In this lesson, learn about standard solutions and the characteristics that define them. Explore what they are, as well as learn the method behind how to make one.
What Is a Standard Solution?
Analytical chemists take on the task of separating, identifying, and quantifying a variety of chemical components in a wide range of materials. Standard solutions are one of the many tools chemists can use when specifically (and accurately) trying to identify these components.
Standard solutions are solutions that contain a known and accurate amount (i.e. concentration) of a substance or element. These solutions are commonly used to help identify and determine the concentration of a substance whose concentration is unknown. When working to analyze, let's say an unknown substance, you want to be as accurate as possible with all results. This is where standard solutions step in. Because these solutions contain accurate concentrations of a chemical component, they will increase confidence regarding the determination of substances with unknown concentrations.
When preparing standard solutions, you might need to dissolve a primary standard in a solution such as distilled or purified water. What is a primary standard? A primary standard is a type of reagent that has specific characteristics. It aids in accurately assessing the unknown concentration of a substance. Think of primary standards as a library. You decide to take your unknown substance to that library with the goal of accomplishing two tasks: identifying what that unknown is and determining its concentration.
A primary standard must:
- Be pure (99% pure)
- Be very stable (i.e. not react with other chemicals)
- Be readily available and inexpensive
- Not change its size (i.e. solid primary standards) if exposed to air
- Be very large in size (mass) to help minimize weighing errors
Given these characteristics, it makes perfect sense why we can think of a primary standard as the library, or point of reference! Understanding what a standard solution is, there is a method you can use when working with standard solutions.
Method For Using Standard Solutions
Keep in mind that as we go through this example, your goal is to create a solution where you not only know what substance is in the solution, but can accurately identify the concentration of this solution. There are multiple ways to prepare a standard solution, but we'll focus on one example commonly encountered in a laboratory.
Say you're in the lab and need to make a standard solution of 1.0 M sodium chloride. You know that you want the volume of this solution to be 400 ml. You also know that the concentration of this standard solution must be 1.0 M. How would you determine, with certainty, the amount of solid sodium chloride to be added to this 400 ml container? The best way to tackle this problem is to use the following equation:
Molarity = mol / L
Molarity is the amount of mols of a substance in a 1 L solution. Here, we have a molarity of 1.0 M, which means 1.0 mol of sodium chloride is in 1 L of solution. We also know that mols of a substance can be used to find the mass of a substance using formula mass (g/mol). Formula mass is simply taking the atomic mass of each atom (which you can find on the periodic table) in your compound and adding them together to get a total mass. For example, with sodium chloride, the atomic mass is 22.98 g/mol for sodium and 35.45 g/mol for chloride. The formula mass of this compound (NaCl) would be 58.43 g/mol.
Putting this all together, we discover the following:
1.0 M = 1.0 mol of NaCl / L
.400 L of standard solution = 1.0 mol / 1 L = .400 mols of NaCl
.400 mol NaCl x (58.5 g NaCl / 1 mol) = 23.4 g NaCl
When preparing this standardized solution, we would need to add 23.4 g of NaCl to a container that has 400 ml (.400 L) of distilled or purified water. This would be our standard 1.0 M solution of NaCl. Now, what method would we use to actually make this solution?
You already know the amount (23.4 g) and exact volume (400 ml) needed to prepare the standard solution. In a laboratory, you would carefully (and very accurately) weigh out 23.4 g of solid NaCl in a weighing glass/boat. This would be transferred to a volumetric flask or your handy beaker, which already contains 400 ml of distilled water (or purified water).
When making standard solutions, you want to be as accurate as possible, which means placing every bit and piece of solid NaCl into the solution. You can accomplish this by taking some of the water from your 400 ml solution to rinse that weighing glass/boat that contains your solid substance. Once your transfer is complete, seal your solution, stir it very well, and you've made yourself one fancy standard solution of 1.0 M sodium chloride.
Let's review. Standard solutions are solutions with a known concentration of a substance. They're used in chemistry, particularly analytical chemistry, to help identify or determine the concentration of unknown substances. Primary standards are often used when preparing standard solutions. One standard solution method involves using molarity to calculate the amount of a known substance that needs to be added to a solution at a defined volume. Accuracy is very important when making standard solutions.
Standard solutions - solutions that contain a concentration of a substance or element
Concentration - a known and accurate amount of a substance or element
Primary standard - a type of reagent that has specific characteristics; can be used to accurately assess the unknown concentration of a substance
Molarity - the amount of mols of a substance in a 1 L solution
Formula mass - the total mass of a compound; found by adding together the atomic mass of each atom (which can be found on the periodic table) within the compound
Ensure that you're able to do the following when you've studied the lesson:
- Define standard solution, primary standard, and molarity
- Recognize an example of a primary standard based on its properties
- Write the formula mass of a compound
- Walk through the method for using a standard solution