The Titration of Acids and Bases

Purpose
This experiment will allow you to gain practical experience in the preparing standard solutions, using a pipette and a buret, and performing standard titrations. You will use this experience to experimentally determine the concentration of acetic acid in an unknown solution. There are three parts to this experiment, they are 1. Preparation a standard solution of oxalic acid (~0.07 M) 2. Preparation a sodium hydroxide solution (~0.1M) which will be standardized using the standard oxalic acid solution. 3. Determination of the concentration of acetic acid in an unknown vinegar solution. (0.1-0.2M)

Introduction

Standard solutions are solutions with known concentrations, generally to four significant figures. There are two different ways to make a standard solution. We can make a primary or a secondary standard. A primary standard is prepared directly by dissolving a known mass of sample to make a known volume of solution. A secondary standard is prepared by dissolving an approximate amount of sample into a volume of solvent and determining its exact concentration through titration experiments. Primary standards are prepared from compounds that are at least 99.9% pure, have a definite composition, are water soluble, are easily weighed, and do not change composition on contact with air. Oxalic acid dihydrate (H2C2O4•2H2O) fits these criteria and therefore may be used as a primary standard. Sodium hydroxide absorbs water when it comes into contact with air and therefore it is difficult to obtain a pure, dry sample to weigh. For this reason the sodium hydroxide solution will be titrated with the oxalic acid standard to become a secondary standard.

In the first part of this experiment you will prepare a solution of known concentration of oxalic acid. The oxalic acid crystallizes with two water molecules per oxalic acid in the crystalline network. For this reason, we will weigh out an appropriate amount of oxalic acid dehydrate to dissolve in water. The water molecules in the crystal network will become part of the water of solution once it is dissolved. For this reason the molar concentration of oxalic acid dihydrate will be the same as the molar concentration of oxalic acid.

In order to standardize the sodium hydroxide solution you will perform a titration. Sodium hydroxide reacts with oxalic acid according to the reaction below: H2C2O4(aq) + 2 NaOH(aq) ( 2 H2O(l) + Na2C2O4(aq)

You will measure a 25.00 mL aliquot of the oxalic acid solution into a flask and add an indicator. An indicator is a substance that changes color when a solution changes from acidic to basic. The common indicator used for acid base titrations is phenolphthalein. Phenolphthalein is colorless in a solution that is acidic and bright pink in a solution that is basic. In this titration the oxalic acid solution is acidic and therefore phenolphthalein will be colorless. The sodium hydroxide solution will be added drop wise from a buret into the flask containing the oxalic acid and indicator. As the sodium hydroxide is added to the flask it will react with the oxalic acid and be neutralized. At the point where all of the oxalic acid is reacted, the next drop of sodium hydroxide will make the entire solution basic and it will turn pink. At this point you have completed the titration.

In order to determine the concentration of acetic acid in the vinegar solution you will titrate it with the standardized sodium hydroxide solution. The equation for this reaction is HC2H3O2(aq) + NaOH(aq) ( H2O(l) + NaC2H3O2(aq)

In order to get the best precision possible, you should repeat each titration until you get 3 trial that are within 1% of each other.

Procedure

Safety Notes
➢ Wear safety glasses at all times.

Preparation of Standard Oxalic Acid Solution
Carefully weigh a 100 or 150 mL beaker and record its mass. Measure between 2.1 and 2.3 g of pure oxalic acid dehydrate crystals into the beaker and weigh again. Add 30-60 mL of deionized water to the beaker and dissolve the crystals. You may gently heat the solution to speed up this process. Transfer the solution quantitatively into a clean 250-mL volumetric flask. Rinse the beaker with 15-20 mL of deionized water and pour this solution into the volumetric flask and repeat. This will ensure that all of the oxalic acid is transferred into the volumetric flask. Fill the volumetric flask to within about 2 cm of the mark and allow it to sit for a minute. This will allow any water clinging to the edges of the neck to drain into the flask.

Using an eyedropper, fill the flask to the mark with water. Stopper the flask and mix the solution by repeated inversion and swirling. This requires about 30 inversions and takes close to 1 minute. Next you will transfer this solution into one of the clean 500 mL bottles in your drawer. To transfer, first pour a small amount (~10 mL) of the standard solution into the bottle and rinse the inside walls of the bottle. Discard this wash solution. Do this two times to prevent the standard solution from being diluted with any pure water that might have remained in the bottle after washing. Calculate the molarity of the oxalic acid solution using the mass of acid used and the volume of the volumetric flask and record the concentration of your solution on the bottle. You label should look like the one pictured in figure 10.2.

