Chapter 2 Acid-base Titration Chemical Technology

A volumetric quantitative analytical technique that is often used to measure how much acid or base is present in a solution is called a titration. Acid-base titrations are based on neutralization reactions. If a solution is acidic, a titration is to add a base to it until the base neutralizes all the acid.

Acid-base titrations can be used for most acids and bases, including hydrochloric acid, sulfuric acid, acetic acid, sodium hydroxide, ammonia, and so on. In particular, it is even possible to determine in one titration the composition of a mixture containing acids or bases of different strengths, such as sodium hydroxide and sodium hydrogen carbonate. Hydrochloric acid and sodium hydroxide are two most commonly used reagents in acid-base titrations.

The reaction follows a stoichiometric relationship. The stoichiometric point in an acid-base titration may be visually determined by use of an indicator which tells us when the titration is completed. Visual detection of completion of the reaction is a key factor in maintaining the simplicity of titration. A visual indicator is an organic compound that changes color when the p H of the solution changes. Such p H-dependent color changes are the result of chemical changes in the indicator with its chemical environment caused by the addition of H3O+or OH-. An example of these changes in the functional moieties of phenolphthalein, a commonly used indicator which changes from colorless to a pink hue at p H 8.0-9.0.

Ideally, the observation of a sudden change in the color of the solution with the addition of a few drops of indicator tells us the completion of the titration. Sometimes color change seems like instant, with a very small drop of the titrant completely changing the color of the solution. However, depending on the concentration of titrant, titrated substance, and the selected indicator, sometimes we have to add even several milliliters of titrant before we see a color change. This confusion makes it difficult for us to determine when we should stop the titration. In some cases, we should look for a completely different indicator if the one selected fails to guarantee accuracy in the measurement. In order to choose a suitable indicator for an acid-base titration, we need to know the p H of the end point before using standard indicator tables. At the end point of the titration the p H of the solution suddenly changes. The end-point p H can be calculated with the aid of the titration equation.

As an example to show the general procedures in an acid-base titration, sodium hydroxide solution is used to titrate a solid acid dissolved in deionized water. The end point is determined by the color change of an indictor.Shows the schematic of the titration setup.

Designed for classical quantitative volumetric analysis, this experiment serves as a good example of conducting a quantitative experiment with the combination of several quantitative techniques. It is one of the most accurate procedures yet one of the simplest in chemistry lab work. In general, a solution of the acid A is added to an Erlenmeyer flask. A buret is filled with titrant, the solution of base B, at the beginning of the titration to its maximum capacity. The volume of the base solution is read before the beginning of the titration. Solution B is then added drop-wise from the buret to Solution A in the Erlenmeyer flask. The titration is completed when the indicator exhibits a permanent color change. The buret is read again to obtain the volume of Solution B added. With the known concentration of B in the titrant, the titrant volume that reacts with all of A in the flask can be used to calculate how much A is present based on stoichiometry. Some of the chemicals and equipment involved in this experiment include a buret, a buret clamp, a pipet, a small funnel, a standardized sodium hydroxide solution, and phenolphthalein indicator.

The beginning of the acid-base titration starts with the dissolution of the solid acid sample in deionized water. Add the end-point indicator, which is phenolphthalein, with two drops to each flask containing the acid sample and deionized water. Properly label the flasks. Be consistent in all of the samples when adding the indicator. Swirl the flasks until the solid acid is completely dissolved. Finally, rinse with deionized water three times around, which is critical to ensure that all solid acid has been removed from the flask walls and dissolved in the solution. All solid particles must be dissolved prior to the titration.

The buret needs to be checked for if it is quantitatively clean, both to avoid contamination and to be sure that titrant volumes are accurately read. Make sure the buret stopcock is closed. Fill the buret with water and then drain it to check the buret, making sure that its walls drain cleanly. Before checking for drainage, wait a minute or two after completely draining the buret. Sometimes droplets appear on the inner walls of the buret after some time, indicating that the buret is not quantitatively clean. In this case, it is necessary to use standard cleaning procedures to clean the buret. If the buret is droplet free, then it is quantitatively clean and can be used for titration.

