Types and Methods of Titration
Introduction
As a chemist, you rely on accurate titration methods to determine the concentration of unknown solutions. Acid-base titrations are fundamental techniques you will use in your work. To perform them properly and obtain precise results, you must understand the types of acid-base titrations and how to execute them.
In this guide, we will explore the major categories of acid-base titrations you may encounter: strong acid-strong base, weak acid-strong base, weak base-strong acid, and polyprotic acid-base titrations. With the right technique and equipment for each method, you will be able to determine unknown concentrations and become proficient in this essential analytical procedure. Continue reading to learn the details of how to perform accurate acid-base titrations.
Types and Methods of Titration
What Is Acid-Base Titration?
Acid-base titration is a method for figuring out how much of an unknown acid or base there is in a solution.. An acid-base indicator or pH meter is used to monitor the pH and determine the endpoint of the titration. There are various kinds of acid-base titrations, including:
Types and Methods of Titration
Strong Acid-Strong Base Titration
This involves titrating a strong acid (like HCl) with a strong base (such as NaOH). At the endpoint, the number of moles of OH- equals the moles of H+. Using the concentration and volume, the unknown concentration can be calculated.
Types and Methods of Titration
Weak Acid-Strong Base Titration
A strong base, such as NaOH, is used to titrate a weak acid, such as acetic acid. The endpoint is determined by the indicator changing color. More OH- is needed to neutralize the weak acid than if it were strong. Calculations factor in the acid dissociation constant (Ka) of the weak acid.
Types and Methods of Titration
Polyprotic Acid Titration
Polyprotic acids like H3PO4 or H2CO3 release H+ in stages, so multiple equivalents of OH- are needed to reach the endpoint. The pH changes more gradually, requiring indicators that span a wide pH range. Additional calculations account for the stepwise ionization.
Types and Methods of Titration
Strong Acid-Weak Base Titration
A strong acid is titrated with a weak base, e.g. NH3. More NH3 is needed to neutralize the H+ than if using a strong base. The dissociation constant of the weak base (Kb) is used in calculations.
Acid-base titrations allow quantification of unknown concentrations when performed carefully with the proper technique, indicators, and calculations. With some practice, these useful methods can become second nature.
Types and Methods of Titration
Strong Acid-Strong Base Titrations
When performing a strong acid-strong base titration, you’ll be working with
concentrated solutions that react vigorously. So proceed carefully! This type of
titration is useful for determining the concentration of an unknown acid or base.
Types and Methods of Titration
Equipment
You’ll need:
- A buret for delivering the titrant
- An Erlenmeyer flask for the analyte solution
- A magnetic stirrer and stirring bar
- A pH meter to monitor the solution’s pH
- Standard solutions of 0.1 M NaOH (the titrant) and HCl (the analyte)
Types and Methods of Titration
Procedure
- Rinse the buret with NaOH and fill it with the standard 0.1 M NaOH solution. Record its initial volume.
- Add about 25 mL of the unknown HCl solution to the Erlenmeyer flask along with a stirring bar. Flask should be set atop the magnetic stirrer.
- Add NaOH from the buret while monitoring the pH. The solution’s pH will remain unchanged and then abruptly increase. This marks the end point. Record the final NaOH volume.
- Calculate the unknown HCl concentration using the formula: M1V1 = M2V2. M1 is the molarity of NaOH, V1 is the volume of NaOH needed to reach the end point, M2 is the unknown molarity of the HCl, and V2 is the volume of HCl in the flask.
- Repeat the titration two more times to confirm your results. The values should be within 0.05 mL of each other.
By following this procedure carefully and accurately recording your results, you’ll be able to determine the precise concentration of an unknown strong acid solution using a strong base titration. With practice, these titrations can become second nature!
Types and Methods of Titration
Weak Acid-Strong Base Titrations
A weak acid-strong base titration involves titrating a weak acid with a strong base. The most common example is acetic acid (CH3COOH), a weak acid, titrated with sodium hydroxide (NaOH), a strong base.
Types and Methods of Titration
Procedure
To perform a weak acid-strong base titration, follow these steps:
- Standardize the NaOH solution. Use a primary standard acid like potassium hydrogen phthalate (KHP) to determine the exact concentration of the NaOH.
- Prepare the analyte, which is your CH3COOH solution of unknown concentration. Dilute an aliquot of the concentrated weak acid solution in a volumetric flask.
- Fill the burette with the standardized NaOH solution. Make sure there are no air bubbles in the tip before starting the titration.
- Add a few drops of indicator to the Erlenmeyer flask containing the analyte. For this titration, use indicators like phenolphthalein or bromothymol blue.
- Titrate the weak acid with the strong base by slowly adding NaOH from the burette while swirling the flask. Add NaOH dropwise near the endpoint.
