Titrating Sodium Hydroxide with Hydrochloric Acid - The technique known as titration is an analytical method commonly used in chemistry laboratories for determining the quantity or concentration of a substance in a solution. In a titration, an analyte the substance whose quantity or concentration is to be determined is reacted with a carefully controlled volume of a solution of accurately-known concentration called a standard solution. The standard solution (also known as the titrant) is usually added to the solution containing the analyte by means of a buret, a piece of volumetric glassware capable of accurately measuring solution volumes.
There are many types of titrations in common use in the analytical chemistry laboratory. Each type uses a different kind of chemical reaction. Examples of titration types include
- acid-base titration,
- reduction-oxidation titrations,
- precipitation titrations, and
- complexometric titrations.
The most commonly studied type is acid-base titration. For this project, we focus our attention exclusively on this type, and we develop a mathematical model that describes the relationship between the volume of added titrant (a base) and changes in concentration of the analyte (an acid). The model presented would need only slight modifications to be applicable to other titration types listed above.
After the addition of each increment of the base, the volume of the base is carefully read from the buret (measured to the nearest hundredth of a milliliter) and the hydrogen ion (H+) concentration of the solution is measured with a pH meter. The pH meter gives the hydrogen ion concentration in terms of pH, which is simply the negative logarithm (common log, i.e., base 10) of the hydrogen ion concentration:
pH = -log[H+],
where [H+] represents the concentration of H+.
The hydrogen ion concentration is directly related to the amount of acid present in the solution at any particular step in the titration according to the following chemical reaction:
HA (an acid) <=>H+ hydrogen ion + A- (the anion of the acid)
The chemical reaction that occurs between the acid and the base allows one to calculate the initial concentration (or amount) of the acid. For example, in the titration of hydrochloric acid (HCl) with a base such as sodium hydroxide (NaOH), the chemical reaction between these two species would have to be known. The reaction is as follows:
HCL + NaOH <=> NaCl + H20
This reaction states that 1 molecule of HCl will react with 1 molecule of NaOH to produce 1 molecule of salt, sodium chloride (NaCl), and 1 molecule of water. The chemical equation allows us to calculate the concentration of a solution of HCl by titration with the base NaOH (where the concentration of NaOH is accurately known).
Let's suppose that our solution is 0.02500 L of an unknown concentration of the acid, HCl. We wish to find its concentration by titration with 0.1000 M NaOH. (M is the notation for the concentration unit called molarity, which is defined as the number of moles of a substance per liter of solution. A mole is equal to 6.022 x 1023 molecules.) By doing the titration and making a plot of the volume of NaOH added versus the resulting pH of the solution, we find that the equivalence point occurs at 0.04398 L of NaOH. (This is the point where the plot appears to increase most rapidly.) Recall that at the equivalence point the amount of base added is chemically equal to the amount of acid present in the solution. According to the chemical reaction (1) between HCl and NaOH, 1 molecule of HCl will react with exactly 1 molecule of NaOH. Therefore, the number of moles of base needed to react with all of the acid presents is the same as the number of moles of acid present in the solution. So, at the equivalence point,
moles HCL = moles NaOH.
The measurement of the hydrogen ion concentration (or pH) at each point in the titration allows us to find the location of the equivalence point, that volume of the base which reacts completely with the unknown concentration of acid. It is at this equivalence point that the amount of base added is chemically equal to the amount of acid present. By chemically equal we mean that the number of molecules of base added is just enough to completely react with all of the molecules of acid originally present -- so that all of the acid molecules are, in a sense, used up. By knowing the volume of the base at the equivalence point, as well as the concentration of the base, one can calculate (among other things) the initial concentration of the acid.
In a typical acid-base titration experiment, the solution containing the analyte (an acid of unknown identity and/or concentration) is placed into a container, and the titrant (a base of accurately-known concentration) is slowly added from the buret in small increments
The most common use of titrations is for determining the unknown concentration of a component (the analyte) in a solution by reacting it with a solution of another compound (the titrant). From the known concentration of the titrant, the volume of titrant added and the stoichiometry of the reaction, the concentration of the analyte can be determined.
While there are many different types of titrations, acid-base titrations are the most common. Consider this example in which a sample of hydrochloric acid (HCl) is titrated with sodium hydroxide (NaOH).
HCl + NaOH <=> NaCl + H2O
During the course of the titration, the titrant (NaOH) is added slowly to the unknown solution. As it is added, the HCl has slowly reacted away. The point at which exactly enough titrant (NaOH) has been added to react with all of the analyte (HCl) is called the equivalence point. Up to the equivalence point, the solution will be acidic because excess HCl remains in the flask. After the equivalence point, there will be an excess of NaOH and the solution will be basic.
An acid-base indicator can be used to help identify when the change from acidic to basic occurs. Phenolphthalein is colorless in acidic solutions and bright purple in basic solutions. As such, it should change from colorless to purple around the equivalence point of this titration. Our approximation of where the equivalence point occurs is called the endpoint. Care must be exercised when an indicator is chosen for titration to ensure that the endpoint coincides as closely with the equivalence point as possible.
The steps below would be used to perform the titration described on the previous page.
1. A known quantity of the unknown solution (HCl) is pipetted into a flask and several drops of an indicator are added. If phenolphthalein is being used as an indicator, the solution should remain colorless at this point. The flask is placed on white paper to make the endpoint easier to see.
2. Make sure the buret stopcock is closed and then rinse the inside with several milliliters of titrant (NaOH). The buret should be held nearly horizontally and rotated so that all of the inside surfaces are contacted by the titrant. Some titrant should also be run through the stopcock to clean it as well. Cleaning is normally performed over a sink.
3. Make sure the stopcock is closed. Place the buret in a buret clamp and fill it carefully with the titrant. Use a beaker with a spout or funnel to reduce the possibility of spilling titrant.
4. Drain some titrant through the stopcock into a waste beaker. Make sure that no air bubbles remain in the stopcock.
5. Read the volume of the buret. This is your initial volume (14.62 ml in this case). Reading is made easier by holding a piece of dark paper behind the buret.
6. Place the flask containing the unknown under the buret. Slowly open the stopcock and add some titrant (usually a milliliter or so). You may notice a temporary color change in the solution near where the titrant was added. Stir the solution thoroughly. Any color change should disappear.
7. Continue adding titrant in small quantities. As the titration progresses, the color change described in step 6 will take longer to disappear. This signals that the endpoint is getting closer and that the titrant should be added in smaller and smaller quantities. Titrant should be added dropwise very close to the endpoint.
8. The endpoint of the titration is signaled when a permanent color change is observed (longer than 30 seconds). It is possible to overshoot the endpoint by adding too much titrant. A correct endpoint is shown on the left, and overshot endpoint on the right.
9. Record the volume in the buret. This is your final volume (26.48 ml in this case). Subtract the initial volume (step 5) from the final volume to determine the volume of titrant added (26.48 - 14.62 = 11.76 ml).
10. Use standard chemical calculation methods to determine the concentration of the analyte.