After the buret is cleaned, it is necessary to rinse it with the titrant, sodium hydroxide solution. Use a clean and dry funnel to add titrant to the buret. Titrant can also be poured into the buret directly with the buret removed from its holder. Small portions of titrant are used to rinse the buret in order to conserve the titrant. Hold the buret on its side and roll it to rinse the internal buret walls thoroughly. The buret tip is rinsed with the buret held over a waste container or sink and all the liquid being allowed to pass through the tip. Remove the last drop of titrant and continue with rinsing. Usually three times of rinsing is needed to remove any deionized water left in the buret.

Titrant can then be filled in the well-rinsed buret. Still use a funnel to add titrant to the buret. Carefully lift the funnel for smooth delivery and to avoid overfilling of the titrant. Similar to the cleaning of the buret, an alternative is to remove the buret from the buret holder, and directly pour the titrant from the titrant bottle. Let some titrant run through the buret tip into the waste container and check whether there are any air bubbles in the tip. The bubbles will cause difficulty in obtaining accurate values of volume if they are not removed. The bubbles can be shaken out by opening the stopcock, firmly holding the buret with both hands, and jerking downward a bit. When bubbles are removed, tip off the hanging titrant drop and mount the buret for titration.

With the titrant filled in the buret, the samples and the buret are ready for titration. First, the initial level of the meniscus should be read. Look directly at the meniscus, and measure the meniscus at eye level from the center of the meniscus. It is critical to use a consistent buret reading procedure throughout the experiment. Use a contrast card to assist in reading the buret consistently. As a standard practice, the reading of the meniscus level should be immediately recorded in a permanent lab book. Taking notes on a scratch paper is not a correct way of recording such a critical observation.

Titration starts with sample flask number 1. Place a white paper beneath the titration flasks to aid in judging the end-point color. The buret is positioned in such a way that its tip is a few centimeters below the flask rim. The sample is titrated, using the disappearance of the indicator color as a guide of the titration rate. At the beginning of the titration, allow the titrant to run full bore into the flask. At the point where the titrant hits the acid solution the color may temporarily turn pink, but this color disappears upon swirling. The color disappearance is very rapid because of the fast production of colorless water by reaction of the base from the buret with the acid in the sample. During the titration process, continuously swirl to ensure proper mixing which leads to fast reaction.

As the rate of color changes slows, titrant can be added more slowly. With more sodium hydroxide from the buret added to the solution in the flask, more acid in the sample is reacted and less acid is available in the solution. When the red indicator color lingers in the flask for a second or two on swirling after addition of titrant, the end point is near and the delivery of titrant should be slowed down. Smaller volumes of titrant should be added carefully into the flask. The rapid addition rate at the start of the titration is consistent with the rapid indicator color change at the start of the titration, so is the slow addition near the end point and slow indicator color change near the equivalence point.

As the end point is approaching, the addition of titrant is reduced to a few drops. It requires patience and skill to locate the correct end point. Carefully watch the rate of color change. The addition should be even smaller if it takes longer for the color to fade away. Continue with ever smaller increment addition of titrant. Rinse the inner flask walls and the buret tip to make sure that no droplets remain in those places and that all the acid is in the solution. The end point is reached when the addition of a final half-drop leads to a persistent color change. The first appearance of a permanent pink coloration indicates the end point, and the solution should appear extremely pale.

Then read the final level of the meniscus in the buret and also record the reading immediately in the lab book. The difference between the initial volume and the volume left in the buret at the end of titration is the volume of the base consumed. It needs to be pointed out again that consistent reading of the buret is important. Be careful not to add too much titrant. If too much base is added and the indicator in the flask becomes deep pink or purple, an error called overtitration occurs. The entire titration needs to be repeated with a new sample.

After the first sample is done with titration, repeat the procedures for the other samples. The average value of the titrant volumes can then be used to calculate the concentration of acid in the sample with the aid of the titration equation. Note that each titration should be an independent measurement. The first or even a rough titration allows the quick determination of the approximate volume of titrant needed to neutralize the acid. Such knowledge can be used to estimate the end point for each sample. However, the predicted end point is only a guide. It should not be the target of the titration.