Volumetric Methods of Chemical Analysis

Introduction to Volumetric Analysis

Analysis of Hydrogen Peroxide using Potassium Permanganate

Determination of the Molar Mass of an Unknown Acid

Analysis of CaCO₃ Content of Marble using a Back Titration

The equivalence point in a titration can be indicated by a colour change. This colour change is accomplished by adding an indicator that changes colour as close as possible to the equivalence point.

The point at which the visual colour change occurs is called the end point. A successful titration depends on the end point being very close to the equivalence point.

Let's examine the titration in a bit more detail. First we need a burette to hold the titrant.

The burette is used to deliver accurately known volumes of titrant. It has a graduated scale that can be read to an accuracy of 0.01 mL.

The burette uses a valve to deliver titrant drop by drop or as a steady stream.

Finally we need a beaker or flask to hold the sample to be titrated.

For the purposes of these computer experiments we will represent the experimental apparatus as zoomed views of the three areas of interest; the titrant meniscus, the stopcock valve, and the beaker containing the sample.

Analysis of Hydrogen Peroxide using Potassium Permanganate

You are employed by the quality control department of Guelph Peroxides, a company that supplies hydrogen peroxide to drugstores.

Recently there have been complaints about the quality of the product and you have been assigned to analyse some of the returned samples.

Although you are under a bit of pressure, you stay calm. You look in your reference books and find that titration with potassium permanganate is the recommended method of analysis.

The equation for the reaction is

2 KMnO_4(aq) + 5 H₂O_2(aq) + 3 H₂SO_4(aq)

    → 5 O_2(g) + K₂SO_4(aq) + 2 MnSO_4(aq) + 8 H₂O_(I)

We will use potassium permanganate as the titrant.

First we need to obtain a 1.00 mL aliquot of the H₂O₂ sample. Use your mouse to select the piece of laboratory equipment you would use to carry out this procedure.

No. The best choice is the pipette. A 1.00 mL pipette will accurately and reliably deliver the 1.00 mL aliquot of H₂O₂ that we require for this experiment.

An analytical balance has sufficient accuracy to measure 1.00 mL of liquid but it is poorly suited to this task. The balance is best suited to tell you the exact weight of a sample but it does not help in preparing a sample which must have some specific weight or volume.

Neither a graduated beaker nor a graduated cylinder have sufficient accuracy to prepare a 1.00 mL aliquot. These pieces of apparatus are usually only calibrated to 0.1 mL.

Yes, a 1.00 mL pipette will accurately and reliably deliver the 1.00 mL aliquot of H₂O₂ that we require for this experiment.

When we perform the titration what indicator should we use?


1) Phenolphthalein

2) Methyl Orange

3) Starch

4) No indicator required

In this case no indicator is required.

Potassium permanganate is a deep purple solution. When all of the H₂O₂ has been used up, the solution becomes pink because of the presence of excess KMnO₄. Thus the endpoint is the appearance of a permanent, faintly pink solution.

Yes, no indicator is required.

Potassium permanganate is a deep purple solution. When all of the H₂O₂ has been used up, the solution becomes pink because of the presence of excess KMnO₄. Thus the endpoint is the appearance of a permanent, faintly pink solution.

Before we begin our titration we will do one last thing.

Since our H₂O₂ aliquot of 1.00 mL is so small we will dilute it with about 20 mL of water. This gives us a larger total volume and makes the observation of the end point easier.

A titration measures the number of moles of H₂O₂ in our sample. Once we have placed our 1.00 mL aliquot of H₂O₂ into the beaker we can dilute it as much as we want since this doesn't change the number of moles of H₂O₂ in the beaker.

Our lab assistant has prepared several standardized solutions of aqueous potassium permanganate that we can use as our titrant. Since we don't know the exact concentration of the hydrogen peroxide we will probably have to try several titrations before we find a titrant that will give an acceptable titrant volume between 10 and 50 mL.

Volumes below 10 mL are not sufficiently precise and volumes in excess of 50 mL exceed the maximum volume of the burette.

You use your mouse to perform a titration.

To add a drop of titrant click on the drop icon near the valve. This adds about 0.02 mL of titrant.

Press and hold down the stream icon located immediately below to add a steady stream of titrant. The rate of addition increases as you continue to hold down the button.

If you know where the end point occurs, you can click on the add volume icon to add a specific amount of titrant. In the popup, type the volume of titrant (in mL) to add.

To retry a titration or switch to another titrant click the restart icon. This will get a fresh sample, let you refill the burette and try again.

When your titration is done click the done icon. Type the burette volume reading when you are prompted. The computer will warn you if you have misread the burette or your end point is too far from the equivalence point.

Your assistant has retrieved a sample for you to analyse. Make sure that you record the sample number as you will need this later. The sample number is 7865.

Your assistant has prepared four aqueous potassium permanganate solutions with different concentrations. Click on the solution with which you would like to titrate.

