Lesson Explainer: Titration Experiments Chemistry • Third Year of Secondary School

In this explainer, we will learn how to describe acid–base titration methods and their use in determining acid and base concentrations.

When we want to determine the concentration of an acid or a base, we can perform a titration experiment. In a titration experiment, a solution with a known concentration is added to an exact volume of a solution with an unknown concentration in the presence of an indicator.

The apparatus necessary for the experiment is shown below.

A buret is used in this experiment because we are unsure of exactly what volume of a solution with a known concentration will be necessary. The stopcock or faucet at the bottom of the buret allows us to easily add a little amount of solution at a time. An Erlenmeyer flask, also known as a conical flask, is used because the flask can easily be swirled without spilling. Notice that the buret and Erlenmeyer flask are placed over the base of the retort stand to prevent the apparatus from toppling over. The white tile is optional but can help us to more easily identify subtle color changes.

A standard solution, also called a titrant, will be placed in the buret. A standard solution is a solution that has a known concentration.

If the solution that we are trying to determine the concentration of is an acid, the standard solution should be a base and vice versa. Standard solutions can be purchased but are often easily made in the laboratory.

Titration experiments are often performed four or more times, so enough standard solution must be prepared to account for all of the trials.

When adding a standard solution to the buret, it is important to ensure that the stopcock at the bottom of the buret is in the closed position, perpendicular to the column.

A buret should always be filled at eye level to prevent spilling corrosive acids or bases into your face or eyes. A funnel can be placed into the top of the buret to further help prevent spillage when filling. To fill the buret, we slowly add titrant, filling to near the 0 mL mark. Then, we remove the funnel. It is not necessary to begin a titration at exactly 0.00 mL.

We should then check the buret for air bubbles, gently tapping to remove them. Next, we place a waste container under the buret and open the stopcock to allow a few milliliters of liquid to drain. We should use a wash bottle to rinse the tip of the buret with deionized water. We can then record the starting volume of titrant. We must remember to read the volume from the bottom of the meniscus.

Example 2: Understanding the Risks Associated with Filling a Buret

Why should a buret always be filled at eye level and never above?

1. To reduce the risk of splashing acid or base onto the eyes or face
2. To know when the beaker or measuring cylinder is empty
3. To make sure the solution is poured into the buret and not on the floor
4. To watch the solution move down the buret
5. To allow the buret to be filled while sitting down

Answer

A buret is frequently used when performing an acid–base titration and is therefore often filled with an acid or base. Acids and bases are corrosive. They can irritate or burn the eyes and skin and can cause respiratory distress. To minimize the potential for spilling such substances on the eyes or face, a buret should never be filled above eye level. The correct answer is choice A.

The solution of unknown concentration is called the titrand or analyte.

An exact volume of titrand should be added to an Erlenmeyer flask using a volumetric pipet. The volume of titrand used is sometimes called an aliquot.

Example 3: Choosing Which Solution to Place into an Erlenmeyer Flask and Buret in an Acid–Base Titration

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?

Answer

The Erlenmeyer flask is filled using a pipet. A pipet is a piece of glassware used to deliver a specific volume of liquid. A buret is a piece of glassware that is filled with a liquid that can then be dispensed. Burets are used when the volume of liquid necessary is not known as the stopcock at the bottom of the buret can easily be opened or closed to control the volume of the liquid delivered.

In this titration experiment, a student is using a known volume of base but does not know the total volume of acid needed. Therefore, the base should go into the Erlenmeyer flask and the acid into the buret.

During a titration experiment, the titrant will be added to the titrand until the acid and base have completely neutralized one another. This is called the equivalence point.

The approximate volume of titrant needed to reach the equivalence point can be visually determined by adding an acid–base indicator to the Erlenmeyer flask. An indicator is a weak acid or base that undergoes a color change over a specific pH range.

There are a variety of indicators that can be chosen for a titration experiment depending on the acid and base used as the titrant and titrand. Near the equivalence point, addition of a single drop of titrant will cause the pH of the solution in the Erlenmeyer flask to change drastically. A good acid–base indicator will exhibit an abrupt color change during this point in the experiment.

The equivalence point of the reaction between a strong acid and strong base will occur at a pH of 7. However, a drastic pH change with the addition of single drops of titrant will occur between a pH of approximately 3.5 and 10.5. Therefore, we should choose an indicator that changes color within this range.

There are two indicators that are commonly used for strong acid–strong base titrations. They are phenolphthalein and methyl orange. We commonly say that phenolphthalein is colorless in acid and pink in base; although technically, this color change occurs between a pH of 8.2 and 10. We commonly say that methyl orange is red in acid and yellow in base; although technically, this color change occurs between a pH of 3.2 and 4.4.

