What You Should Know About ISE Measurements of Ammonia

While direct measurement of ammonia is one of the most popular ion selective electrode (ISE) measurements performed today, it is also one of the most challenging. This technical bulletin will discuss many of the most common problems and offer suggestions to overcome these difficulties.

How does the ammonia ISE work?

In order to understand the limitations of ammonia measurement, it is important to gain a fundamental understanding of the how the ammonia ISE works.

In water, ammonia exists simultaneously as two forms: as free unionized gaseous ammonia (NH₃) and as the ionized ammonium ion (NH₄⁺). The ratio of NH₃ to NH₄⁺ that exists in water depends mostly on the pH and to a lesser degree the temperature;

At 25°C a pH 7.25 sample is almost all NH₄⁺

At 25°C a pH 9.25 sample is equally NH₃ & NH₄⁺

At 25°C a pH 11.25 sample is almost all NH₃

Dissolved NH₃ is gaseous and passes thru the hydrophobic membrane of the ammonia ISE. The pH change of the electrolyte solution on the other side of the membrane caused by the diffused gas is sensed by the inner body of the ammonia electrode—a pH electrode. The pH change is relative to the amount of dissolved NH₃ present and can be measured with a pH or Ion meter capable of mV readings or, better yet, direct ion concentration.

Why is ammonia measurement more difficult than pH measurement?

Unlike pH, users often prepare their own ion calibration standards—a major source of error.

Ionic strength adjuster (ISA) is required for calibration solutions and samples.

Ammonia calibration standards become unstable once ISA is added.

Ammonia readings often take several minutes while pH measurements are generally much faster. Low ammonia readings can take many minutes.

Temperature compensation for pH is easily accounted for. However, it is infinitely more difficult with ion measurement and generally not used. Ion samples which differ in temperature from the calibration standards will subsequently differ in reading.

Ammonia measurement requires more electrode maintenance than pH. It requires filling and changing the electrolyte solution as well as occasional membrane replacement.

The range of interest for ammonia is usually <5 parts per million.

Ion measurement isn't nearly as repeatable or precise as pH—especially ammonia.

General DOs

Do prepare calibration standards using serial dilution of a certified standard for best results. Calibration standards prepared independently from the same certified standard are not as repeatable. Higher precision can be attained by weighing (gravimetric) standards instead of using a volumetric flask (or much worse—a graduated cylinder!). Consider weighing 100.00 grams of DI water vs. filling to the 100 mL line on a flask using your eyeball. When using volumetric measurements it’s a good idea to see how much variation you may have. Take what you think is 100 mL of water measured by volume and weigh it—is it 99.2 grams? 101.4 grams? Use only deionized (DI) or comparably clean water for dilution and standard preparation.

Start with 1000 ppm certified ammonia calibration standard
To prepare 1L solutions of:	Option 1 Good	Option 2 Serial Better	Option 3 Serial Gravimetric Best
100 ppm	100 mL 1000 ppm 900 mL DI water	100 mL 1000 ppm 900 mL DI water	100.0 grams 1000 ppm 900.0 grams DI water
10 ppm	10 mL 1000 ppm 990 mL DI water	100 mL of 100 ppm 900 mL DI water	100.0 grams 100 ppm 900.0 grams DI water
1 ppm	1 mL 1000 ppm 999 mL DI water	100 mL of 10 ppm 900 mL DI water	100.0 grams 10 ppm 900.0 grams DI water

Do prepare at least two serial dilutions ideally a decade apart (factor of 10, such as 1, 10 and 100, or 5, 50, 500, etc). Keep in mind that standards below 1 ppm are more difficult, requiring long stabilization, and are not as linear (reduced slope). To help overcome low standard and sample measurement, try diluting the filling solution 1 part to 9 parts DI water for faster response.

Do begin calibrating with your lowest standard and then your next lowest standard, etc., finishing with your highest standard.

Do keep track of the corresponding mV values of your calibration standards during calibration. The mV values should decrease by 50-60 mV or so with each tenfold increase in concentration. This mV difference is referred to as “slope.” Slope can be expressed as a percentage of 59.16 per decade, or as mV value. Note that when three standards are used i.e. 1, 10, and 100 ppm, there are two slopes; between 1 & 10 and 10 & 100. Typically, an electrode will have similar slopes—it’s not likely that an electrode would be 98% slope between 10 & 100 and only 82% slope between 1 & 10. This would suggest a problem with the 1 ppm solution rather than an inherent electrode problem. Note that electrodes may not be linear at extreme ends of the measuring range.

Do keep the sample conditions as close as possible to those of your calibration standards in every aspect. Temperature, volume, stirring, container material and shape, and immersion depth each can influence the results.

Do use the same % addition of ISA. Usually 2% (2 mL ISA per 100 mL) in both standards and samples. Keep in mind that samples preserved with acid will require more ISA than those that aren’t preserved. Periodically test the pH of some samples to ensure it is high enough after ISA. Wait until you are ready to measure or calibrate before adding ISA. Standards and samples with ISA added should not be used after two hours. Addition of (ISA) essentially dilutes. If you dilute samples by varying amounts or an amount different from the calibration standard, you’ll need to adjust your readings correspondingly.

Do take care of the membrane. The glass pH electrode should gently protrude against the membrane. However, it is thin and can be easily torn if it comes in contact with the bottom of a beaker or stir bar, or if wiped dry. The good news is that the membrane only requires replacement when damage occurs. When it fails, it is often obvious. A damaged probe may drip electrolyte from the tip. Additionally, the mV values will remain very negative in all solutions (a result of the high pH value from ISA). Membranes should generally be replaced according to performance, not a schedule.

Do keep your electrode filled with the correct electrolyte and change it at regular intervals. Note: filling solution (0.1 Molar NH₄Cl) is added to many 12-mm diameter electrodes by unscrewing the body (not the cap).

Do disable the Hold or Auto Read function of your instrument due to the slow response of gas sensing electrodes. If not disabled, automatic calibration of standards and measurements of samples are usually accepted too quickly—often resulting in lower than expected slope. With slow responding electrodes, instruments have a hard time distinguishing when a reading is “stable.” Use best judgment instead.

General Don’ts

Don’t become too preoccupied with slope. The goal is to get an accurate, repeatable reading. Readings are always more important than the slope %. Generally, a slope of 90-102% is acceptable. After calibration, test calibration standards as if they were unknown values. Select a reasonable variation you can live with and incorporate this check into QC checks. If an electrode goes from 97% to 93%, this is not cause for immediate concern—especially if the electrode passes the “reasonable variation” check in known standards as previously discussed.

Don’t try to utilize too many standards—especially if they are similar concentrations! Less IS more. Attempting to calibrate with too many standards (i.e., 0.1, 0.2, 0.5, 1, 2, and 5 ppm!) will waste much time and will not improve accuracy. Choose one calibration standard above and one below the expected sample range. Try to incorporate factors of 10 (1 ppm and 10 ppm, 0.5, 5.0 and 50 ppm, etc).

Don’t assume a problem is meter-related. If the meter turns on, it likely works fine. If the same connection of your meter works with a pH electrode, the problem isn’t meter-related. Similarly, don’t assume your TV is broken when you lose the picture during a thunderstorm! Meter problems are not usually intermittent.

Don’t give up! Contact a technical support representative who can help. Gather calibration data, model numbers, and serial numbers, along with any other relevant information. When calling, be prepared to calibrate during the call and have your equipment and a notepad handy.