Your assistant has filled the burette with your selected titrant and has prepared a sample for us to titrate by pipetting 1.00 mL of the hydrogen peroxide sample and then diluting to 10 mL with distilled water.

With the titration results you've obtained you will be able to calculate the concentration of the hydrogen peroxide solution.

Determination of the Molar Mass of an Unknown Acid

This next experiment gives another application of volumetric analysis.

Our unkown acid will be called HX. It reacts with NaOH by the following equation.

NaOH_(aq) + HX_(aq) → NaX_(aq) + H₂O_(I)

Thus 1 mole of NaOH reacts with 1 mole of HX.

How should we carry out this experiment?

1) Pour about 20 mL of NaOH from a volumetric flask into a beaker and then add enough acid to neutralize it.

2) Add exactly 20.00 mL NaOH from the burette to a beaker and then add enough acid to neutralize it.

3) Weigh out exactly a small amount of HX and then add enough NaOH from a burette to neutralize it.

Methods 1 and 2 don't work because we have no way to accurately 'titrate' the NaOH sample with solid HX. Method 1 also does not accurately determine the amount of NaOH used.

Only method 3 where a weighed sample of HX is titrated with standard NaOH from a burette will work.

That's right. Only method 3 where a weighed sample of HX is titrated with standard NaOH from a burette will work.

You have been given an acid sample in a small vial. Be sure to record the number on the vial in your laboratory notebook.

Your sample number is 1234.

The first step is to weigh the vial containing the acid unknown. Later we will remove some of the acid to use as our sample and then we will weigh the vial again. The difference between the before and after weights will tell us the weight of the sample.

We will measure the weights using an electronic analytical balance.

The balance is controlled in the following manner.

The balance is turned on and off using the on/off button. When the balance first turns on the display may not read exactly 0 grams. Press the tare button to zero the display.

Press the + (plus) button to place the vial on the balance. The - (minus) button removes the vial. Finally, when you are done press the done (check) button to have the computer check your value and continue.

Now we will add some of the contents of the vial to a beaker. We will transfer about 0.5 g.

Since we have now removed all of the HX that we want to use we must weigh the vial again. The difference between this new weight and the previous weight of the vial will tell us how much HX we removed and put into our beaker.

Now that we know the exact weight of the HX sample we are ready to titrate it.

First we will add about 10 mL of water to the sample to dissolve it completely. We will also add a few drops of an acid base indicator.

Our lab assistant has once again prepared a standard titrant solution. In this case it is a 0.09156 mol/L NaOH solution.

Be sure to record its concentration in your laboratory book.

Your assistant has filled the burette with the titrant for you.

We've transferred about 0.5 g of HX into a new clean beaker and have dissolved it in about 10 mL of water. You will need to reweigh the vial to determine exactly how much HX we transferred. The difference between the new weight and the previous will be the weight of the sample.

Using the information that you have gathered during this experiment you will be able to determine the molar mass of this sample.

Since we know that the sample is pure, this information could lead to the identification of the acid although we will not attempt to do so in this case.

Analysis of CaCO₃ Content of Marble using a Back Titration

The chemical reactions involved in this analysis are:

1) CaCO_3(s) + 2 HCl_(aq) → CO_2(g) + H₂O_(I) + CaCl_2(aq)

The CaCO₃ is dissolved in HCl. Note that the dissolution of the CaCO₃ in HCl is a chemical reaction. In order to be sure that all the CaCO₃ dissolves an excess of HCl is deliberately added.

The next step involves the titration of the excess HCl using a standard NaOH solution.

2) HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(I)

Our sample consists of a fine white powdered sample of marble contained in a small vial. Remember to record the sample number in your lab book.

Your sample number is 1234.

The first step is to weigh the vial containing the marble powder. Later we will remove some of the marble to use as our sample and then we will weigh the vial again. The difference between the before and after weights will tell us the weight of the sample.

Now we will add some of the powdered marble sample to a beaker. We will transfer about 0.25 g.

After doing this we must reweigh the vial again.

Now we know the exact weight of the marble powder that we placed in the beaker. The next step is to completely dissolve the sample using 50.00 mL of 0.1007 mol/L HCl delivered by a pipette. We will also add a few drops of an acid-base indicator.

Now that our sample is ready we can titrate the excess HCl (the unreacted portion) with a standard NaOH solution. Our lab assistant has prepared a standard NaOH solution with a concentration of 0.02510 mol/L.

Be sure to record these concentrations in your laboratory book.

Your assistant has filled the burette with the titrant for you.

We've transferred about 0.25 g of marble dust into a new clean beaker and have dissolved it in exactly 50.00 mL of 0.1007 mol/L HCl. You will need to reweigh the vial to determine exactly how much marble dust we transferred.

Having completed the titration we can now calculate the percentage of CaCO₃ in the marble sample. Different types of marble have CaCO₃ concentrations ranging from 50 to 100% by weight. Using our results we could identify the specific type of marble that we were given.