If you've grown hydroponically then you've probably noticed that when you initially set up your reservoir and adjust your pH levels they will slowly rise throughout the week.
Why does pH rise as my water sits in the reservoir?
Before we can answer this it's important to know the definition of pH. Straight from the Merriam-Webster dictionary, pH is a measure of acidity and alkalinity of a solution that is a number on a scale on which a value of 7 represents neutrality and lower numbers indicate increasing acidity and higher numbers increasing alkalinity.
A simple way to define pH is how acidic or alkaline (basic) your water is.
It's important to remember that if a solution has a low pH then it has more acid in it and a solution with a high pH has less acid in it.
Now that we know what pH is let's get down to business on why pH rises.
It's fairly common knowledge that most things are more soluble when added to hot water rather than cold but are you aware that the opposite is true for gases? In cold water, gases are more soluble.
So, when your tap water splashes through and flows through the pipes at a cooler temperature of around 50F, you have a lot of Co2 and oxygen that dissolves into the water.
When Co2 dissolves in water it forms carbonic acid.
As Jesse Pinkman from the famous chemistry show Breaking Bad would say, "Science b*tch!"
Once you put the tap water in your reservoir it will begin to rise to room temperature which ideally is around 72F in bloom. So, our tap water is warming up in our reservoir and as this is happening the carbonic acid starts to turn back into Co2.
pH levels begin to rise as carbonic acid turns into Co2. The Co2 begins evaporating out of your water and your pH balance will start to level out at a higher number as the water equilibrates with its surroundings.
Here's a tip:
Have a reservoir full of tap water that sits for a few days before you add any nutrients or pH balance chemicals. This will allow the pH balance to level out and some of the chlorine to evaporate (some types of chlorine do not and HighTimes has a great article on this here).
Other important things to remember when dealing with high pH.
Your plants absorb different nutrients at different pH levels. Generally, it is ideal to keep your pH levels between 5.5 and 6.5 as the majority of the nutrients are absorbed at those levels.
It's ok if your pH balance drifts between 5.5 and 6.5!
It's also important to note that abrupt shifts in pH levels can kill your plants.
So, when dealing with plants we are going to be careful and try to use as little pH up and pH down as possible while attempting to keep the levels stable. The pH levels are going to fluctuate naturally and as long as they aren't above or below 5.5 or 6.5 you are most likely going to be fine.