A titration is a procedure for determining the concentration of a solution (the analyte) by allowing a carefully measured volume of this solution to react with another solution whose concentration is known (the titrant). The point in the titration where enough of the titrant has been added to react exactly with the analyte is called the equivalence point, and it occurs when moles of titrant equal moles of analyte according to the balanced equation between the analyte and titrant. There are many types of titrations. In this experiment, you will be performing an acid-base titration.
Actually, today you will be performing a back-titration. Back titration is also titration. It is called back-titration because it is not carried out with the solution whose concentration is required to be known (analyte) as in the case of normal or forward titration, but with the excess volume of reactant which has been left over after completing reaction with the analyte. Back-titration works in the following manner.
In today’s lab you will analyze antacids which are common medications used to neutralize stomach acid. Antacids are weak bases, and the goal is to measure how much acid can be neutralized by a particular antacid (in other words, you will determine the number of moles of H+ neutralized per gram of each antacid.) But because antacids are insoluble in water, it is problematic to perform a normal titration. Instead, you will react the antacids with a known amount of HCl such that the products of this reaction will be soluble in water. After the reaction, the solution will contain left over HCl, and you will perform a titration (a back-titration) to calculate how much HCl is left over. The difference between the total amount of HCl and the amount of left over HCl (measured by the back-titration) will yield the amount of HCl required to neutralize the antacid.
To obtain accurate results, the titration must be stopped precisely at the equivalence point. If too much titrant is added accidentally during a normal titration, the results usually are discarded and the titration must be started over. However, during a back-titration, it is possible to rescue a bad titration by adding more HCl to the failed titration, resuming the titration, and stopping at the new equivalence point. Afterwards, simply recalculate the total amount of HCl used, and the difference between the total amount of HCl and the amount of HCl for the back-titrations will yield the amount of HCl required to neutralize the antacid. Please realize that a second back-titration is only necessary if too much titrant (NaOH) is added during the first back-titration and the equivalence point is exceeded.
After completing this experiment, the student will be able to:
- Determine the number of moles of H+ neutralized per gram of each antacid.
- Calculate the cost effectiveness of each antacid.
>>> Get cost data from the instructor during lab!! <<<
Acid indigestion is a common ailment caused by the overproduction of stomach acid, HCl. Over-the-counter antacids provide some relief from the symptoms of acid indigestion. They are generally made up of some mixture of weak bases such as Mg(OH)2, Al(OH)3, and CaCO3 that can react with HCl as shown in these net ionic equations:
H+(aq) + OH-(aq) 🡪 H2O(l) (Equation 1)
2 H+(aq) + CO32-(aq) 🡪 H2O(l) + CO2(g) (Equation 2)
In this experiment, the method of titration will be used to determine the number of moles of H+ neutralized per gram of antacid. In the “back-titration,” a portion of antacid will be mixed with an excess of HCl. The H+ that has not reacted with the antacid is then titrated with standardized NaOH in the presence of the indicator bromophenol blue to a blue end point.
The end point is defined as the volume of OH- needed to see a color change. Because only the tiniest excess of OH- over H+ can cause the color change of an indicator, the end point is a close approximation of the equivalence point. (The difference between the end point and the equivalence point is known as the titration error.) At the equivalence point, the number of moles of OH- added is equal to the number of moles of excess H+ that had not been neutralized by the antacid. By knowing the total moles of HCl added, one can then calculate the number of moles of H+ neutralized by the antacid.
Total moles of H+= moles of H+ neutralized by antacid + moles of H+ neutralized by NaOH
Because the antacid may include both OH- and CO32-, it is not possible to calculate the number of moles of each of these ion species independently. Instead, the number of H+ neutralized by the antacid is found. The amount of antacid required to neutralize one mole of H+ neutralized is said to be one “equivalent.”
Total equivalent of antacid = Total moles of H+ neutralized
The more cost-effective antacid is the one that costs fewer dollars per equivalent.
References and further reading
Technique G: Buret Use
- SAFETY PROCEDURES AND WASTE DISPOSAL
3.0 CHEMICALS AND SolutionS
4.0 GLASSWARE AND APPARATUS
5.1 Preparation of Antacid Sample
- Choose an antacid and record its name on the data sheet.
- With a mortar and pestle, crush one tablet of antacid to as fine a powder as possible.
- Weigh out between 0.3 – 0.4 g of the powdered antacid into a pre-weighed Erlenmeyer flask. Record the mass of the antacid sample to 0.0001g.
5.2 Preparation of Burets
- Obtain a buret, label it “NaOH” and make sure that it is clean and does not leak. If necessary, clean the buret with a buret brush and soapy water and rinse with laboratory water.
- Rinse the buret several times with a few milliliters of your NaOH solution (transfer the NaOH to the buret from a beaker using a funnel to minimize spillage), making sure that the stopcock and buret tip are also thoroughly rinsed with NaOH. (Why rinse with NaOH? To remove water that would otherwise dilute and change the concentration of your NaOH solution!)
- Fill the buret with your NaOH solution making sure that there are no air bubbles and no leaks in the stopcock or tip. Record the initial volume of the buret (reading the meniscus to 2 decimal places).
