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pH and SH: A Complete Guide by Ryan Cash

Ryan Cash

UAMCC Associate Member
Does the pH of my surfactant matter? Will Bleach still kill plants if it sits on a roof for a long time? How does Sodium Hypochlorite work? Why is it so hard to clean a roof in the wintertime?

I think these are some pretty common questions a lot of us have as we're getting into exterior cleaning. Let's dive into them, starting with taking a look at what exactly bleach is and how pH and temperature effects it's performance.

Conditions that affect Sodium hypochlorite stability and performance.

  1. pH—affects how bleach works.
  2. Heat—heat decomposes bleach faster but also makes it work faster. Depending on pH, different compounds are made by heat with different effectiveness.
  3. pH and heat—this combination dictates how aggressive chlorine bleach will be.

There are many other factors as well, and not all chlorine bleaches are created equal, but we’ll stick with what is important to an exterior cleaning application.

1. pH and Chlorine bleach (Sodium hypochlorite)

1. At pH above pH-12.00, Chlorine bleach contains Sodium Chlorate and Sodium hypochlorite at equilibrium. Sodium chlorate is a strong oxidizer, and eventually decomposes into oxygen and table salt (Sodium chloride). Sodium hypochlorite decomposes into water, table salt and oxygen.

Chlorine bleach at pH-12.00 and above is quite stable and is a moderate oxidizer.

2. At pH-11.00, Chlorine bleach begins to decompose fast into Hypochlorous acid and hydrogen. This decomposition makes the Chlorine bleach more acidic, thus continuing to lower its pH making more Hypochlorous acid and Hydrogen faster. Hypochlorous acid is a strong oxidizer. Stronger than the Chlorine bleach (Sodium hypochlorite) and Chlorine gas itself. So as the pH drops, the bleach starts becoming more unstable.

Chlorine bleach between pH-11.00 and pH-8.00 is a very strong oxidizer. Stronger than at pH-12.00 and above.

3. Below pH-5.00, Chlorine bleach decomposes into Hydrochloric acid (swimming pool acid), water and Chlorine gas. At this pH, Chlorine bleach becomes ineffective as an oxidizer, and toxic due to the formation and release of Chlorine gas.

In conclusion, never mix Chlorine bleach with any acid (or ammonia) because of the formation of toxic Chlorine gas. And it becomes ineffective as an oxidizer.

pH and Heat

Heat accelerates rate of reactions (Bleach decomposition and oxidation).

As a rule-of-thumb in Chemistry, for every 10 degrees increase in temperature, the rate of a reaction (Chlorine bleach decomposition and oxidation) doubles or triples, depending on the type of reaction.

We now can see that depending on pH and how much heat, Chlorine bleach can become either damaging or ineffective.

For example, above 140 F, Chlorine bleach decomposes too fast to work effectively. Additionally, typically when SH drops below 40 F, it's oxidation process slows down too much to be effective.

The trick for using Chlorine bleach safely and effectively is to use the right pH at the right temperature for the type of job be we want to accomplish.

If we properly balance the pH and temperature for our application, Chlorine bleach will do its job (oxidation) at a specific time and decomposes during that time into table salt, oxygen and water. Or table salt, water and hydrogen—depending on the pH chosen for the job.

For exterior cleaning, having the Chlorine bleach at pH above pH-12.00 and above, and at a temperature between 70 F and 130 F would be ideal.

This can be be achieved by storing bleach in a stable temperature and using the proper surfactants to help maintain a high pH when cleaning.

Now, let's talk run off and plant health as the bleach degrades:

About 1% of the bleach in a gallon is Caustic soda, and that is a strong reactive base.

The pH of the bleach is not coming from the bleach itself (Sodium hypochlorite), but from the Caustic soda added to bring the pH up to above pH-12.00. Usually, bleach is pH-13.5

So, even though the bleach may be completely decomposed by reacting with the organic matter, sunlight, time, etc. as long as the pH is 11.00 and above, the Caustic soda will burn the plants.

Plants are also very pH sensitive in the soil, and very small changes will affect plants.

The same goes for the table salt created with the bleach decomposition. Plants don’t like sodium salts like table salt in the soil.

Since pH is a measure of concentration of an acid or a base, by adding water, we dilute the pH. But there’s an important issue with diluting pH. pH increases and decreases logarithmically. For example, pH-9.00 is ten time stronger than pH-8.00. pH-10.00 is twenty times stronger than pH-8.00 and so on. Thus, pH-13.00 is 50 times stronger than pH-8.00

This means that it takes a lot of water to drop pH-13.00 down to pH-8.00. Maybe 50 time more water than the 5 times it would take to dilute the bleach concentration itself.

So whats the solution? Dilute! Before during and after a cleaning makes sure that you’re soaking plants with water. Use the least amount of SH possible when cleaning and avoid run off/overspray as much as possible.

The absolute best way to protect plants is to avoid getting SH on them in the first place!