What is Ammonia?

Ammonia is a simple inorganic molecule with the formula NH3. It has many important industrial and agricultural uses. Ammonia’s  low molecular weight (17.031 vs water 18.01528) makes it useful a detection tool  but safe handling is required since it can be toxic. Two simple and inexpensive ammonia leak detection methods are discussed.

See our other blog for Water Leak Detection.

Ammonia Leak From a Fridge?

Ammonia is an efficient refrigerant but its toxicity limits it to industrial applications such as freezing the ice in hockey rinks.  Phenolphthalein test paper changes from colorless to pink/fuchsia in the presence of ammonia. The test papers are first dipped in distilled water & then exposed to the suspected gas leak. The test result is a qualitative yes or no answer although the speed of the color change is proportional to the leak rate.

Ammonia Refrigerant Leak Detection

Phenolphthalein is a chemical indicator that changes pH in the presence of ammonia & is useful for the detection of ammonia leaks in refrigeration systems.

Ammonia & Phenolphthalein

Phenolphthalein can indicates the presence of ammonia by changing colors at an alkaline of ~pH 11.6 .  Test papers infused with it need to be moistened with water first so that airborne ammonia can dissolve onto the strip.


Ammonia Leak Detection Cloth

The ammonia molecule’s size allows it to get through holes smaller than water can.  A cloth infused with the chemical indicator bromothymol blue can help pinpoint leak location much more accurately than a simple phenolphthalein test paper.

More demanding applications such as those in the nuclear industry use even smaller compounds such as tritium which is radioactive & needs specialized equipment.

Lab Glove Box

Glove boxes are used for handling dangerous materials that are radioactive, chemically toxic or biologically active such as viruses.

Ammonia leak detection cloth  can be used to detect leaks in equipment designed to be sealed or air-tight, such as incubators, pharmaceutical isolators, glove boxes, etc.

A July 2012 article published in the journal Clean Air and Environment1 discusses the difference between leak rate and leak detection and the ISO classification of leak tightness based on volume loss per hour2, 3.   It  describes methods for conducting leak detection including ammonia leak cloth.

Ammonia leak cloth works on the principle that the cloth will turn from yellow-orange to blue in the presence of a very small amount of ammonia gas.   Pharmaceutical isolators can be tested with a small dish of liquid ammonia placed inside the isolator which is then pressurized.  The ammonia leak detector cloth is placed at any suspect seal point & if ammonia “leaks” through the seal the cloth will turn blue.

Ammonia leak cloth is excellent for leak tests since it is a very small molecule that can pass through very small holes.   Liquid ammonia is readily available, inexpensive, easy to clean up and the indicator chemicals in the cloth are very sensitive.   An added benefit is that the test is reversible.  The cloth will revert back to the original yellow-orange color once it is removed from the ammonia source.

Ammonia Leak Detection Cloth

Bromothymol Blue Molecular Structure

Ammonia leak detection cloth uses Bromothymol blue as the indicator. Click on the image below for more information.

Bromothymol Blue Molecular Structure Model


1)      Coles, Tim (2012).  Leak Rate Measurement for Pharmaceutical Isolators:  Practical Guidance for Operators and Test Engineers.   Clean Air and Containment Review Issue 11, pages 8-12.

2)      ISO Standard 10648-2:1994.  Containment enclosures—Part 2:  Classification according to leak tightness and associated checking methods.

3)      ISO Standard 14644-7:2004  Cleanroom and associated controlled environments—Part 7:  Separative devices (clean air hoods, gloveboxes, isolators and mini-environments).

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