- Obtain a second buret, label it “HCl” and make sure that it is clean and does not leak. If necessary, clean the buret with a buret brush and soapy water and rinse with laboratory water.
- Rinse the buret several times with a few milliliters of your HCl solution, and fill the buret with your HCl solution making sure that there are no air bubbles and no leaks in the stopcock or tip. Record the initial volume of the buret (reading the meniscus to 2 decimal places).
- Record the concentrations of NaOH and HCl on your data sheet.
- The buret does not need to be cleaned or rinsed between titrations. Simply refill the buret with the same solution prior to beginning a new titration. Be sure to record the new initial volume of the buret (reading the meniscus to 2 decimal places).
5.3 Addition of Excess HCl to the Antacid
- Using the buret, add approximately 40 mL of standardized HCl to the prepared antacid sample. Record the final volume of HCl in the buret to 0.01 mL.
- In order to dissolve as much as of the antacid as possible and to drive off as much dissolved CO2 as possible, gently boil the mixture of antacid + HCl for about two minutes.
- Cool the mixture to room temperature.
- There may be a significant amount of substances that are not dissolved in your mixture. Because the active ingredients of an antacid are quite water soluble, the solids will not affect the results. (The solids are a mixture of inactive ingredients such as the coating and some binding compounds).
- Add 6 - 8 drops of bromophenol blue indicator solution. Swirl to mix.
- Check that the solution is yellow because there should be excess H+ in solution. If the solution is green, blue, or a mixture of both green and blue, then there is excess base (antacid) in solution and more HCl must be added. Helpful tip: In a separate container, mix about 25 water with about 4 mL HCl and add 2-3 drops of bromophenol blue indicator solution. This solution will be yellow, and you can use it for comparison with your antacid solution.
- If more HCl must be added, add about 20 ml of HCl from the buret, again recording to 0.01 mL. Make sure there is enough HCl in the buret. Remember, it is not possible to measure the volume of HCl if the level drops below the 50.00 mL mark!
- Assess the color of the solution. If it is not yellow, repeat the addition of HCl until it is.
5.4 Titration of Excess HCl (Back-titration)
- Begin the titration by carefully opening and closing the buret stopcock to allow the NaOH solution to drain into the antacid solution while swirling the flask. Stop the flow of NaOH frequently to assess the titration by observing its color.
- You may wash the sides of the flask with your squirt bottle containing laboratory water during the titration.
- As the endpoint approaches, the indicator will change color from yellow to green and finally to blue. However, the solution may change from blue back to green as you swirl the flask. Slowly continue the titration if the blue does not persist.
- As the endpoint gets closer, add NaOH one drop at a time while swirling the reaction mixture well before adding another drop. Stop the addition of NaOH as soon as one drop causes the solution to change permanently to blue (about 30 seconds) — this is the endpoint!
- Record the final volume of the buret (reading the meniscus to 2 decimal places).
Tip: Identifying the endpoint is not easy. When you suspect that you have reached the endpoint, proceed to record the final buret volume. Then, add one more drop of NaOH and assess the titration. If the color does not change or if the color becomes darker blue, then the endpoint was reached before that drop was added and your recorded volume was correct. But if the color changes to blue, then the true endpoint has now been reached—cross out the previously recorded buret volume and write in the new final volume of the buret. You may repeat this process if you are still uncertain of the endpoint.
5.5 Second Back-titration – only necessary if too much NaOH was added
- If your solution is intensely blue – you’ve added too much NaOH! Don’t panic, this error can be corrected.
- If too much NaOH was added, add enough of the standardized HCl to make the solution yellow. (About 5 - 8 mL) Record the new total volume of HCl.
- Again from the buret, add NaOH slowly (as described earlier) until the solution turns blue.
- Record the final volume of the buret (reading the meniscus to 2 decimal places).
Repeat the procedure with another brand of antacid. Make every effort to reach an endpoint that is equivalent in its “blueness.”
6.0 DATA RECORDING SHEET
Mass of Antacid
Addition of HCl
Titration with NaOH
Additional Data – to be provided by your instructor (*see note on next page)
*Note regarding the ”Additional Data – to be provided by your instructor”
During lab, be sure to obtain from your instructor the “Cost of antacid per package” and “Mass of antacid per package” for each brand of antacid that you analyzed. If necessary, contact your instructor or classmates after lab if this data was not obtained earlier. If you do not have enough time to obtain this data from the instructor or classmates, then it is suggested that you conduct an internet search at a website such as Amazon, CVS, Walgreens, or other online store in order to find prices and weights to obtain the “Cost of antacid per package” and “Mass of antacid per package” for each brand of antacid that you analyzed. Cite the website and the data taken from the website. It is at the discretion of the instructor to accept (or not) this alternate data.
7.0 DATA ANALYSIS
Below, show your work for each of these calculations for the first antacid that you analyzed.
- Moles of H+ neutralized by the antacid sample
- Equivalents of antacid in sample analyzed
- Equivalents per gram of antacid (in eq/gram)
4. Cost of antacid per equivalent (in $/eq)
Which antacid is the better buy? Explain.