Aims and Objectives of the Experiment
This experiment aims at studying the characteristics of titrations with the strength of different acids used in these titrations. This is done by drawing the titration curves of the different titrations that are carried out in this experiment and interpreting those curves. The units of measurement for this experiment are the standard units that are used in titrations. These are molarity (moles/liter) of substances while the quantities are measured in mL (milliliter).
Theory and Principles
Conductometry is a type of electroanalytical method in analytical chemistry. While other electroanalytical methods use equilibria or electrodes, conductometry is different and employs the conductance of materials to study the reactions (Skoog, Holler, & Nieman, 1998).
Conductometric titrations are used to analyze titration reactions by monitoring the capability of the analyte, or the chemical, to conduct an electrical current. Ohm’s law states that electric current is inversely proportional to the resistance. The inverse of resistance is conductance which is a measure of the ability of a material to conduct an electric current (Britannica, n.d.).
Conductometric titrations work on the principle that the conductivity of the solution varies with the course of the titration due to the difference in ionic conductivity of the two ions, one is initially used while the other is introduced in replacement of the first, participating in the titration (Tel Aviv University, n.d.).
Conductometry may be used for simpler purposes such as to study the titration of acids and bases and for complex purposes such as the determination of sulfate (Garcia & Schultz, 2016), charge density quantification (Farris, Mora, Capretti, & Piergiovanni, 2012), and others.
Instrument Used
The Metrohm 660 conductometer is used in this experiment to carry out the six conductometric titrations as mentioned in the lab manual. The Metrohm 660 conductometer is a digital meter that measures conductivity. It features a clamp, a stand, and a probe. The cell constant for the probe is 0.74 cm-1. The Metrohm 660 features automatic switching of range with an option for fixed ranges (Labequip, n.d.). The conductometer is shown in figure 1.
Figure 1: The Metrohm 660 Conductometer
Another piece of equipment used along with the conductometer is the Metrohm E649 magnetic stirrer which is really useful as the solution needs to be constantly stirred in order to keep it uniform. The stirrer is shown in figure 2.
Figure 2: The Metrohm E649 Magnetic Stirrer
Advantages and Disadvantages of the Conductometric Method
The method of measuring the end point in conductometric titrations is with the conductivity meter, therefore, no indicator is required. Another benefit that it offers is that it can work for colored solutions. The accuracy of results obtained by conductometry is high. Conductometry may be used for dilute solutions including weak acids and weak bases. Yet another advantage of using conductometry is that it may be used for turbid solutions.
Along with all the benefits that the method of conductometric titration has to offer, it also extends a few disadvantages to the experimenter. The first one of them is that the level of salt, sometimes, increases in the solution which creates a hindrance in the path of the electric current and the value of conductivity hence noted may be inaccurate. Another disadvantage that the experimenter has to cope with when using conductometric titrations is that the high concentration of ions, sometimes, creates hindrance in the path of the electric current, thereby, reducing the conductivity reading in the conductometer and inducing inaccuracy in the results obtained (Pharmstuff4u, n.d.).
Experimental Procedure
Calibrate the instrument by placing potassium chloride in the cell. The cell constant potentiometer is adjusted so that it shows a value of 1.4 mS/cm which is the approximate value of the electrolytic conductivity of such solution. The electrodes are then rinsed. The test solution is added to the titration vessel, and diluted with a known volume of water. Fill the burette and place it above the vessel so that the free flow of titrant is established. Measure the conductance until approximately twice the equivalent volume of the titrant has been utilized. Plot the conductivities against the volume of the titrant. Conduct the six titrations, as mentioned in the lab manual with different titrants each time, in the same way.