- When the indicator’s color changes, the endpoint has been achieved. Note the volume of NaOH added.
- Repeat the titration two more times to get consistent results. Calculate the concentration of the weak acid using the NaOH concentration and the volume at the endpoint.
Types and Methods of Titration
Key Points
- The pH at the endpoint is greater than 7 due to the weak acid’s buffering capacity. The pH range depends on the acid’s Ka value.
- Multiple indicators can be used since there is a large pH range for the color change.
- An excess of NaOH is needed to overcome the buffer and reach the endpoint.
- The weak acid concentration is calculated from the NaOH concentration and volume.
- Titrations of this type can also be performed with other weak acids like acetic acid, ammonia, and sulfurous acid. The general steps remain the same but the indicator and pH range will differ depending on the analyte.
Types and Methods of Titration
Weak Base-Strong Acid Titrations
A weak base-strong acid titration involves titrating a weak base with a strong acid. The most common example is ammonia (NH3) which is a weak base, titrated with a strong acid like hydrochloric acid (HCl).
Types and Methods of Titration
Procedure
To perform a weak base-strong acid titration, follow these steps:
- Standardize the strong acid solution using a primary standard base like potassium hydrogen phthalate (KHP). This will determine the exact concentration of the strong acid.
- Prepare the weak base solution of known concentration. For ammonia, weigh out a sample and dilute with distilled water.
- Add a few drops of indicator to the weak base. Methyl red indicator is commonly used for ammonia titrations, changing from red to yellow in the end point range.
- Fill the burette with the standardized strong acid solution.
- Add the strong acid from the burette to the weak base solution while swirling, until the indicator changes color.
- Note the volume of strong acid added, which is the titre value. Use it to calculate the concentration of the weak base.
The titration curve will be steeper near the equivalence point compared to a strong acid-strong base titration, due to the weak nature of the base. An indicator with a more gradual color change is required. This method can also be applied for other weak bases like methylamine.
Compared to a strong acid-strong base titration, an excess of strong acid needs to be added to overcome the weak base’s resistance to change in pH. More care and precision are also required in detecting the end point. However, the same principles of acid-base neutralization and stoichiometry are applied in the calculations.
By following these steps carefully, you can accurately determine the concentration of a weak base solution using a strong acid titration.
Types and Methods of Titration
Polyprotic Acid-Base Titrations
Polyprotic acid-base titrations involve acids or bases that can donate or accept more than one proton. These titrations require multiple equivalences points to reach the endpoint.
Types and Methods of Titration
Types of Polyprotic Acids and Bases
Common polyprotic acids include:
- Phosphoric acid (H3PO4) which has three acidic protons
- Carbonic acid (H2CO3) which has two acidic protons
- Sulfurous acid (H2SO3) which has two acidic protons
Types and Methods of Titration
Polyprotic bases include:
- Carbonate (CO3 2-) which can accept two protons
- Phosphate (PO4 3-) which can accept three protons
Titration Curves
The titration curves for polyprotic acids and bases have multiple equivalence points. Each equivalence point corresponds to the neutralization of one acidic proton (for acids) or the acceptance of one proton (for bases). The pH at each equivalence point depends on the strength of subsequent protons or lone pairs.
For example, the titration curve for the weak diprotic acid H2CO3 with NaOH has two equivalence points. The first occurs at pH 8.3, when one proton from H2CO3 is neutralized. The second occurs at pH 10.6, when the second proton is neutralized. The pH increases more gradually after the second equivalence point due to the weak conjugate base (HCO3-).
Titration of the weak base CO32- with HCl also has two equivalence points, corresponding to the acceptance of the first and second protons by CO32-. The pH decreases more gradually after the second equivalence point due to the weak conjugate acid (HCO3-).
Types and Methods of Titration
Applications
Polyprotic acid-base titrations are useful for:
- Determining the amount of a polyprotic acid or base in a solution
- Analyzing the composition of complex acid/base mixtures
- Elucidating the stepwise dissociation of polyprotic acids and protonation of polyprotic bases.
Polyprotic acid-base titrations provide insight into solution equilibria that involve multiple protons. With practice, these titrations can be successfully performed to gain a deeper understanding of acid-base chemistry.
Types and Methods of Titration
Conclusion
Through this overview, you should now have a solid understanding of the major types of acid-base titrations and how to perform them. Whether dealing with strong or weak acids and bases, these titrations allow you to determine unknown concentrations with a high degree of accuracy when done properly.
While the calculations and use of indicators may seem complex, with regular practice these techniques can become second nature. Acid-base titrations provide a straightforward way to gain valuable insights into solutions and compounds. With this knowledge under your belt, you’ll be well equipped to dive deeper into analytical chemistry.
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