Instances where preparation of standard solution?

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.

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:

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.

Note: one mole is equal to 6.022 × 10²³ units of that substance.

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:

The process for making a standard solution using the dilution method is as follows:

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

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.

The process for making a standard solution using the weighing method is as follows:

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:

A standard solution can be prepared by weighing method in the following way. (a) The mass of solute needed is calculated and weighed. (b) The solute is dissolved in some distilled water in a beaker. (c) The solution is transferred into a volumetric flask.

How to prepare for titration

This video explores all necessary information for preparing and setting-up an acid/base titration including:

Titration requires a solution of accurately known concentration called a standard solution. If the unknown solution is basic then the standard solution will be acidic and vice versa.

Titration is considered to be a volumetric analysis as it measures the volume of standard solution required to exactly neutralise the unknown solution This required volume is called the titre or titre volume.

Properties of a primary standard include:

Examples of substances unsuitable for making primary standard solutions

Examples of good standard solutions

Note that anhydrous compounds are prepared by drying in an oven until all water content is evaporated (mass remains constant).

What is a secondary standard?

A secondary standard is one whose concentration is accurately determined via titration with a primary standard solution prior to its use. For example, NaOH is often used as a secondary standard as its concentration can be determined by titrating with a primary acid standard solution e.g. oxalic acid.

Secondary standards should be avoided if possible as the process of standardisation introduces more sources of error, therefore affecting the accuracy of results.

Step 1: calculate and weigh the correct quantity of the substance

The first step of making a standard solution is to calculate the quantity of the substance we want to make the solution out of.

For example, if we want to make 250.0 mL of a 0.0500 mol/L solution of sodium carbonate:

$$n = c \times V$$

$$n(Na_2CO_3) = 0.0500 \times 0.250 = 0.0125 \hspace{0.1cm} mol$$

$$m = n \times MM$$

$$m(Na_2CO_3) = 0.0125 \times (2 \times 22.99 + 12.01 + 3 \times 16.00)$$

$$m(Na_2CO_3) = 1.33 \hspace{0.1cm} g \hspace{0.1cm} (3 s.f.)$$

Therefore, 1.33 g of sodium carbonate is required to make this solution.

Step 2: dissolve the solid

From step 1, 1.33 g of sodium carbonate is dissolved in a small amount of distilled water in a beaker of appropriate size. Use a stirring rod to help completely dissolve the solid.

Step 3: transfer the standard solution to a volumetric flask

Transfer the solution to a 250.0 mL volumetric flask using a funnel. Ensure that the volumetric flask is rinsed with distilled water prior to use. It is appropriate to clean the volumetric flask with water because distilled water will be added to the flask eventually to make the standard solution.

Ensure complete transfer of sodium carbonate by rinsing the beaker, stirring rod and funnel with distilled water and discarding the rinsing into the volumetric flask.

Step 4: add distilled water until required volume

Once the solution is transferred to the volumetric flask, add distilled water until the level reaches 1 cm below the graduation mark of the flask.

Use a plastic pipette to add distilled water dropwise until the bottom of the meniscus is level with the graduation mark.

Step 5: homogenise the solution

Stopper the flask and invert it up to 10 times.

A burette is a piece of volumetric equipment used in all types of titrations. it usually can contain up to 50.0 mL of a particular substance. The volume measurements are marked such that 0.0 mL is at the top of the burette while 50 mL is at the bottom of the burette.

The titrant is the solution to be added to the burette. The burette is washed with distilled water, followed by a small amount of titrant prior to use. The distilled water and titrant should coat all the entire interior surface of the burette and be discarded via the tip (ensure the stopcock is open).

It is important to finish the washing process with the titrant as residual amounts of water would dilute the concentration of the titrant. Residual amounts of titrant will not affect the concentration of the titrant.

The titrant of a titration can either be the standard solution or the unknown solution to be analysed. If the standard solution is to be added to the burette, then the unknown solution will be added to the conical flask.

After the burette is clamped to a retort stand and filled with the titrant, record the initial volume - this is the reading that is level with the bottom of the meniscus. After each titration trial, record the final volume.

The titre volume equals to the difference between the initial and final volumes recorded.

A pipette is used to transfer a specific volume of either the standard solution (from the volumetric flask) or the unknown solution to the conical flask. This specific volume is known as the aliquot.

The pipette is rinsed with the solution that is to be transferred. For example, if the standard solution is to be transferred using the pipette, it must be rinsed with the standard solution prior to use.

The conical flask is to be rinsed with water prior to use. It is appropriate to use distilled water for rinsing because any dilution of the solution in the conical flask will not affect the number of moles of the substance. As long as the pipette is rinsed appropriately and the correct quantity of the solution is transferred into the conical flask, its concentration in the flask will not affect the titration itself.

After the solution is transferred, add a few drops of an appropriate acid/base indicator and place the conical flask on a white tile. The white tile allows for a better visualisation of the indicator's endpoint.

An experiment is conducted to investigate the concentration of a solution of HCl. Exactly 50.00 mL of HCl is required to neutralise 20.00 mL of a 0.0500 mol/L solution of sodium carbonate.

In this example, the titrant is the HCl solution and the titre volume is 50.00 mL.

Therefore,

• Sodium chloride – table salt is used as a primary standard solution for reactions involving silver nitrate.
• Powdered zinc – powdered zinc is often used to standardise ethylenediaminetetraacetic acid or EDTA, which acts as a stabiliser in many pharmaceutical products.