A universal indicator is commonly found in the chemistry laboratory and is often used to test a solution to determine an approximate pH.

However, the universal indicator is not used in titrations because it changes color over a wide range of pH values and the color changes are less abrupt than other indicators.

Example 4: Determining the Color of an Indicator at One pH Value and Which Indicator to Use When the pH Changes from 8 to 11

The table below shows the color range for several different indicators.

1. Which indicator is blue at a pH of 5?
2. Which indicator would be best to show that the pH of a solution has changed from 8 to 11?

Answer

Part 1

The graph shows the pH range in which each indicator changes color. For example, bromophenol blue changes color from yellow to blue between a pH of 3 and 4.6. Below a pH of 3, bromophenol blue will be yellow and above a pH of 4.6 bromophenol blue will be blue.

At a pH of 5, bromophenol blue will be blue, bromothymol blue will be yellow, cresolphthalein will be colorless, and alizarine yellow will be yellow. The indicator that is blue at a pH of 5 is bromophenol blue.

Part 2

The indicator that would be best to show that the pH of a solution has changed from 8 to 11 will be an indicator that undergoes a color change in this pH range. Bromophenol blue and bromothymol blue will both remain blue as the pH changes from 8 to 11. Alizarine yellow will be yellow between a pH of 8 and 10 and will begin to turn orange as the pH gets closer to 11. There is color change; however, it occurs at too high of a pH. Cresolphthalein is colorless at a pH of 8 and purple at a pH of 10. As cresolphthalein undergoes a drastic color change in the desired pH range, it would be the best indicator to use for this experiment.

Let us consider a titration experiment where a base titrant, acid titrand, and phenolphthalein indicator are used. At the start of the experiment, phenolphthalein and the acid titrand will be in the Erlenmeyer flask. As phenolphthalein is colorless in acid, the flask will appear colorless.

When the base titrant is added, a bright-pink spot will appear in the Erlenmeyer flask as the phenolphthalein reacts with the base and turns pink. However, the color will quickly disappear as the acid titrand in the flask reacts with the base, neutralizing it. It is important to continue to swirl the flask, to allow the acid and base to react completely during this process.

As more base is added to the flask, the pink color will remain for longer periods of time. This is because as the experiment progresses, there is less acid in the solution and the acid–base neutralization reaction takes longer to occur. Once all of the acid has been neutralized, any additional base added will remain in the solution and can react with the phenolphthalein. When adding a drop of base causes the solution to turn faintly pink, the end point of the experiment has been reached.

It is important to recognize that the end point of the experiment and the equivalence point are often not the same. The equivalence point occurs when all of the acid and base have been neutralized, but the end point occurs when the indicator has changed color.

The final volume of titrant in the buret should be recorded when the end point has been reached. If the Erlenmeyer flask appears a dark-pink color, too much base has been added and the experiment will need to be repeated.

The table below shows the desired end point for the phenolphthalein indicator and methyl orange indicator.

Example 5: Recalling the Colors of Methyl Orange

Fill in the blanks: Methyl orange is a useful indicator, particularly for titrations against acids. It is in strongly acidic solutions and in basic solutions.

Answer

Methyl orange is an indicator that is red in a solution with a pH less than 3.1 and yellow in a solution with a pH greater than 4.4. Solutions are acidic when the pH is less than 7, neutral when the pH is equal to 7, and basic when the pH is greater than 7.

This means that methyl orange will appear yellow in all basic solutions and red in strongly acidic solutions. We should fill in the first blank with the word red and the second blank with the word yellow.

It is very easy to add too much titrant during a titration. We can get an estimate of the amount of titrant needed to reach the end point by first performing a rough titration.

Subtracting five milliliters from the volume of titrant used in a rough titration tells us how much titrant we can safely add quickly during a good titration. Never use the rough titration titrant volume in any titration calculations.

There are several common errors that can occur when performing a titration experiment. Measurement errors include inaccurate measurement of the titrand or misreading the initial or final buret volumes. Air bubbles in the buret will affect the volume readings as well. We can remove air bubbles by gently tapping the buret. In addition, if extra unreacted titrant is on the buret tip or in the neck of the flask, we can record a titrant volume that is too large. We can eliminate this error by rinsing the tip of the buret and the neck of the flask with deionized water when we are close to the end point of the titration.

If the indicator never changes color during the titration, the wrong indicator may have been selected or the same solution may have been placed in both the buret and Erlenmeyer flask. If the Erlenmeyer flask remains colorless during a titration involving phenolphthalein, double-check that the indicator was added to the flask.