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CSEC Chemistry: Volumetric Analysis

Most of the time in chemistry, we're searching for information we don't know. For example, if we want to know if chlorine gas is an oxidizing agent, we react it with potassium iodide solution, which will turn brown if chlorine is an oxidizing agent (spoiler: it is).

Other times, it has nothing to do with what a certain compound can do, but only specific details about a mixture or a certain reaction. Think back to mole concept, where using mole ratios we could tell how much of a certain substance would be needed to react with another substance to produce a certain amount of product.


Volumetric analysis is an approach that helps us to determine either the concentration of a certain solution or the mole ratio of a certain reaction between two compounds.

The name sounds kinda scary, as if it is some kind of dizzyingly complex set of procedures and mathematical formulae that would take a large textbook and doctoral studies to understand. But luckily for us, it's not that difficult, and you can learn about it by reading this post for a couple minutes.


Volumetric analysis involves using the volumes of two compounds to calculate something. That something is usually either the molar concentration (mol/dm^3) or mass concentration (g/dm^3) of one of the compounds or the molar ratio (ratio of moles) of two reactants in a reaction.

Cool, we know what it is, but how does it actually work?


Well, volumetric analysis requires that we have two solutions: one aqueous acid and one aqueous alkali. (Note: aqueous just means 'in solution', and an alkali is just a soluble base).


To perform volumetric analysis, we also need to know the concentration of at least one of the solutions. If we know the exact concentration of a solution, we call that solution a standard solution. Regardless of whether we know the concentration of the standard solution in molar concentration or mass concentration, these values can still be used.


The final thing we need for volumetric analysis is a titration. A titration is a process where a solution of known concentration (which can be called a standard solution or titrant) is slowly added from a piece of equipment called a burette to a known volume of another solution until the reaction reaches a neutralization point. We can tell when the reaction reaches the neutralization point using an indicator.


There was a bit of new vocabulary in that definition, so let's go through each term. A burette is a long glass tube with markings for measurements and a tap on its lower end that allows us to add specific volumes of one solution to another.


You can see an example of a burette on the left. If you look closely, you will see that it starts with 50ml at the bottom and ends with 0ml at the top. So you have to read it from the top down. Strange right? That seems completely backwards- the opposite of a measuring cylinder, where 0 is at the bottom, and you read from the bottom up.

However, if you think about what we use it for, it makes perfect sense. We turn the tap (also called a stopcock) to allow some of the liquid in it to drip out. If it is filled all the way up to zero, the burette will always show how much has been used every time we open the tap.








Let's look more closely at how a burette might be used in a real experiment.

Most of the time, the burette isn't completely filled to the zero mark, so we have to determine how much was used based on the difference between the initial reading ind the final reading.

In the diagram above, you will notice that the readings are taken at the bottom of the meniscus of the liquid in the burette. The meniscus is a little curve that occurs at surfaces of molecular liquids (like water) when they come into contact with another material. Whenever we measure liquids, we always read from the bottom of that little curve.

Note as well that the burette readings are written to two decimal places (two numbers after the decimal point).

So, since we found the initial reading and the final reading, we just have to subtract to find the total volume used:

Volume used= Final Reading-Initial Reading


Volume used= 3.50 ml - 1.20 ml

Volume used= 2.30 ml


In summary, there are only 4 things you need to remember abut reading burettes:

  1. Read from the top downwards

  2. Read from the bottom of the meniscus

  3. Write your readings to two decimal places

  4. Volume used= Final Reading - Initial Reading


In our definition of titration, we also mentioned the neutralization point. If you remember types of reactions, the reaction between an acid and a base is called a neutralization reaction. In this type of reaction, hydrogen ions (also called protons, H⁺) from the acid react with hydroxide ions (OH⁻) from the base. At the neutralization point, all of the hydrogen ions have completely reacted with all of the hydroxide ions, and neither is in excess. This could also be called the end-point of the reaction, as all neutralization is complete.


When we are adding one solution of an acid or alkali to another solution (whether alkali or acid respectively), we are unsure of when the neutralization point will occur. We need a way to tell when the reaction is complete. This is why we use indicators in volumetric analysis.

Indicators are substances which change colour in response to changes in pH. For example, phenolphthalein, which is colourless in acidic solutions and pink in basic solutions.


