‘TomToo’ very recently — as in a few minutes ago — asked about chlorine testing and which chlorine test he should use:

“Saw your chlorine sale announcement and have a question. The water in our beach house stinks really bad of chlorine when we first get there and we want to test to make sure it’s safe to use but you have free and total chlorine strips. How do you know which to use?”

Thanks for your inquiry, TomToo, and believe us when we say others share your confusion when it comes to knowing which type of chlorine they need to test for.

Lucky for you, though, we have simple answer if your concern deals only with worrying about having too much chlorine in the water. You should test for total chlorine.

The USEPA set the Maximum Contaminant Level for chlorine in drinking water at 4 ppm (parts per million) and that number takes into account ALL of the chlorine in a water sample, meaning both free and combined chlorine.

Moral of the story?

If you need to know the FULL amount of chlorine in a drinking water sample, testing for total chlorine makes the most sense.

At this time we carry 100 packs of DPD-1, DPD-3 and DPD-4 in the Water Testing Blog Test Kit Store.

Differences between DPD-1, 3 and 4?

For the answer to that question, we suggest that you take a look back at one of our earlier postings called What is DPD?. Not to brag, but we think that article does a decent job of answering the question.

To sum things up, though, below you will find the most common uses for DPD-1, DPD-3 and DPD-4:

• DPD-1 — Used by itself in a water sample to determine the free chlorine concentration in the sample.

• DPD-3 — Used after DPD-1 in a water sample to determine the sample’s total chlorine concentration.

• DPD-4 — Used by itself in a water sample to determine the total chlorine concentration in the sample.

So, before purchasing DPD, you may want to verify the type of chlorine testing that you will need to do — else run the risk of accidentally purchasing the incorrect DPD reagent(s)!

Where to purchase DPD?

As we stated earlier, you can purchase 100-packs of DPD-1, DPD-3 and DPD-4 from our Water Test Kit Store.

As for where to find other forms of DPD reagents, at this time we know of places to purchase two other forms of DPD: tablets and powder.

Need a new meter to test chlorine levels, too? We suggest looking at the eXact Chlorine Photometer available on the FilterWater.Com web site. It offers the reliability of DPD-based chlorine testing and the convenience of not having to match colors against a color chart.

Want accurate chlorine test results but don’t want to deal with DPD reagents and meters? Take a look at the SenSafe Free Chlorine water Check product currently available in our Water Test Kit Store. It bears USEPA Approval for use in municipal water testing and offers superior resistance to monochloramine interference that can sometimes affect the results obtained when testing for free chlorine residual using DPD test reagents.

Free (available) chlorine refers to the concentration of chlorine molecules residing a water sample that have not, yet, oxidized contaminants. As a general rule you want to maintain a free chlorine residual in a body of water or water source.

Therefore, if a water sample has a total chlorine residual but no free, available chlorine, then the sample may or may not be ‘safe’. Some public (municipal) drinking water systems use only combined chlorine Why would they do that?

It seems counter-intuitive that a public water systems would choose combined chlorine over free chlorine given free chlorine’s superior disinfecting (oxidizing) properties, but monochloramines do still have oxidizing capabilities and they do not create compounds (disinfection by-products, tri-halomethames, TTHM’s, etc.) that may cause cancer.

So… if you want to know how much free chlorine you have versus how much combined chlorine you have, simply subtract the free chlorine concentration from the total chlorine concentration:

(Total Chlorine) – (Free Chlorine) = Combined Chlorine

For pool water you can use a test strip like the Pool Check 6-Way to determine the free and total chlorine concentrations of a sample.

For drinking water you can use test strips like SenSafe Free Chlorine Water Check to determine the free chlorine concentration of a water sample and SenSafe Total Chlorine Water Check to determine the total chlorine concentration of a water sample.

OR, for an even easier test procedure, you can use the WaterWorks 2 Free & Total Chlorine Test Strip which tests for both free and total chlorine at the same time.

As a general rule, if your water has a chlorine smell to it, then you most likely have, at the very least, chloramines (combined chlorine) present. This does not, however, rule out the possibility that your water also may contain a free chlorine residual, too.

For the record, health officials and scientists agree that chlorine gas does NOT do the body any favors and in too great a quantity can cause lung and eye irritation… so we suggest not performing the ‘sniff test’ if you can help it.

Taste test: Not all water that contains chlorine will smell like a swimming pool. While combined chlorine (chloramines) has a strong odor associated with it, free chlorine does not. Therefore water that may not stink like chlorine may still possess a strong chlorine taste.

We do NOT advocate the taste test method for any reason. We mentioned it just so that we could talk about why water can lack a strong chlorine smell and yet still have a strong chlorine taste.

Chemical test kit: We know of many different test kits that the general public can use to determine the free and/or total chlorine concentrations in their drinking water.

“Wait… Did you just talk about two different types of chlorine?”

Yes, because as we discussed during a few moments ago, one can have free chlorine and/or combined chlorine in their water. Typically free chlorine acts as the main sanitizing and/or disinfecting agent in a chlorinated water supply and once the free chlorine finds an organic contaminant to ‘kill’, it becomes entangled with the contaminant and forms a combined chlorine molecule.

Therefore, it stands to reason that a chlorinated water supply may contain both free and combined chlorine at the same time if the free chlorine molecules have successfully neutralized all biological contaminants and the effort did not require the use/exhaustion of all free chlorine molecules present in the water supply.

Getting back to chemical test kits, please…

Now that we have established the existence of different types of chlorine which may or may not exist in a water supply, we will now discuss different ways of testing for chlorine in a water supply.

• Test Strips — Perfect for field testing because they require the user only to dip the strip into the water and compare the color of the test pad/area to a color chart that came with the package of test strips. Products exist for the detection of both free and total chlorine. People can even get a single test strip to test for both at the same using a product called WaterWorks 2
• ‘Wet’ Chemical Test Kits — Considered by most as the traditional, tried and true method for testing the quality of water. Most ‘wet’ kits typically require dissolving DPD as a tablet, powder or liquid into water samples and either comparing the colors of resultant solutions to color charts or adding drops of an additional chemical until a second color change reaction occurs.

  Simpler ‘wet chemistry’ test kits for chlorine (i.e. 2-way pool water test kits) use a different chemical called OTO (Orthotolidine) and detect total chlorine (free chlorine + combined chlorine = total chlorine).

  Problem w/ using OTO: No way exists to use OTO in a way that will allow a person to determine the free chlorine concentration.

  The DPD reagent system does, however, allow for differentiation between the two if a water sample has DPD-1 added first, gets analyzed, and then has DPD-3 added… and then get analyzed again.

• Water Testing Meters — Most water testing meters for chlorine (i.e. the eXact Chlorine Photometer) require the use of DPD, a chemical indicator which turns a shade of pink/red in the presence of chlorine molecules. DPD-1 indicates the presence of free chlorine and a combination of DPD-1 plus DPD-3 indicates the presence of free and/or combined chlorine (total chlorine).

So… Which method of chlorine testing will work best for you? Ask yourself the following questions:

1) Do you need to test for free chlorine, total chlorine or both?

2) How much accuracy and precision will you need in your results?

3) If performing testing for reporting purposes, what do the rules and regulations governing your profession say you must use as a testing methods?

Once you have answered those three questions you will have a clear idea of whether you should use chlorine test strips, wet chemistry test kits for chlorine testing or chlorine testing meters to determine chlorine concentrations in your water.