Ammonia ISE analysis by Laboratory Based ASTM D1426 98 Method B & Instran

Comparison	Laboratory	Instran
Calibration Checking	Every 3 hours or daily	Every measurement – 15 minutes normal frequency
Volumetric Dosing Accuracy	0.08ml (for 2-10ml pipette)	0.015ml
No of points on calibration curve	3	2
Stock Solution shelf life	3 months	3 months

Delivering

 Accurate

Reliable

Repeatable

Online ISE Ammonia Measurement

Requires Auto-Calibration

 & Auto-Cleaning

The Solution is Already Here!

·       Tests can be performed every 15 minutes.

·       Consumables NaOH & SKA – 0.5ml per test or 1.5 litre / month.

·       2-Channel systems available as an option.

·       Note: Analyser can be calibrated in range < 1mg/l to better the ASTM D1426 98 Method B Precision and Bias Data at low concentrations.

·       Maintenance and Servicing every 6 months – approx. 1-2 hours on site. Sensor replacement BNC fitting, Tubing if contaminated, 3*Solenoid routing valves, Syringe and Syringe Protection Winding.

·       Analyser can also be set up for Colorimetric and Titration Analysis for up to 20 compounds. For more data please visit

The BEST Analyzer Solution for Monitoring Water - Instran