pH Test

You might know how lemon juice is acidic, but did you know there is a scale that can help measure the acidity of a liquid? The pH scale measures how acidic or basic a substance is and determines how it will interact with the world around it.

pH tests can help determine where different solutions fall on this scale. Solutions that fall below the number seven on this scale will be bitter and acidic! Some examples of other acidic solutions are coffee and hydrochloric acid, but you can only drink one of these liquids! Substances higher than seven are considered basic. Examples of bases are baking soda and various soaps and detergents.

So, without further ado, let's learn about the pH test!

• First, we will look at the definition of a pH test, acids, and bases.
• Then, we will talk about the different pH test methods.
• After, we will explore some uses of the pH test.
• Lastly, we will look at an at-home pH test experiment.

pH Test Definition

Let's start by looking at the definition of a pH test.

If a substance is acidic, it will have a pH value of less than seven; if a substance is basic, it will have a pH value larger than seven.

• Pure water is a seven on the pH scale and is used as a neutral point of comparison.

Acids are able to "donate" one of their hydrogen ions, or they can form a covalent bond with a pair of electrons. They also have different properties, such as having a sour taste, conducting electric currents, reacting with active metals (some examples being lithium, sodium, or potassium) to produce hydrogen gas, and changing the color of certain solutions during pH tests.

Depending on the strength of the acid, it can be dangerous to touch them without protective gear.

Bases have a bitter taste, and, depending on the type of base, such as laundry detergent or soap, they can feel slippery. Also, unlike acids, bases do not react with metals. Similar to acids, they can also alter the color of pH test solutions.

Can Bases be Dangerous?

While you probably know that acids can be dangerous, you might not know that bases can be dangerous as well.

Once bases make contact with the skin, it is sometimes not noticeable until it is too late, and they have already caused damage. Also, it is harder to remove bases from the skin due to their slippery nature.

Calculating pH and H⁺

There are formulas we can use to calculate the pH and the H⁺ of a solution.

• Calculating the pH uses the formula: -log₁₀[H⁺]

• Calculating the H⁺ uses the formula: 10^-pH

pH Test Methods

There are multiple different ways to test for pH such as pH indicators, pH papers, and pH meters.

pH indicators

pH indicators are a type of weak acid that aid in finding pH. Different chemicals can help aid in this because once the proper pH is reached, the solution will change color. Some of these chemicals are bromothymol blue, and phenolphthalein, each covering a different range on the pH scale.

While pH indicators are inexpensive, quick, and easy, it can be difficult to get an accurate read on the pH of a solution, and the original solution must be as colorless as possible.

Fig. 1. pH indicators can change the color of an acid or a base.

pH papers

Different papers can give readings on a solution's pH as well. The most common kind of paper is known as litmus paper. Litmus paper comes in three kinds: acidic, basic, and neutral. The type of litmus paper used depends on the solution. While a litmus paper can be useful in narrowing down a pH, there are more expensive papers that can get even better results. These pH papers are also easy, cheap, and quick to use.

Fig. 2 pH Test Paper.

pH meters

While the most expensive option, pH meters offer the most accurate results. pH meters work by assessing the electrical potential of the solution compared to a reference. The electrical potential is created via the concentration of hydrogen (H) ions in a solution. There are two kinds of pH meters, one is handheld which is great for quick readings and/or fieldwork, and one is used on a lab bench which is better for more accurate readings.

pH test importance

pH tests are important for many reasons. They monitor medical conditions for people, food quality and safety, and water quality.

pH test uses

Different industries use pH tests for different purposes.

It is also important in the food industry because it can identify the freshness of food and prevent the spread of spoiled food. pH tests are also used with soil for crop management because certain crops will grow best in certain soil pHs. pH tests can also be used for urine testing or acid reflux.

pH test colors

Different colors on pH tests will display the pH of a solution.

• Acidic colors are red and progress from orange to yellow as the pH becomes neutral.

• Basic pHs are cool colors like blues and/or purples.

At-home pH test experiment

Let's finish off by looking at a fun experiment you can do at home in order to develop an understanding of pH. This simple experiment uses red cabbage, baking soda, and lemon juice!

Materials

• Saucepan

• Cups

• Red cabbage

• Lemon juice

• Baking soda

Steps

1. Boil chopped red cabbage in a saucepan and boil for five minutes. Cover and rest the cabbage for thirty minutes.

2. Put the red cabbage liquid into different cups.

3. Put lemon juice in one of the cups and baking soda in another. Look at how the colors in the cups have changed!

Understanding the experiment

The lemon juice will turn the originally purple cabbage juice into a red color because it is acidic. Once the baking soda is added, it will turn blue because bases are cool colors.