Determining pH: Methods & Classification

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  • 0:04 Acid vs. Base
  • 1:07 What Is pH?
  • 1:38 Methods of pH Determination
  • 4:14 Lesson Summary
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Lesson Transcript
Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson we discuss different diagnostic tests that are used to determine the pH of a solution. Our specific methods include conductivity meters, pH meters, indicators, and reactions with active metals like magnesium or zinc.

Acid vs. Base

Have you ever thought about what's going on inside your stomach? For example, when you eat some food, what happens to it once it gets to your stomach? How your body takes the food that you ingest and processes it into useful energy and substances it needs is nothing short of amazing. You may not realize that the inside of your stomach is very acidic. So much in fact, that the acid it produces for the breakdown of food is capable of stripping the paint off of an automobile!

The acidity, which is the concentration of acid, or basicity, which is the concentration of base, of a solution is quantified by what's known as pH. The specific value of the pH tells us a great deal about the relative amounts of acid or base that are present in a given solution. The focal point of this lesson is how this pH value can be determined by various measuring techniques in the chemistry lab. Our main methods of determining pH include discussions on conductivity meters, pH meters, indicators in solution, and finally reactions with active metals such as magnesium and zinc. Let's strip some paint!

What Is pH?

Before we talk about how pH is measured, let's briefly discuss what pH values mean. pH is actually an abbreviation for 'potential of hydrogen.' It is a numerical value assigned to a solution that tells us how acidic or basic that solution is. The pH scale ranges from 0 to 14; pH values that are less than 7 are defined as being acidic, while pH values greater than 7 are defined as basic. A pH that is a perfect 7 is said to be neutral and is neither acidic or basic.

Methods of pH Determination

Now that we know what pH is and have some insight into its numerical values, let's discuss the various methods and instrumentation available for measuring the pH of a solution.

1. Conductivity Meters

A conductivity meter is an instrument that detects the presence of electrical current within a solution. By definition, acids release hydrogen ions in solution, and bases release hydroxide (-OH) ions. Both of these species possess an electrical charge, and it's the job of the conductivity meter to not only detect the charge but also calculate the relative concentration of the ions. By knowing the concentration, the instrument is able to calculate the pH of the solution.

2. pH Meter

Another way chemists determine the pH of a solution is by using a pH meter. A pH meter is very similar to a conductivity meter. However, the pH meter specifically looks for and detects the hydrogen ion concentration within a solution. Usually a pH meter must be calibrated to ensure its accuracy, and this is typically done by using a standard solution whose exact pH is known. The meter is set to that specific value and then unknown pH values are determined in reference to the standard.

3. pH Indicators

What if we didn't necessarily need to know the exact numerical value of the pH of a solution, but just if it were acidic or basic? A pH indicator is a compound that, depending on the nature of the solution environment it's in, causes the solution to be different colors. For example, methyl orange is an organic compound that turns the solution different colors based on the pH.

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