Making a standard solution is a core skill all chemists and lab technicians need to know. A standard solution is simply a solution in which the exact concentration of a specific element or compound is known. They’re used as the ultimate reference point to measure other unknown substances.
Keep reading to learn more about standard solutions and discover the two primary methods for making them in a laboratory setting.
Key Takeaways
A standard solution is a substance with a known or predetermined concentration
Standard solutions are used in analytical laboratory tasks such as titration
There are two main methods of preparing a standard solution: dilution and the weighing method
Standard solutions are used to calculate the concentration of an unknown substance in a solution
Calculations are made based on the proportionality of molar concentrations
In this post:
Introduction to Standard Solutions
Standard solutions are vital in analytical chemistry, as they can help determine the exact concentration of an unknown substance. Analytical lab techniques like titration depend on standard solutions to achieve precise and accurate results.
Sometimes standard solutions serve as reagents and buffers. To make a highly accurate standard solution from scratch, you usually start with a primary standard. This is an extremely pure, stable compound that doesn’t absorb moisture from the air and typically has a high molecular weight.
Not every chemical qualifies as a primary standard. A good example is sodium hydroxide, which quickly absorbs moisture and carbon dioxide from the air, making it impossible to weigh with perfect accuracy. When using such substances, the resulting solution must be verified against a primary standard before use.
Methods for Making a Standard Solution
Let’s look at the two main methods of preparing a standard solution, the dilution method and the weighing method.
Both involve assigning a desired concentration based on the proportional relationship between the molarity and volume of the standard and stock solutions.
The Dilution Method
Chemists use the dilution method when they already have a highly concentrated stock solution on hand and want to make a weaker solution. Labs routinely prepare standard bench acids like hydrochloric acid, sulfuric acid, and nitric acid in this manner.
When you add more solvent to a stock solution, you decrease its molar concentration. However, you don’t change the total number of solute particles; you just spread them out across a larger volume. Because the total moles of solute stay constant, you can use a simple conservation of mass formula:
M1V1 = M2V2
The process for making a standard solution using the dilution method is as follows:
- Calculate volumes – Use the formula to find out exactly how much stock solution you need to draw.
- Add partial solvent – When working with strong acids, always add water to the flask first. Never pour water directly into a concentrated acid, as it can cause dangerous splashing and extreme heat.
- Measure the stock solution – Use a precise volumetric pipette or Mohr pipette to draw up your calculated volume of stock solution.
- Mix and fill to the mark – Transfer the stock solution into a volumetric flask. Fill the rest of the flask with distilled water until the meniscus touches the line, then invert to mix.
Acid Dilution Example
It doesn’t really matter what type of solution you want to prepare, provided you know the concentration of the stock solution and you’ve set your desired concentration of the diluted solution.
Let’s say you want to prepare 50mL of a 1.0M sulphuric acid solution from a stock concentrated solution of 2.0M. You can calculate this by using the formula: M1V1 = M2V2.
By simple transposition, you can isolate V1.
- V1 = M2V2 / M1
- V1 = (1.0M x 50mL) / 2.0M
- V1 = 25mL of stock solution
The Weighing Method
Chemists use this method when they need to dissolve a solid chemical (the solute) into a liquid (the solvent, usually distilled water).
The process for making a solution using the weighing method is as follows:
- Calculate the required mass – Determine the exact mass of the solid required based on the target molarity.
- Accurately weigh the solute – Use an analytical balance to weigh the solid solute into a clean, dry weighing boat or small beaker.
- Dissolve the solid – Transfer the solid into a beaker, add a small amount of distilled water, and stir until the solid dissolves completely.
- Transfer to a volumetric flask – Pour the dissolved solution into a clean volumetric flask using a glass funnel. Rinse the beaker and funnel multiple times with distilled water, pouring the rinse water into the flask too.
- Fill to the mark – Add distilled water until the bottom of the liquid’s curve (the meniscus) rests exactly on the etched line of the flask’s neck. Stopper the flask and invert it 10 to 15 times to mix it thoroughly.
Base Solution Example
Set the desired concentration before preparing the solution. Unlike the dilution method, the weighing method requires you to calculate the molar mass of the substance you’re preparing.
The following example demonstrates how to calculate the mass needed using the weighing method. Note that because sodium hydroxide (NaOH) is not a primary standard, the resulting solution would need to be verified against one before being used as a true standard solution.
- Na – 23 u
- O – 16 u
- H – 1 u
- Total: 40 u or 40 grams per mole
If you set the concentration at 1M of NaOH, you’ll need to dissolve 40 grams of NaOH in one litre of water.
Preparing a standard solution involves calculating the molar values of solutes and solvents. This can be done through one of two methods: dilution or the weighing method.
Tips for Accurate Solution Preparation
In analytical chemistry, precision and accuracy dictate your success.
Accuracy means your measurement hits the true, correct target. Precision means you can repeat that measurement reliably down to tiny increments. Together, accuracy and precision can ensure exactness when you prepare solutions.
The Right Equipment and Calibration
The two main pieces of equipment you’ll need are precise volume-measuring glassware (such as a volumetric flask and pipette) and a mass-measuring device (such as an analytical balance). Avoid using graduated cylinders for final volume measurements, as they are not accurate enough for standard solution preparation.
The accuracy of your measurements depends on the calibration of your instruments. For example, analytical balances should be calibrated regularly using certified standard check weights.
Relying on uncalibrated equipment or trying to cross-check highly precise digital instruments with less precise analogue ones can introduce systematic errors.
Accuracy in preparing a solution can also be achieved by preparing several samples consistently. You must also ensure that certain factors, such as temperature, humidity, and pressure, are maintained at standard conditions.
It’s also crucial to avoid impurities. Make sure your glassware is properly cleaned and free from contaminants.
Conclusion
Whether you utilise the dilution method or the weighing method, mastering the preparation of solutions is fundamental to achieving reliable results in analytical chemistry. By prioritising the calibration of your instruments and maintaining rigorous attention to detail during calculations, you ensure the accuracy and precision necessary for successful laboratory experiments.
