Leak Detection: Water, Ammonia, H2S

Leak detection may trigger thoughts of NSA Internet snooping or community swimming pools but this blog is devoted to the earth needs of homeowners or safety & product quality assurance departments who discover water & other leaks & need to find & fix them.

Water Leak Detection

Water can get into places you can’t see or reach & do serious damage if left unattended. Special paper treated with water “sensing” chemicals can offer a simple & inexpensive solution for finding roof or dishwasher leaks, water incursion into car doors, lab equipment & much more.

As might be expected with water being the “universal” solvent that it is, no one solution works for every problem but the 3 three we offer cover most cases.

Water Leak Detection-Cobalt Chloride

Cobalt chloride sheets are the most commonly used. CoCl changes from blue to pinkish white at relative humidity above 55% as can be seen in the image below. At 99% it changes very quickly & with liquid water it is immediate.

You can lay out or tape sheets as needed to find single or multiple leaks in ceilings, under appliances or in packaging. It can also be cut into strips & used to find moisture in hard to find places like cracks & crevices in circuit boards or concrete blocks.

Cobalt chloride sheets main drawback is that the change is reversible. Traces of water revealing roof leaks will disappear if a heat wave beats you to the inspection.

Cobalt Chloride Moisture & Water Leak Detection Paper

Chlorophenol red test sheets color from yellow to bright red on contact with water as seen below or when used with a vacuum assembly can aid in spot plate leak testing.

Chlorophenol red, however, doesn’t change color if the pH of the water is less than ~5, so  it is not recommended for acid rain studies.

Chlorophenol Red Water Leak Detection Indicator Paper


Chlorophenol Red Molecular Structure

Chlorophenol Red is in water treatment processes. For chemistry students, it is an example of a C3 point group. Click on the image to read more about this molecule.

Chlorophenol Red Molecular Structure Model

Water Leak Detection-Universal pH Paper

For both a permanent record & detection of alkaline or acidic water, the best choice is the universal pH indicator water test paper sheet.

Universal pH Paper Water Leak Detection

 Gas Detection

There are of course many gases which leak & most need far more sophisticated methods than simple test papers. However, the methods listed below have many practical applications.

Ammonia Leak Detection Cloth

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 further describes several different methods for conducting leak detection activities with a brief discussion of 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.  To test equipment such as pharmaceutical isolators a small dish of liquid ammonia is 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 an excellent choice for conducting leak tests since ammonia is a very small molecule that disperses easily and can pass through very small leaks.  Liquid ammonia is readily available, inexpensive, easy to clean up and doesn’t take much to run the test since 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


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 envirnments—Part 7:  Separative devices (clean air hoods, gloveboxes, isolators and mini-environments)

Bromothymol Blue Molecular Structure

Ammonia leak detection cloth uses Bromothymol blue as the indicator.  It is another example of a “propeller” rotation C3 point group. Click on the image below for more information.

Bromothymol Blue Molecular Structure Model

Phenolphthalein Test Paper-Ammonia Gas Leaks

Chlorofluorocarbons (e.g. Freon) were largely banned as refrigerants when they were implicated in ozone depletion in the earth’s atmosphere. Ammonia has been used in its place but as it is a very small molecule it can easily leak through small gaps in gas line fittings.

Phenolphthalein test paper is a well known indicator which changes color in response to ammonia but works best when the strip is wet. The image below shows a wet strip on the left compared to a dry one in the middle.

Phenolphthalein Test Paper Ammonia Refrigerant

Phenolphthalein Molecular Structure

Phenolphthalein is a commonly used indicator & another example of a C3 point group. Click on the image for more information.Phenolphthalein Molecular Structure Model

Lead Acetate Paper-Hydrogen Sulfide

Hydrogen sulfide smells like rotten eggs & can be produced by bacterial breakdown of organic matter in the absence of oxygen.  It is toxic at concentrations of ~300ppm or higher.

Lead acetate infused test strips can be used to indicate the presence of hydrogen sulfide gas at concentrations as low as 5ppm. The strip works best when wet &  is an inexpensive safety method for sewer or oil well drilling workers.

It can of course also be used at home to test drinking water.

Lead Acetate Test Strips Detection of Hydrogen Sulfide 

Article Name
Leak Detection of Ammonia & Hydrogen Sulfide Gas
Leak detection can involve identifying simple water leaks to potential dangerous chemicals such as ammonia & hydrogen sulfide. In many cases this can be achieved with simple, low cost methods which are outlined here.
Indigo Instruments