Shock Talk

Why do you need shock? It is not because we want to sell you more chemicals… It’s the opposite actually, we want to sell you less! And you want to use less! Shocking your pool weekly ensures the most efficient chemical usage. 

Proper shocking practices are the least talked about and least understood element of pool chemistry. It is slightly more confusing than normal Chlorine and PH testing, so many pool owners gloss over when it is mentioned. The test for it does not say “shock level”, it is more complex than that. So let's simplify the math: When we shock a pool we are hoping to make combined chlorine a zero

Free Chlorine + Combined Chlorine = Total Chlorine 

Free Chlorine is the amount of chlorine in the water that has not yet come into contact with contaminants, so it is still free to kill things. Think of it as a soldier with a loaded weapon. 

Combined Chlorine is the amount of chlorine in the water that has already exhausted its killing potential, so it is not helping to sanitize the water anymore. Think of this as a soldier with a weapon that has no bullets. Oftentimes when you pool water smells like too much chlorine or even has a rotten smell, it is because of the presence of high combined chlorine. (1)

Total Chlorine is the sum of these two numbers. Think of this as the amount of soldiers on the battlefield. We can test total chlorine and we can test for free chlorine but we cannot test for combined chlorine. So we reconfigure the equation to look like this:

Total Chlorine - Free Chlorine = Combined Chlorine  

Having a high combined chlorine level means that our chlorine is not free to do its job and kill the bacteria in the pool. If our soldiers do not have ammo then we do not have an effective fighting force.  In order to keep our families safe, we need a fully supported army of chlorine. One part per million of free chlorine can do the work of 25 parts per million of combined chlorine. (1) In order to maintain a clean pool we recommend 1-3 ppm free chlorine, so based on this math we would need as much as 75 ppm combined chlorine to keep our pool clean. 

So let's restock our ammo, and get rid of our combined chlorine. We do this by shocking the pool, or adding an oxidant to the water. To get technical, when electrons are pulled from the contaminants in the water, those contaminants are essentially broken down at a chemical level. In other words they are completely ‘oxidized’ out of the water. (2) 

Once these contaminants are gone the chlorine is once again “free” to do its job and return to killing more bacteria. 

 When shocking a pool we have three main chemical options: 

1: Calcium Hypochlorite

2: Sodium Di-Chlor

3: Potassium Monopersulfate

Calcium Hypochlorite is an incredibly potent chemical and a chlorine product. It is by far the most powerful and concentrated of these three chemicals. For this reason we typically reserve the use of this compound for extreme situations. Usually incredibly high combined chlorine or excessive amounts of algae present in the water. Calhypo will result in very high chlorine levels for 4-8 hours and it is not recommended to swim on the same day as addition of the chemical. (2)

Sodium Di-Chlor as you may have guessed is also a chlorine product. It is used to super-chlorinate the water. To get rid of combined chlorine we need to raise the free chlorine to a level that is 10x the combined chlorine level. This means that more of this chemical is needed to achieve the goal. This can be an issue because di-chlor is ‘stabilized’ with another chemical called conditioner. While some conditioner is good, it is possible to have too much. So one must be careful to not overdose the pool with di-chlor.  Packaging of these different compounds can be virtually the same, so it is important to read the chemical name on the product and match it exactly with what you  need.

Potassium Monopersulfate is actually an oxygen based chemical and does not contain chlorine or conditioner. This means it is much less harsh and makes it harder to overdose the pool. It is a quick dissolving chemical and only requires about an hour before you can return to swimming. For those of us with sensitivity to high chlorine the use of this chemical is a no brainer. We prescribe this chemical in most cases because of these factors and it is our recommendation for use most commonly. (2) In spas we recommend adding this product after each use. 

That’s it, that is the summary of what you need to know about shocking a pool. The only thing not covered here is dosing, but that will vary from pool to pool based on specific conditions. Please consult with a pool professional to have your water tested and help determine the proper dosing for your situation.