 

Now that we understand all of these terms, let's see how it all comes together in a real titration.

In this example, we want to find the molar concentration of a certain solution of potassium hydroxide (KOH).


Procedure


STEP 1

Measure out a fixed volume of the solution whose concentration you do not know using a pipette and transfer it to a conical flask. This is known as the analyte, since it is what you want to analyse.

In this case, that solution is aqueous potassium hydroxide, so you may choose to use a volume of 30 ml (30cm³) based on the size of the pipette.

A few drops of an indicator are also added to the conical flask. In this example, we will use phenolphthalein.


(A pipette looks like this if you're wondering)



STEP 2

Place a burette on a stand and fill it with a dilute acid whose concentration you do know. This acid will be the standard solution.

Position the conical flask with the sodium hydroxide underneath the lower end of the burette. For our example, we will use sulphuric acid (H₂SO₄) of concentration 0.25 mol/dm³.


By now, the setup for the titration should look something like this:



STEP 3

Read the initial reading of the burette and record it in a table.

Open the tap of the burette peridoically and allow small volumes of the titrant to drip into the analyte until the colour changes. Swirling the flask may help to better mix the reactants.

In this example, the colour should change from pink to colourless. As soon as the indicator is colourless, the tap should be turned off and the final reading recorded in a table as well.


This first titration is called the rough titration, and it gives us and idea as to the approximate volume which results in the neutralization point.

Steps 1-3 are repeated until the volume of acid added for each titration is within 0.1 cm³ of each other.


The table we record the values in looks like this:

Finally, to begin to use this data to find the concentration of the analyte, we need to find the average of the titrations within 0.10 cm³ of each other (we usually exclude the rough titration), which gives us the volume of sulphuric acid needed to neutralize 25.0 cm³ of potassium hydroxide solution.


Average= (25.00 cm³ + 25.00 cm³)/2= 25.00 cm³

 

If we think of everything we need for volumetric analysis as a checklist, we can mark off everything that we have:

  1. Two solutions (one acid, one alkali)? Check.

  2. Concentration of at least one solution? Check.

  3. Volume of analyte? Check.

  4. Volume of titrant needed? Check.

This is all the information we need to do our volumetric analysis. Now all that's left to do is calculate the concentration of the analyte.


Calculation

Before starting our calculations, we should convert all volumes to dm³. We do this by dividing any cm³ values by 1000.

Volume of analyte= 30 cm³/1000= 0.030 dm³

Volume of acid needed= 25.00 cm³/1000=0.025 dm³


Part 1: Find the number of moles of reactant in the volume of acid used.

Number of moles= molar concentration × volume used


Number of moles of acid= 0.25 mol/dm³ × 0.025 dm³

Number of moles of acid= 0.00625 mol


Part 2: Use the balanced equation to find the mole ratio between the analyte and the titrant.

We first write the balanced equation for the reaction between the analyte and the titrant:

2KOH(aq) + H2SO4(aq) → K2SO4(aq) + 2H2O(l)

We are interested in the ratio between the KOH and the H2SO4, so:

mole ratio= 2KOH : 1H2SO4

2 moles of KOH react with 1 mole of H2SO4

So, there isas much KOH as there is H2SO4.


Part 3: Calculate the number of moles of the analyte that reacted using the mole ratio and the number of moles of titrant.

We can write the information we know as a mathematical statement:

2 mol KOH = 1 mol H2SO4

x mol KOH = 0.00625 mol H2SO4

We can cross multiply to solve these equations:

2(0.00625)=x

x = 0.0125 mol KOH


Part 4: Divide the number of moles of analyte by the volume of analyte to find the concentration.

Molar concentration= 0.0125 mol/0.030 dm³= 0.417 mol/dm³


Sometimes you will also be asked to find the mass concentration of the analyte. In this case, you first find the molar mass of the analyte:

molar mass of KOH= (39+16+1) gmol⁻¹ = 56 gmol⁻¹

Then, you multiply the molar mass by the molar concentration to give you the mass concentration in g/dm³:

0.417 mol/dm³ × 56 gmol⁻¹= 23.25 g/cm³


Note as well that this same procedure can be used to convert from mass concentration to molar concentration if you divide the mass concentration by the molar mass instead.

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