Lesson Video: Titration Experiments Chemistry

Video Transcript

In this video, we will learn how to describe acid–base titration methods and their use in determining acid and base concentrations. In this video, we’re specifically going to be looking at acid–base titrations. So, what’s the purpose of an acid–base titration?

When doing an acid–base titration, our main purpose is to work out the concentration of an acid or base, which is currently at an unknown value. We do this by using an acid or base that we do know the concentration of. Of course, if our unknown solution is an acid, we want our known solution to be a base and vice versa. So, let’s have a look at how to set this up.

Here is an example setup of the equipment you’ll need to carry out an acid–base titration. At the bottom, you’ll need a conical flask. This is sometimes called an Erlenmeyer flask. The shape of this flask makes it ideal for swirling liquids. This is something you’ll need to do during the titration. By using a conical flask to swirl your liquids, you’re far less likely to splash anything out of the flask than if you were using something with a wide neck like a beaker. You’ll also need a buret. Burets are good for accurate liquid measurements in instances where you don’t know at first quite how much liquid you’re going to need. Of course, if you know exactly how much liquid you’ll need, you would use a pipet.

You’ll also need a retort stand, sometimes called a clamp stand. This has a heavy metal base and a metal pole that extends upwards. It’s onto this pole that you can attach clamps with which to secure your buret. Of course, you don’t want to make this too tight and risk damaging the glassware. When setting up your retort stand, you need to be careful about which orientation you have it in. You want to make sure that everything that you’re clamping is overhanging the large part of the base. If you have it the wrong way round, you risk everything toppling over. This can be both dangerous and expensive. You may also decide to place a white tile directly beneath your conical flask. This can be helpful if you’re looking for subtle color changes.

Now that we have our basic setup, we need to work out how to run this titration. Let’s begin by looking at our two main solutions. Our two solutions are referred to as the titrant and the titrand. You may also hear the titrant referred to as the standard solution. This is the solution that we do know the concentration for, and this is the solution that we must place in our buret. On the other hand, our titrand might be referred to as the analyte. This is our solution which has an unknown concentration, and it’s this solution which should go into your conical flask. Bear in mind that even though we aren’t certain of the concentration of our titrand, we must use an exact known volume. This will be important for the calculations that you perform after you’ve done the experiment. We won’t go into the calculations in this video.

So, how could you remember which goes where? You could remember that the unknown concentration is a known volume. And since you have to know the volume at the start, that must go into the conical flask. There’s no point in using a buret to add an exactly known volume. Alternatively, you could remember that the titrant ending in T goes in the top and the titrand ending in D goes down at the bottom. Once we’ve added both of our solutions to the correct glassware, it’s time to add a few drops of indicator. We’ll look at how to select the right indicator later on. We’re now ready to run our titration.

It’s a good idea if your first run is very rough. So rather than adding your titrant dropwise right from the beginning, add it a few centimeters cubed at a time until you notice the indicator change. Once the indicator has changed color and swirling it doesn’t make it turn back, you’ve got a rough idea of how much titrant you need. Let’s say that in this example, it took about 40 milliliters before the indicator changed. Now, we’ll need to clean out our conical flask and refill everything. Our subsequent runs will now need to be much more careful. We can start our more accurate run by adding a bit less than in step one. As an example, let’s say we add about 35 milliliters.

Next, we need to add the titrant a couple of drops at a time, making sure that we give our conical flask a good swirl. We continue adding a drop or two of our titrant at a time and swirling and then adding a few more drops and swirling until our indicator reaches the end point. This means that the indicator is just turning whichever color it’s supposed to. Of course, you’ll need to repeat this experiment several times to make sure that you’re getting the right result. You want your results to be both accurate and precise.

Now that we have a rough idea of how to carry out an acid–base titration, let’s take a closer look at our titrant. Remember that the titrant is also called the standard solution, and this is the solution for which we know the exact concentration. So how do we make up a standard solution? Often, it depends what type of solution you are preparing. It could be that you need to add an exact known mass of a solid to a volumetric flask and make it up exactly with your liquid. For example, you might need an exact mass of sodium hydroxide pellets and make it up to the line on your flask with water. Alternatively, it could be that you need to mix two liquids together. This might be the case, for example, if you need to prepare a hydrochloric acid solution. It could be that the hydrochloric acid you already have is too concentrated, so you need to mix it with some water to dilute it.

Either way, the key points are that you have to know exactly how much of each substance you’re putting into your standard solution. To measure exact masses, of course, you’ll want to use a balance. To measure volume, you’ll want a pipet or volumetric flask. However you make up your standard solution, make sure that you make plenty of it. You’ll need to run your titration for maybe five times, so you’ll need to have enough standard solution for all of these runs. You probably want a little bit extra as well, just in case.

Preparing the titrand is much more straightforward. You simply add an aliquot of your titrand to the conical flask. Aliquot is simply the scientific way of saying a small sample. Remember that since your titrand has an unknown concentration, we must use a known volume. You could use something like a measuring cylinder, but a volumetric pipet is much more accurate. You could also use a graduated pipet.

