The Deep End

DEAL OR DUD? READ THE DATE CODE BEFORE YOU BUY LIQUID CHLORINE

Big Box Store Pool Supply Wake-Up Call: Read the Date Codes

DFW pool owners, this is your reminder that a “great deal” on liquid chlorine may not be a great deal at all.

This jug of Pool Essentials liquid chlorine was recently purchased from a big box store in a land far, far away. At first glance, it looks like a bargain.

Now let’s get educated.

Pool Essentials liquid chlorine uses a date code format of:

YY DDD

That means the first two digits are the year, and the next three digits are the day of that year.

So, for example, a code showing 24 174 means it was bottled on the 174th day of 2024. That chlorine was bottled at 10% strength when it was fresh.

Here is the problem: liquid chlorine loses strength over time.

A good rule of thumb is that liquid chlorine has about a 6-month half-life, with a suggested use window closer to 3 months. After 6 months, it can lose roughly half its strength. Six more months, it loses half of what remains. This continues over time.

So liquid chlorine that started at 10% can break down roughly like this:

Fresh: 10%
6 months: 5%
12 months: 2.5%
18 months: 1.25%
24 months: 0.625%

At that point, the plastic jug may still have some value, but the liquid inside is close to worthless for actually sanitizing your pool.

This is why date codes matter.

Big box stores often buy huge quantities. Product may sit in distribution warehouses, then go through more warehousing, then sit in the back room of a store before it ever reaches the shelf. Eventually, you buy it thinking you scored a deal — but you may be paying for weak, old chlorine. If it’s fresh, then it’s a deal. If it’s old, you are just helping them to not fill up their dumpster.

Local swimming pool supply retailers typically receive chemical deliveries more frequently, and their liquid chlorine is often bottled at 12.5% to 13%, meaning it starts out about 25% stronger than 10% chlorine. When stock is properly rotated, you are not only getting a stronger product, but usually a much fresher one.

The takeaway:

Read the date codes before you buy liquid chlorine.

Cheap chlorine is not cheap if it has already lost most of its strength. A fresh, stronger product will usually do a better job, require less guessing, and save you frustration trying to keep your pool clear.

-Deep End Frank