Does the pH of a stormwater sample change after 15 minutes?

Written By: Doug Ruhlin | May 16, 2014

Time to Read 6 Minutes

Learn why so many permits require pH analysis within 15 minutes of sample collection.

We've been diving into pH in stormwater lately because we've been getting a lot of questions on it. It can be somewhat confusing, so hopefully we're helping to ease the problems with testing the pH in your stormwater sample with these posts.

In most NPDES stormwater permits, you're required to monitor your stormwater discharges for pH. The USEPA has established specific protocols and standard methods for how to monitor the pH of a stormwater sample. One criteria of these methods is the time limit set for which you can perform the analysis, which is 15 minutes. As soon as you obtain your stormwater sample to be analyzed, the clock starts and you have 15 minutes to run the test. As a State Certified Environmental Laboratory, we know just how fast those 15 minutes disappear.

Here are the reasons why 15 minutes matter, and how to make them count!

pH of Stormwater Samples Changes After 15 Minutes

So why 15 minutes? USEPA Standard Methods.

The specific testing method protocol for testing the pH of a stormwater sample is SM 4500-H-B-11. This method describes how to measure pH and since most NPDES permits require you to monitor for it, you’re going to have to comply with these methods. It tells you how to calibrate your meter, how to obtain your sample, and that you only have 15 minutes to perform the analysis.

But do 15 minutes really matter of a pH test?

The answer is yes! The USEPA set 15 minutes as the window to perform a pH analysis of a liquid because the pH of a liquid can actually change over time. The longer a sample sits reacting with the air around it, the greater the chance the pH value of that liquid will change.

It’s a simple chemical equation. The air surrounding us contains carbon dioxide (CO2), and when given the chance to be absorbed by a liquid (H2O), a weak acid will be produced, carbonic acid (H2CO3).

Carbonic acid changes the pH value of any liquid. More interaction time means more change, which is why the USEPA set 15 minutes for the time limit to perform the pH test.

What you can do to make the most of 15 minutes!

Have your equipment set and ready to go. Since you only have 15 minutes it is very important to be prepared to perform the analysis. We strongly recommend that you have your pH meter calibrated before obtaining your sample. We also recommend you use an accurate and reliable pH meter. The calibration of a pH meter can take upwards of 15 minutes alone, so trying to squeeze that in after you get your sample while the clock is ticking is not a good idea. Have your meter calibrated and have any paperwork or forms ready to be filled out as you perform the analysis.

The clock starts ticking the second you collect your sample, so make sure you know what you're doing! After all, that 15 minutes goes quick, and you won't have time to refresh yourself on how to calibrate and work your meter once you've collected your sample.  Online pH training can be a great way to familiarize yourself with exactly what you'll need to do to perform a pH analysis, so you can be ready to go the next time a storm hits!

And practice. Practice practice practice. Everyone has heard that before, but practice performing the analysis, either by sampling random liquids, or learning how to use the meter through the calibration process, will give you the ability and knowledge to perform the test fast and smoothly each time. Since 15 minutes is all you get, learn to make each minute count!

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Need help with stormwater permit compliance?

For a lot of folks out there, this is a pain to deal with. After all, you have a business to run! If you're overwhelmed, you're not alone. RMA has been actively involved in helping companies get and stay in compliance with stormwater permits across the United States since our founding in 1992. Long story short, we know the ins and outs of the environmental problems industrial and commercial facilities face and can help you get into compliance with stormwater permits and other applicable environmental laws and regulations, ensuring your business stays out of trouble and in compliance.

Our staff members have been on-site at thousands of operations across the country, so when we say we've seen it all and done it all, we mean it. We've helped everyone, from globe-spanning, multi-national organizations to small "mom & pop" operations. No matter your size or location, we'd love to learn how we can help.

So, if you're having any type of issue at your facility and need the help of an environmental consulting firm with a proven track record, reach out. Even if we can't help, we’ll do our best to steer you in the right direction. Feel free to contact us at, click here to contact us, or give us a call anytime at 888-RMA-0230 to learn how we can help your facility deal with stormwater permits and other environmental regulations.

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