Result
0.1 M HCl with 0.1 M NaOH:
Figure 3: Conductometric Titrations: HCl (End-point: 10.71 ml)
0.1 M CCl3COOH with 0.1 M NaOH:
A sample of commercial vinegar:
Gram of vinegar solution = density of vinegar solution X volume of vinegar solution
= 1 g/ml X 5 ml= 5 grams
% acetic acid in vinegar solution = (grams of acetic acid/ grams of vinegar solution) X 100
= (0.924/5) X 100 = 18.48 %
Molar mass acetic acid = 2 × 12.01 + 4 × 1.008 + 2 × 16.00 = 60.052 g mol-1
mass acetic acid = moles acetic acid x molar mass acetic acid = 0.0154 × 60.052 = 0.9248008 g
Density (g/mL) = mass (g) ÷ volume (mL)
volume (mL) = mass (g) ÷ density (g/mL)
Density of acetic acid = 1.049 g mL-1 (at 25°C)
volume of acetic acid = mass acetic acid ÷ density acetic acid
volume acetic acid = 0.925 ÷ 1.049 = 0.881 mL
Discussion
This experiment measures the change in conductivity of the solution with the change in the number of ions that conduct electricity. The total conductance of the solution is also dependent on other ions present in the solution. The contribution of the ions to the conductivity of the solution depend upon the mobility of each ion. Therefore, the change in conductivity is measured rather than predicted.
When acid base titrations are carried out, the hydrogen ion from the acid and the hydroxyl ion from the base react to form water. The conductivity of the solution, thus, depends upon the hydrogen ions present. Before the base is added, the hydrogen ions are most mobile. As the base is added, the hydrogen ions are converted to water and, as a result, the conductivity decreases. Adding more base increases the conductivity gradually.
When strong acids are titrated with strong bases, the conductance is high before the titration due to the presence of hydrogen ions. The conductance decreases with the addition of the base due to the conversion of hydrogen ions into water. This decrease is observed until the equivalence point. After the equivalence point is crossed, the conductance starts to increase because of the increase in the number of hydroxyl ions.
It may be seen from the plots of the experiments that the conductivity decreases sharply until the equivalence point in the first titration of HCl and NaOH, and then increases. The second titration shows similar response, and the conductivity decreases until a certain point and then increases in case of the titration of CCl3COOH with NaOH, but the starting conductivity is less as compared to the first case. The third curve shows the titration between CHCl2COOH with NaOH. The third curve shows similarity with the second curve as similar acids were used. The fourth curve which is the titration curve for CH2ClCOOH with NaOH shows very small levels of conductivity showing the weakness of the acid. The conductivity increases after the equivalence point due to the hydroxyl ions from NaOH. The second last curve is for the titration of acetic acid with NaOH. The initial conductivity is almost zero and increases after the equivalence point due to NaOH. The last curve is of the titration of commercial vinegar with NaOH and shows an almost constant increase in conductivity.
Conclusion
The conductometric technique is used by measuring the conductivity of the solution as the titration progresses. The titration is analyzed by interpreting the plots from the titration data. The general trend that was observed was the decrease in conductivity until the equivalence point after which the conductivity increases. The conductometric technique is used with a wide variety of substances and therefore is also favorable over other techniques in specific cases.
References
Britannica. (n.d.). Chemical Analysis - Classical methods. In Britannica.com. Retrieved from https://www.britannica.com/science/chemical-analysis/Classical-methods#ref621214
Farris, S., Mora, L., Capretti, G., & Piergiovanni, L. (2012). Charge Density Quantification of Polyelectrolyte Polysaccharides by Conductometric Titration: An Analytical Chemistry Experiment. Journal of Chemical Education, 89(1), 121–124. https://doi.org/10.1021/ed200261w
Garcia, J., & Schultz, L. D. (2016). Determination of Sulfate by Conductometric Titration: An Undergraduate Laboratory Experiment. Journal of Chemical Education, 93(5), 910–914. https://doi.org/10.1021/acs.jchemed.5b00941
Labequip. (n.d.). Metrohm 660 Digital, Bench-model Conductivity Meter. Retrieved January 29, 2017, from http://www.labequip.com/metrohm-660-digital-benchmodel-conductivity-meter.html
pharmstuff4u. (n.d.). Advantages and Disadvantages of Conductometry in Chemistry. Retrieved January 29, 2017, from http://www.pharmastuff4u.com/2014/06/advantages-and-disadvantages-of-conductometry.html
Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of instrumental analysis. Saunders College Pub.
Tel Aviv University. (n.d.). Conductometric Titrations. Electroanalytical Methods.