Now that we have our two main solutions ready, we should have a look at indicators. So, why do we need an indicator for our acid–base titration? In this example, let’s imagine that our titrant that is the solution in the buret is a base and our titrand or analyte in the conical flask is an acid. Let’s say that the starting pH of our acid is four. As we add drops of base and swirl, our pH will start to increase. We’re aiming to add just enough base that will neutralize our acid, leaving us at pH seven. As we get close to neutral, our indicator may start to just change color. But as we swirl, the color changes back. As we exactly neutralize the acid with our base, we would expect the color to remain permanently even when we swirl. And we just want a hint of color change. This moment is our end point. If we were to carry on adding our titrant, the pH will continue to rise, and the color will become much darker. This means we’ve gone too far. If this happens, we’ll have to start again.

Indicators help us to recognize this end point. The indicator in this example is phenolphthalein. Phenolphthalein is colorless in acids but turns pink in bases. So when using phenolphthalein as an indicator, you can recognize the end point because you have just a tiny hint of pink which doesn’t go away with swirling. Of course, if you’re doing this the other way round and your titrant is an acid and your titrand is a base, what you’re really looking for is for that lovely pink color to have just disappeared completely. Another commonly used indicator is methyl orange. Methyl orange is red in an acid and yellow in a base. The end point for this indicator is when your solution is a hint of orange. The way that indicators work is that they themselves are actually weak acids or bases, and this is what helps them to change color.

There are, of course, many, many other indicators, but these are the two most common. It should also be noted that not all indicators change color at pH seven, and you could also use a pH meter instead of using an indicator. We don’t tend to use litmus paper or universal indicator for acid–base titrations. Using litmus paper is just too fiddly and not accurate enough. And universal indicator changes colors over a wide range of pHs, which is not very helpful for titrations.

Now that we have a solid understanding of titrations, let’s have a look at some of the things that might go wrong. The first potential source of problems in titration experiments is the equipment. Incorrect use of the equipment can cause your results to be incorrect. An inaccurate buret reading is a common problem, and there are lots of reasons that you might get an inaccurate reading. Imagine you need to read the value from this buret. Where do we take the reading from? Which of these is correct? The answer is the bottom one. You should always take your reading from the bottom of the meniscus.

Other tips for always ensuring an accurate buret reading are get rid of any bubbles, make sure that you always know your starting value before you run your titration — usually, this is at zero, but not always — never start using your buret if the liquid is above zero mLs, and if you filled your buret with a funnel, always remove the funnel before beginning your experiment. This is because the end of the funnel can display some of the liquid and thereby change your readings or the funnel could contain extra drops of liquid which will also alter your readings.

But what else could go wrong with the equipment? If you don’t use an exact volume of titrand in your conical flask, that can also affect your results, as can incomplete mixing. If you end up of droplets of titrand stuck on the neck of the conical flask, this has been accounted for in your buret reading but has not mixed with your titrand. This means that your results will not be accurate. You can avoid these droplets by being careful how you use your buret and by ensuring that you give your titrand a good swirl.

You also want to avoid droplets stuck to the end of the buret. If this happens, gently touch it onto the side of the conical flask and swirl it in. Muddling up the solutions is also a possibility with titrations. To avoid this, make sure everything is carefully labeled. This will hopefully stop you putting the wrong solution in the wrong piece of glassware.

The indicator can also cause issues. For example, you might have chosen an indicator where it’s really difficult to detect the color change. An inappropriate indicator or simply not being careful with your experiment could mean that you miss your end point. There are also safety hazards associated with titrations. Doing these experiments means using acids and bases, many of which are harmful or corrosive, so care must be taken. You should always make sure that your equipment is secure and safe before you start your experiment. This will prevent your buret and all of your equipment from tipping over.

You should also always fill your buret safely. This means never filling your buret above eye level. To do this, you could place your buret on a stool or on the floor. This prevents you from splashing potentially harmful chemicals onto your head, face, or into your eyes. You may also choose to use a funnel to fill your buret to avoid spillages. This is perfectly fine, but remember to remove the funnel before starting the experiment. Now, let’s have a look at some questions.

A student wants to use titration to determine how much acid is needed to neutralize a known volume of base. They set up the experiment as shown. At the start of the experiment, which solution should go into the Erlenmeyer flask and which should be used to fill the buret?

This question is asking about an acid–base titration. This type of experiment is used to determine an unknown concentration. The solution which fills the buret is called the titrant. You can remember that the t at the end of titrant could stand for top, meaning it goes in the top. The titrant has a known concentration, and it’s sometimes referred to as the standard solution. Our titrand goes into the Erlenmeyer flask, sometimes called a conical flask. This is the solution with an unknown concentration, sometimes called the analyte.

The question tells us that we have a known volume of base. Our titrand with the unknown concentration must have a known exact volume. So, in this case, the base is our titrand and should go into the Erlenmeyer flask. This means that the acid should go into the buret.

So, let’s summarize. We know that the titrant with known concentration should go in our buret, while the titrand with an unknown concentration goes in our flask. And there’s a range of indicators we can use to find our end point. And the keys to a successful titration are a careful setup, dropwise addition, and plenty of swirling.