Identifying this organic acid was an extensive task that involved several different experiments. Firstly, the melting point had to be determined. Since melting point can be determined to an almost exact degree, finding a close melting point of the specific unknown can accurately point to the identification of the acid. In this case the best melting point range was 207-209 degrees Celsius. Melting point, while very helpful in identifying an unknown organic acid, is simply not enough. Next, a series of titration experiments had to be done. In the first series, a titration is completed of sodium hydroxide (NaOH) with KHP. Three reps were completed and the resulting data led to the finding of the molarity of the sodium hydroxide. The second part of the titration series involved titrating the same prepared sodium hydroxide solution with Hydrochloric Acid (HCL).
This is how the molarity of the HCL was determined. The last part to the titration series was necessary to determine the equivalent weight of the acid. The equivalent weight for this unknown was calculated to be ~197 g/mol H+ Once these titration series experiments were complete, enough data is accumulated to complete a computer search. Based on the results of the computer search the next experiment involved conducting at least one trial of pKa and, since the results contained acids that had N,Cl,Br, or S, chemical testing by sodium fusion had to completed. With all of this information combined, it was then simple to correctly identify the organic acid unknown.
A. Melting Point
To complete this experiment, start by gathering 3 capillary tubes and a melting point apparatus to measure the melting point of the organic acid unknown. Next, take one of the capillary tubes and dip it into the acid until about 3mm of the tube is filled. Tightly pack the capillary by dropping it on a hard surface so that the acid is packed at the bottom of tube down to 2mm. This first tube will be used to figure out a rough melting point for the unknown. Set the temperature of the melting point apparatus to 100 degrees Celsius. Press start once. Insert packed capillary tube into the apparatus and observe it while the temperature rises. Sample does not melt when apparatus reaches 100 degrees Celsius, so the apparatus must be turned off and then back on, setting the temperature 20 degrees higher and press start. Keep repeating this step until a rough melting point is found.
B. Equivalent Weight
Introduction: The equivalent weight of an acid is a quantitative value that aids in the identification of the unknown organic acid. The formula for equivalent weight is equal to the molecular weight of the acid divided by the number of acidic hydrogen’s present per molecule of the acid, and is defined as the weight of the acid in gram units. The quantity is easily determined by experimentally pairing the acidic hydrogens of the unknown along with known quantities of hydroxyl ions through the process of titration. For this experiment a three-part series of titrations are completed Equivalent weight of an acid is found using a series of titration methods: i. The first part of the titration series involves titrating a NaOH solution with KHP. Start by making the NaOH solution. The solution is made by diluting 19.1 Molarity (M) of concentrated NaOH down to 800mL of approximately 0.1M. A beaker is filled with 800mL of deionized water, and a graduated cylinder is used to measure out 4.2mL of the 19.1M NaOH. Pour the 4.2mL NaOH into the beaker with deionized water.
Mixture is stirred well with a glass rod. This is now the ~0.1M NaOH solution. Three reps of titration of NaOH with KHP can now be completed, as well as a blank titration. To begin, three sets ofabout 0.3000g of KHP are weighed out on an analytical balance. Put the three sets of KHP into three separate, labeled flasks. All three sets of the KHP is then dissolved with approximately 50mL of deionized water. Next, a buret is used to start the actual titration. Buret is initially filled to 0.00mL mark with the NaOH solution, this is recorded as initial volume. Next, add 2-3 drops of phenolphthalein indicator into each of the three flasks containing KHP. A magnetic stir bar is then added to the first flask, and placed above a stir plate. Everything is positioned under the buret. Stirrer is put on medium speed and the titration can start. Slowly release the NaOH into the KHP flask. As the end point is reached, a pink color will be seen in the flask.
When the lightest pink possible remains in the solution for more than 30 seconds titration is complete. The final volume is recorded, and the same steps are taken for the other two sets of KHP solution. Finally, blank titration is completed to determine deviation. ii. The second part of the titration series involves titration of NaOH with Hydrochloric acid (HCL). Again, three reps of titration and a blank titration have to be completed. A volumetric pipet is used to measure 10.00mL of HCL into three labeled conical flasks. Then the flasks are filled with deionized water until about the 50mL mark. A buret is filled with NaOH solution up to 0.00mL. The titration is complete using a few drops of phenolphthalein indicator and a stir plate in the same way as the KHP titration. Lightest pink color also indicates end point for these trials.
