A standard solution is a chemical solution where you know the concentration of a particular element or compound. It’s prepared from a standard substance by weighing the substance and mixing it with a specific volume of solvent to achieve the desired concentration. There are two main methods making a standard solution:
- The dilution method
- The weighing method
Standard solutions are vital in analytical chemistry as they can help determine the amount of substance in a solution of unknown concentration. Laboratory analytical methods like titration depend on standard solutions to achieve precise and accurate results.
The concentration of a standard solution is typically measured in moles per litre or mol/L, which is commonly abbreviated to simply M. Other units, such as moles per cubic decimetre and kilomoles per cubic metre, can also be used. Smaller subunits are used for small samples while bigger units tend to be used for larger samples, such as in the case of chemical manufacturing.
Standard solutions can also serve as reagents and buffers. These are prepared from primary standards, which are extremely pure substances. A pure substance is stable, non-hydrated and has a high molecular weight. Not all substances can be used as a primary standard.
In this post:
The main parameter you need to calculate when preparing a standard solution is the concentration of solute in proportion to the solvent.
You can do this by first measuring the mass of the standard substance and the volume of the solvent. The measurement should be based on the molar concentration you want of the solution. Molar concentration is simply the ratio of the number of moles of a solute, expressed in molar mass (grams), and the volume of a solvent in litres.
For standard solutions, the volume of the solute is fixed at 1L, which is equivalent to 1dm3. This simplifies the calculations and maintains a high level of precision.
To prepare a standard solution, you’ll also need to know the total molar mass of a substance. You can calculate this by writing the chemical formula and then adding the atomic weights of the constituent elements, as stated on the periodic table.
It’s important to remember the difference between atomic weight and atomic mass. The former refers to the average weighted mass of all the isotopes in an element, while the latter is the mass of a single atom.
Let’s take sucrose, or table sugar, as an example. To calculate the molecular mass of sucrose you will need to:
- Write the chemical formula
- Sucrose: C12H22O11
- Determine the total molar mass of the substance
- C = 12 u atomic weight x 12 carbons = 144 units
- H = 1 u atomic weight x 22 hydrogens = 22 units
- O = 16 u atomic weight x 11 oxygens = 176 units
- Total Molecular Mass = 342 units or 342 grams per mole
- Assign a concentration value
If you want a 50% mole concentration of table sugar, you’ll need half of the total grams of the substance’s mole value to be dissolved in one litre of water.
- 50% x 342 grams = 171 grams = 0.5 M when dissolved in 1 L of water
Note: one mole is equal to 6.022 × 10²³ units of that substance.
Making a Standard Solution Using the Dilution Method
You can prepare a standard solution by using what’s known as the dilution method. This involves adding additional solute to a substance of known concentration to reduce the molarity concentration of the solution.
However, this method doesn’t change the mole value of a substance as the same number of particles are still present. This means you can use the conservation of mass to calculate the dilution. The bench acids – hydrochloric acid, sulphuric acid, and nitric acid – are all prepared from concentrated stock solutions using the dilution method.
M1V1 = M2V2
- M1 – the desired molarity of the diluted solution
- V1 – the desired volume of the diluted solution
- M2 – the molarity of the concentrated stock solution
- V2 – the volume of the concentrated stock solution.
Process of the Dilution Method
The process for making a standard solution using the dilution method is as follows:
- Step 1: Set the desired molar concentration (M1) of the diluted solution at a given desired volume (V1).
- Step 2: Calculate the volume of the stock solution (V2) that needs to be added to achieve the desired diluted concentration.
- Step 3: Carefully measure the volume of the stock solution. Use a pipette for more precise and accurate measurements.
- Step 4: Mix the new solution in a flask.
Example – Acid Dilution
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 50 mL of a 1.0 M sulphuric acid solution from a stock concentrated solution of 2.0 M. You can calculate this by using the formula M1V1 = M2V2.
By simple transposition, you can isolate V2.
V2 = M1V1 /M2
V2 = (1.0 M)(50 ml)/2.0 M
V2 = 25 ml of stock solution
Making a Standard Solution Using the Weighing Method
Another way of preparing a standard solution is to follow the weighing method. Typically used to prepare basic standard solutions, the weighing method requires a pure substance that can be dissolved in a solvent such as water.
Process of the Weighing Method
The process for making a standard solution using the weighing method is as follows:
- Step 1. Set the molarity of the solution you want to prepare.
- Step 2. Determine the molar mass of a substance in grams based on the chemical formula.
- Step 3. Weigh the equivalent amount of the pure substance in grams.
- Step 4. Dissolve the substance in water.
- Step 5. Add water until you reach the correct volume.
Example – Base Solution
You can 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.
For example, if you’re preparing sodium hydroxide (NaOH), the molar mass is the sum of all the atomic weights of the constituent atoms:
- Na – 23 u
- O – 16 u
- H – 1 u
- Total: 40 u or 40 grams per mole
If you set the concentration at 1 M of NaOH, you’ll need to dissolve 40 grams of NaOH in one litre of water.
The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).