How To Make A Standard Solution

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:

1. The dilution method
2. 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.

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 amount of a substance in a solution of that you don’t know the concentrations of

• Calculations are made based on the proportionality of molar concentrations

Introduction to Standard Solutions

Standard solutions are important in analytical chemistry. They are used to analyse sample solutions of unknown concentrations by comparing them to the standard solutions. 

The concentration of these sample solutions can be precisely determined using standard solutions, which is important if you want to determine a substance’s purity. 

Aside from comparing concentrations, standard solutions are also used to prepare reagents and buffers. These are important in detecting specific substances and maintaining the chemical equilibrium of solutions. 

Standard solutions are prepared by calculating the molar values of solutes and solvents. This can be done through one of two methods: the weighing method or dilution.

Calculations for Standard Solutions

Calculating standard solutions is based on the proportionality of concentrations within a substance. This applies to both the dilution method and the weighing method. 

You need to first measure the molar mass of your solute and assign the desired molar concentrations. 

You should use the 1M concentration as a reference point, which is equivalent to one molar mass of a substance dissolved in one litre of water. You can then assign the desired concentrations based on volume or mass.

Background Calculations

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. 

This formula can also be written as:

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:

1. Write the chemical formula

• Sucrose: C₁₂H₂₂O₁₁

2. 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

3. 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.

Methods for Making a Standard Solution

There are two main methods of preparing a standard solution:

1. The dilution method 
2. The weighing method

In both methods, you can assign your desired concentration based on the proportional molarity and volume of the standard solution and the stock solution. 

In the dilution method, you can adjust the volume of the solvent. If you use the weighing method, you can adjust the mass of the solute based on its molar mass.

Method #1: Dilution of Standard Solutions

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

Where:

• 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

Method #2: Standard Solution Preparation by Weighing

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.

Tips for Accurate Standard Solution Preparation

Accuracy and precision are both important in chemistry for getting the correct conclusion and results. These concepts are crucial in objective measurements in all experiments. 

Accuracy refers to the correctness of the measurement,  while precision refers to the way in which measurements can be refined down to smaller units of detecting minute differences. Together, accuracy and precision can ensure exactness when you prepare standard solutions.

The accuracy of your measurements will highly depend on the reliability of your instruments. For example, instruments such as weighing scales must be properly calibrated. You can use several instruments to measure the same parameters; if your instruments agree, then the measurement is likely to be accurate. It can also be helpful to use analogue instruments to compare the results measured by the digital instruments.

Accuracy in preparing a standard 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 – and it’s also crucial to avoid impurities. Make sure your glassware is properly cleaned and free from contaminants.

Required Equipment

The two main pieces of equipment you’ll need in preparing a standard solution are volume-measuring glassware, such as graduated cylinder, and a mass-measuring device, such as a triple-beam balance. Avoid using spring-based weighing scales. 

Conclusion

Standard solutions are crucial in many analytical tasks in a chemistry laboratory. Accuracy and precision are important in preparing these solutions. The reliability of your instruments and your meticulousness when measuring the molar concentrations are paramount. That means you need proper calibrations and skills.