Why it is important to rinse out the conical flask with distilled water before it is used in a titration?

Anyone who’s studied chemistry will be overly familiar with titrations. It’s an analytical technique that can be used to find the concentration of a solution (the amount of a solute dissolved in it). I put this graphic together primarily to remind my A level students of some of the key aspects of titrations, but as well as being a handy guide for them, it’s a useful introduction to the technique for non-chemists too!

Students are usually introduced to titrations in the context of reactions between acids and alkalis. As many of you might well recall, this is known as a neutralisation reaction. Titration allows us to work out the concentration of, for example, an acid of unknown concentration, by using a fixed volume of it and measuring how much of an alkaline solution of known concentration is needed to react with all of it.

To measure the fixed volume of the acid solution, chemists often use pipettes. These are long glass tubes which measure a fixed volume, and are also more precise than using measuring cylinders. Pipettes are filled using pipette fillers, which come in a variety of shapes and sizes; some older chemists might remember the practice of mouth-pipetting, which is now frowned upon for fairly obvious reasons! Pipettes aren’t just dunked straight in to the bottle of the solution – this could introduce contamination, so we first pour a suitable amount of solution into a beaker, then use the pipette to measure a precise amount of it. Prior to this, the pipette should have been rinsed with distilled water, followed by the solution it is to be filled with, again to avoid contamination.

Once the solution is in the pipette, it’s then transferred to a conical flask. Conical flasks are better than beakers for this procedure because they can be easily swirled without risk of the contents spilling. Pipettes are actually calibrated to retain a very small amount of solution in the tip when emptied, so although it’s tempting to force this out by blowing down the pipette or by squeezing the pipette filler, this is actually detrimental to the results gained from the titration.

Once you’ve got your acid of unknown concentration in the conical flask, it’s time to set up the burette with your alkali of known concentration. Burettes are tall, thin, graduated glass tubes, with a tap at the bottom that can be opened and closed to allow the solution inside to flow out. The scale on its side allows the amount of solution that’s been allowed to flow out to be read off.

As with the pipette, the burette should be rinsed with distilled water followed by the solution it is to be filled with to avoid contamination issues. It’s easiest to fill with a small funnel on top, though you also need to take care when doing this otherwise it’s easy to send your solution fountaining out of the top of your burette! The burette should be filled up to above the zero line, and then lowered down to it by opening the tap. The meniscus, the bottom of the water level in the burette, should be level with the zero mark.

Filling the burette this way is also useful because it means the space under the tap is also filled with liquid. This is important, as the burette is calibrated to include this volume. If you don’t do this, or if an air bubble is present in the space under the tap, the volume of solution you record as being added will be slightly higher than the amount you’ve actually added, leading to incorrect titre values that will affect your calculated results.

With both conical flask and burette filled, you’re ready to start the titration. First, an indicator is commonly added to the conical flask. For acid-alkali titrations, this is a chemical that undergoes a colour change at certain acidities. Two commonly used examples are phenolphthalein and methyl orange. The indicator changes colour at the end point, when all of our acid has reacted with our alkali. Without it, both solutions are colourless, so it would be impossible to tell!

Alkali solution is run from the burette into the acid solution in the conical flask, swirling the flask as it is added. When the end point is reached, the burette tap is closed, and the volume of alkali added is recorded. A white tile can be placed underneath the conical flask to aid with the ease of spotting the end point colour change. The volume of alkali added is referred to as the titre value; multiple titres are usually taken until concordant results are obtained. These are results that are no more than 0.10 centimetres cubed away from each other.

Once concordant results are obtained, we now know the volume of our known concentration alkali needed to react with a known volume of our acid of unknown concentration. We can now use this to work out the acid’s concentration. To do this, we use a very simple equation: n = cv. In this equation, n is equal to the number of moles, c is the concentration in moles per decimetre cubed, and v is the volume in decimetres cubed.

First, we’ll work out the number of moles of alkali we added from the burette. We know the volume we added, and the concentration of the solution, so we simply multiply these together to find the number of moles. Note that there is one catch: in this equation, volume needs to be in decimetres cubed, not centimetres cubed, so we’ll need to divide our volume in centimetres cubed by 1000 to get it into decimetres cubed. Then it’s good to go in the equation.

Once we know how many moles of alkali we’ve used, we need to know the equation for the reaction so we can work out how many moles of acid it should react with. Let’s take the reaction between sodium hydroxide and hydrochloric acid as an example:

What does this equation tell us? Well, there aren’t any numbers in front of any of the chemical formula, which means that it reads: “1 mole of sodium hydroxide reacts with 1 mole of hydrochloric acid to give one mole of sodium chloride plus one mole of water.” This tells us that the sodium hydroxide and the hydrochloric acid react in a one-to-one ratio. However many moles of sodium hydroxide we had, it reacts with the same number of moles of our hydrochloric acid.

So, we now know the number of moles of hydrochloric acid, and the volume we used. Now, we simply need to use another form of the n = cv equation to calculate the acid’s concentration. By rearranging it we arrive at c = n/v; plugging in the numbers, first remembering to convert the volume to decimetres cubed from centimetres cubed, gives us our unknown acid’s concentration. Job done!

Of course, chemists will be at pains to point out that acid-base titrations are by no means the only use for the titration technique – they can be used for a number of other reactions too, but that’s beyond the remit of this post. There are also back titrations, which are a whole different beast!

Got your own titration tips and tricks you want to share? Drop them into the comments below!

Enjoyed this post & graphic? Consider supporting Compound Interest on Patreon, and get previews of upcoming posts & more!

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

This is part of the HSC Chemistry course under the topic Quantitative Analysis

• Conduct practical investigations to analyse the concentration of an unknown acid or base by titration

How to prepare for titration

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

• standard solution
• burette
• pipette
• conical flask

What is Titration?

• Indicator-based titration: uses colour of indicator to measure equivalence point
• Conductometric titration: uses electrical conductivity to measure equivalence point

What is a Standard Solution? 

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:

• High purity
• Accurately known chemical composition
• Free of moisture (does not absorb water, which would reduce purity)
• Chemically stable
• Readily soluble in pure water
• High molar weight (variations in mass have reduced effect on moles)

Examples of substances unsuitable for making primary standard solutions

• Hydrochloric acid (HCl) of very high concentration because it becomes volatile, resulting in losses as fumes
• Sodium hydroxide (NaOH) because it is hygroscopic (absorbs water)

Examples of good standard solutions

• Oxalic acid
• Benzoic acid
• Anhydrous sodium carbonate
• Anhydrous sodium hydrogen carbonate

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. 

Preparing a Standard Solution for Titration

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.

Preparing the Titrant & Burette

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. 

Preparing the Conical Flask

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.

Example Titration

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,

• Volumetric flask rinsed with distilled water
• Pipette rinsed with sodium carbonate solution (standard)
• Conical flask rinsed with distilled water
• Burette rinsed with HCl solution (titrant)