pH Change

pH Change Meaning

So what is pH, and what does it matter when the pH changes?

So what is acidity, or lack of acidity?

Acidity is described as such:

Acidic solutions have a high concentration of H⁺ ions.

On the other hand, basic solutions (alkaline solutions) can be described as:

Basic solutions have a low concentration of H⁺ ions in solution.

If the pH tells us about the level of H⁺ ions in the solution, which will be their concentration, we can identify the solutions and label them as acidic or basic.

A pH change will indicate that there has been a change in the concentration of H⁺ ions in the solution, which means that a pH change will be a change in the acidity of the solution. This can be an increase in acidity or a decrease. Through this, which we will discuss later, a pH change will indicate the shift from an acidic solution to a basic one, or the other way around.

So how do we determine what is an acidic or basic solution? Is there a way to quantify those numbers and understand pH changes in another way? In the next section, we will cover how to quantify the concentration of H⁺ ions and relate it to the pH by calculations.

pH Equation

pH is measured by the following formula:

\[ pH=- \log[H^+] \]

As you can see, the pH is a logarithmic scale of the concentration of H⁺ ions in solution. Because it is logarithmic, it means that every whole number step represents a whole step in a logarithmic fashion, in this case a change by a factor of 10.

This means that a solution that has a pH = 3 and a solution of pH = 4 differ by 10x in the concentration of H⁺ ions, yet a solution of pH = 3 and one of pH = 5 will differ by 100x.

Using calculations and concentration values for a solution, we can tell that acidic solutions have a low pH number and basic solutions have a high pH value. This is due to the fact that it is an inverted logarithmic scale because the log has a minus sign in front of it. In the next section, we shall discuss how these pH values fit onto a scale to use for pH change.

You can also modify the pH equation to your use:

• To find out the concentration of the H⁺ ions in the solution from the pH value. You can rearrange the equation to formulate the following equation: \[ [H^+]=10^{pH} \]. Using this equation, you can find out the concentration of H⁺ ions in a solution just by knowing the pH. This can be useful if you have a digital pH meter and you want to find out the total number of H⁺ ions, as the digital pH meter will only tell you the pH value.

• Another formula you can use is the pOH formula, which accounts for the concentration of hydroxide ions (OH^-) in the solution. The formula of which is: \[ pOH=- \log[OH^+] \]. The pOH value is the inverse value of the pH reading, further which we will discuss in the next section.

pH Scale

The pH scale was created to make sense out of arbitrary pH values. Quantifying the concentration of H⁺ ions in solution is an effective tool for labelling acids and bases, yet they have to be put in a certain comparative scale to be used. The pH scale provides this, and can be used to compare different solutions.

Below is the pH scale:

pHFig. q. pH scale. Source: jansanconsulting.com

There's two things which you can notice from the scale: the colours and the values.

You will notice that the scale goes from 0 to 14. But why is that? This is because water is often determined to be the midpoint as it is neutral. Water naturally has a pH of 7, which comprises the midpoint of the scale. Anything below 7 becomes acidic and anything above 7 becomes basic (or alkaline).

This scale can be used to plot different solutions and compare them. If you have a database of solutions you are using and their pH values, you can arrange them in order of either increasing or decreasing pH to understand the relative acidity and H⁺ ion concentrations within solutions.

Causes of pH Change

As discussed above, pH changes are changes in the acidity of a solution. They can be either a decrease or an increase in H⁺ ion concentration, so to understand why the pH changes we need to take a look at the sources of H⁺ ions in different solutions.

It is key to mention that a change in pH can be in both directions, this means that the pH can increase or decrease. Most importantly, the key aspects of pH change relate to the way a solution is either acidic, neutral, or basic. If a pH change is drastic, it can result in a shift of its chemical properties, which will have large implications for its chemical uses.

There are two ways to look at this, either in isolated chemical environments or in real-life scenarios.

Isolated Chemical Environments

In an isolated chemical environment, such as a beaker with a specific acid or base, a pH change can only come if there is another solution added to it.

There are three possible solutions which can be added: an acid, a base, or a neutral solution such as water. If you add more acid to an acid, the pH will go down, but if you add a base to an acid, the pH will go up (remember that the logarithm of the pH formula has a minus sign before it, meaning that the scale is inverted: the more acidic the solution, the lower the pH). Depending on the strength of your starting solutions and the amount of them, the pH could either stay within the acid range, basic range, or become neutral. This is because it can undergo a neutralisation reaction.

If you add a neutral solution, such as water, then a dilution of your base or acid will take place. More on dilution later on.

pH Change from Dilution

Here we will discuss the consequences on pH levels from dilution.

We know that pH is correlated to the concentration of H⁺ ions in solution. This is based on a formula stated above.

Thus is you dilute the basic or acidic solution, which can be done by adding a fixed amount of water to the solution, the pH will change.

Dilution of acidic or basic solutions always bring the pH level closer to 7, as it is the neutral midpoint. Thus, adding water to an acidic solution will increase the pH, while adding water to a basic solution will decrease the pH.

The key thing to remember during estimating the pH levels during dilutions is that the amount of H⁺ ions in solution will remain the same, but the amount of solvent (ie. water) will change. This means that there will be a fixed amount of H⁺ ions in solution but the volume of water will change, which will give rise to a different concentration. Ultimately, this will change the pH of the solution.

Due to the logarithmic state of the pH scale, to get from one pH value to another by dilution you will need to you 10x the amount of water.

I hope this article illuminated the concept of pH changes, and that now you have a grasp on how different factors can influence the pH level in chemical contexts as well as nature.