Preparation of Sodium Hydroxide Solution
Calculate the volume of concentrated sodium hydroxide solution you must use to prepare 500 mL of approximately 0.1-0.15 M sodium hydroxide solution. (Check the label on the reagent bottle to determine its approximate concentration. The concentration will be 6M) Measure an appropriate amount using your graduated cylinder. Try to measure to within 1 mL of the desired amount of reagent. The exact amount is not important because you will be standardizing this solution later. Pour the concentrated base solution into a clean (need not be dry) 500 mL bottle and fill the bottle up to the shoulder with deionized water. Shake the bottle well and label it as above recording the concentration to 1 significant figure.

Titration
1. Pipette 25.00 mL of oxalic acid solution into a clean but not necessarily dry Erlenmeyer flask. (You may do three samples as you will be doing at least 3 titrations.)

2. Add 2-3 drops of phenolphthalein to the Erlenmeyer flask. (Do not forget this step or you will not see any endpoint.) 3. Set up a buret using your sodium hydroxide solution as the titrant. Titrate your oxalic acid solution to a pink phenolphthalein endpoint as described in the box . Repeat until you have three titrations that differ from the average (median?) by no more than 0.05 mL of titrant.

4. Pipette 25.00 mL of vinegar solution into a clean but not necessarily dry Erlenmeyer flask and titrate this sample. Add 2-3 drops of phenoththalein and repeat until you get three titrations that differ from the average (median?) by no more than 0.05 mL of titrant. 5. Calculate the molarity of your standard sodium hydroxide solution and your unknown acetic acid (vinegar) solution and complete the excel spreadsheet for this experiment. You should be able to determine the concentrations of both the sodium hydroxide and vinegar solutions to 4 significant figures

Prelab Exercise

1. You have weighed out precisely 2.471 g of oxalic acid dehydrate, and diluted it to 250.0 mL. What is its molarity?

2. What are volumetric pipets and burets and what are they used for?

3. To what volume are you to record the values from the buret?

4. What is the name of the indicator you are to use and what color does it turn at the end point?

5. You have diluted 9 mL of nominally 6M NaOH to 500 mL. What is the approximate molarity of the base? (Pay attention to significant figures)

6. A 20.00 mL sample of the standard oxalic acid solution (from 1 above) requires 24.16 mL of your base to reach the end point. Write the equation for the reaction.

7. Calculate the molarity of the base in question #6.

8. 24.37 mL of an unknown acid is titrated with 40.62 mL of the standard base (from #8 above). What is the molarity of this acid?

9. What is the meaning of the term “end point” in a titration and what is true about the system at the end point? Experiment 10 – Volumetric

Analysis
The Titration of Acids and Bases

Data Sheet

Record all of your original data on this sheet in ink and you may transfer it to the lab book pages in pencil to do the work-up.

Preparation of Oxalic acid Standard

Mass of oxalic acid and beaker

Mass of Beaker

Mass of oxalic acid

Concentration of Oxalic acid solution

Sample Calculations

Preparation of Sodium Hydroxide Standard

Molarity of concentrated NaOH stock solution
Volume of NaOH stock solution used
Final volume of dilute NaOH solution
Approximate concentration of NaOH solution

Sample Calculations

Standardization of NaOH Solution

Problems

1. A 27.86 mL sample of 0.1744 M HNO3 is titrated with 29.04 mL of a KOH solution. What is the molarity of the KOH?

2. A 0.2977 g sample of pure H2C2O4•2H2O crystals is dissolved in water and titrated with 23.73 mL of a sodium hydroxide solution. What is the molarity of the NaOH?

3. A 0.3811 g sample of KOH will just neutralize what volume of 0.2000 M H2SO4?

4. A 0.7500 g sample of commercial lye, NaOH, is dissolved in water and titrated with 35.00 mL of 0.5377 M HCl. What is the percent purity of the lye sample? (i.e., what is the mass percent of NaOH in the lye?)

5. A 30.0 mL aliquot of 0.300 M H3PO4 is mixed with 90.0 mL of 0.200 M KOH, and the mixture is evaporated. Will the salt that crystallizes out be K3PO4, K2HPO4, or KH2PO4? Show your calculations. (Hint: when a polyprotic acid such as H3PO4 is mixed with a base, the protons will react one at a time. For example, if the one mole of OH-1 was mixed with one mole of H3PO4, it would produce one mole of H2PO4-1 and one mole of water. The next proton on the H2PO4-1 would start to react only when more OH-1 was added, over and above the first mol.)