A blank titration can then be completed without using HCL. Initial and final volumes are recorded for each set as data. iii. The last part of this titration series is used to determine the equivalent weight of the organic acid unknown. Begin by determining the solubility of the unknown. Test about 0.1g of the unknown with deionized water. This particular acid does not dissolve right away so ethanol is to be added. The ratio ends up being a 4:1 ethanol to water ratio. Titration can begin (NaOH vs. organic acid). Put about 0.3000g of organic acid in a conical flash and dissolve with solvent system identified before. Complete titration in same way and with same set up as KHP and HCL titrations. After three good trials, complete a blank titration not using organic acid solution. Mass of organic acid as well as initial and final volume from all trials are recorded for data.
C. Data Search
Once melting point and equivalent are determined, a search must be completed using an internet search engine. Since the results for this specific unknown contained acids that have N, Cl, Br, or S, Sodium fusion or chemical testing must be done in addition to one trial of pKa experiment. D. pKa
For this experiment, an analytical balance to weigh out about 0.2000g of the organic acid. This is then put into a beaker along with a stir bar. Acid is dissolved with specific solvent determined in the last part of the titration series. Once organic acid is dissolved, a buret is filled with NaOH. A pH meter is then used to read pH values. Before the pH meter has to be calibrated with a premade pH 4 and premade pH 7. Once calibrated to insure accuracy, the pH meter was placed into the beaker filled with organic acid solution. Initial pH is recorded. Add about 1.00mL of NaOH from buret for about 30 reps, recording the new pH values each time. The data must then be graphed. E. Sodium Fusion (chemical testing)
To complete this experiment certain materials have to be gathered initially: returned test tube solution, a 100mL beaker, fluted filter paper, plastic funnel, a squirt bottle filled with deionized water, 1 disposable pipet, 2 clean test tubes, a glass rod, and a second larger beaker to hold everything. Sodium fusion solution needs to be filtered via gravity filtration (recrystallization). Use a funnel, place it into the 100mL beaker with fluted filter paper that is made wet with the deionized water. The solution is poured through filter paper/ funnel and collect clear solution into beaker. This is what will be tested. Chemical testing can now be conducted. A nitrogen test and a halogen test were conducted with the help of a TA under the hood.
These results were found using a melting point apparatus for measurement and tightly packed vials of the organic acid. The rough melting point was used to determine approximately what temperature the organic acid begins to melt. Once the rough melting point was determined the accuracy of the apparatus needed to be checked, and since 210 degrees Celsius is within the range 209-211 degrees Celsius, this standard bottle was chosen. The standard organic acid and the organic acid unknown were both packed and placed into the apparatus to be measured simultaneously.
The organic acid unknown began to melt at exactly 207 degrees Celsius and was completely liquefied by 209 degrees Celsius. The standard organic acid began to melt exactly at 209 degrees Celsius and was completely liquefied at 211 degrees Celsius. The results of the melting point range for the standard organic acid matched exactly with the label on the bottle, so no correction was needed for the melting point range of the organic acid unknown. Therefore the melting point for the organic acid unknown was accurately determined to be a range of 207-209 degrees Celsius.
The results of the chemical testing by sodium fusion, indicated that there was no nitrogen present and there were halogens present. Since the computer results concluded that two out of three of the potential acids did contain nitrogen. This leads to the conclusion of the experiment, since all information approximately matches the results from the computer search. Therefore, Dichlorobenzoic Acid (Cl2C6H3COOH) is the name of this organic acid unknown. The accuracy of this assumption is determined from the data from three of the experiments: melting point, equivalent weight, and pKa. The melting point range for the acid found by the computer search, matches almost exact (with a smaller range) to the results obtained from experimentally finding the melting point.
The equivalent weight is about 10 units off; this could be due to the tools used and the accuracy of rounding in the calculations. Despite the gap, this equivalent weight value is closer to that of the organic acid unknown than the other two choices. The pKa for the assumed acid from the computer search is very close to the pKa calculated. The error could simply be in a difference in rounding. While the other results also have data very similar to the data taken from experimentation, the sodium fusion testing still proved that there was in fact not any nitrogen found in the organic acid unknown. This eliminates both of the acid names that do contain Nitrogen, leaving Dichlorobenzoic Acid as the identity of the organic acid unknown.