Remember to use a milligram balance and record all masses to the nearest 0.001 g.

Pipetting a Liquid

Volumetric pipets are useful for quick delivery of liquids with greater accuracy and precision than a graduated cylinder. The volumetric pipet delivers a single specific aliquot or volume of liquid.

1. Preparation of the Pipet – Obtain a clean pipet from the stockroom. A clean pipet should have no water droplets adhering to its inner wall. Inspect the pipet to ensure it is free of chips or cracks. Transfer the liquid that you intend to pipet from the reagent bottle into a clean, dry beaker; remember never to put a pipet directly into a reagent bottle. Dry the tip of the pipet with a paper towel. Draw 2-5 mL of solution into the pipet using a pipet bulb. This will be used to rinse the inner walls of the pipet with the liquid and ensure that it is not diluted by the water adhering to the pipet walls. Repeat this rinse step one or two more times making sure that you dispose of the rinse liquid. 2. Filling the Pipet – Place the pipet tip well below the surface of the liquid.

Using a pipet bulb, draw the liquid into the pipet until the level is 2-3 cm above the “mark” on the pipet. Remove the bulb and quickly cover the top of the pipet with your finger. Wipe any liquid clinging to the outside of the pipet with a dry towel and allow some liquid to drain out of the pipet until the meniscus is at the mark in the pipet. (You may wish to practice this step with deionized water until you are able to control the flow easily with your finger.) 3. Delivery of the Liquid – Deliver the liquid to the receiving vessel by releasing the finger from the top of the pipet. Hold the pipet tip to the wall of the receiving vessel to avoid splashing. Do not blow or shake the last remaining bit of liquid out of the pipet. Do not blow any remaining solution out with the pipette bulb, as these pipettes are designed to retain a small amount of liquid in the tip when they have dispensed the appropriate amount of reagent. 4. Clean-up – Once you are finished with the pipet, rinse it several times with deionized water and replace in the pipet container.

A buret is a hollow, graduated cylindrical tube equipped with a regulating device for controlling the flow of a liquid. Since it has a narrow, uniform diameter, it is possible to deliver small quantities of liquid with a high degree of accuracy. The graduations on a buret begin with zero and increase in number from the top down, but do not extend to the bottom.

1. Preparation of the Buret – Obtain a clean buret from the stockroom. Close the stopcock and fill with deionized water. Place a beaker beneath the tip and allow the water to drain. Check to see that the water flows freely and that there are no leaks at the stopcock. The walls of the buret should be free of water droplets. Close the stopcock and add about 5 mL of the reagent you will use for the titration or the titrant. Tilt and roll the buret so that the titrant rinses all of the interior surfaces of the buret. Dispose of this rinse solution and repeat 1 or 2 more times to ensure that all interior surfaces of your buret are wet with the titrant solution. Repeat. Rinse the buret with several aliquots of water followed by several rinsings with the titrant. 2. Filling the buret – Clamp the buret into the buret clamp, close the stopcock, and fill it with titrant. Allow the titrant to drain into a waste container until there is a solid column of titrant extending down to the tip, completely free of air bubbles. Allow 10-15 seconds for the titrant to drain from the walls and record the initial volume in the buret to the nearest 0.01 mL. 3. Performing the Titration – Pipet an aliquot of the solution to be titrated in an Erlenmeyer flask, add indicator and place it on a white surface underneath the buret. A color change is much easier to detect on a white background.

Begin adding titrant to the reaction vessel. You will swirl the flask with the one hand and manipulate the stopcock with the other. (Generally left hand will control the stopcock for right-handed individuals.) As you titrate you will see the indicator begin to change color near the point of addition of the titrant. This color will fade quickly as you swirl the solution. Near the endpoint, the color will take longer to fade away. As you reach this point, rinse the walls of the titration flask with deionized water and slow the rate of titrant addition until a drop (or less) makes the color change of the indicator persist for 30 seconds. Stop, allow 10-15 seconds for the titrant to drain from the buret walls, read the final volume of the buret to the nearest 0.01 mL, and record. • To add less than a drop of titrant to the receiving flask, suspend a drop from the buret tip, touch it to the side of the receiving flask, and wash the wall of the Erlenmeyer flask with deionized water. • Often it is helpful to hold a white card with a wide black mark on it behind the buret to see the meniscus more clearly. 4. Clean-up – After completing a series of titrations, drain the titrant from the buret, rinse the buret with several portions of deionized water, and drain each rinse through the tip. Fill the buret with deionized water and return to the stockroom.