POTENTIOMETRIC TITRATIONI. IntroductionAcid-base titrations, by concept, is also called a neutralization reaction, since it involves an acid and a base (that is regardless of strength, either one is weak or one is strong or either weak or both strong and ad infinitum), and regularly have the products of salt and water. The completion of the reaction is reached when the number of moles of the acid is equal to the moles of the base, or technically the equivalence point is reached. In this point, if the stoichiometric ratios of the acid and base are equal, we can solve for the concentration of the unknown by using the dilution equation, that is, M1V1=M2V2. For diprotic and triprotic acids and bases that are titrated with a strong acid/base, however, have many equivalence points, depending on how protic it is(i.e. a diprotic acid/base has two equivalence points, whereas a triprotic acid/base has three) since the acid/base has to converted to its less acidic/basic form, or its intermediates. A good example would be a diprotic acid titrated using a strong base (in this case, sodium hydroxide), or H2A. The reaction would proceed, forming HA-, and ultimately forming the base, A2-. But in this titration, you cannot determine its actual pH at a certain aliquot of the titrant, since what we are using in this type of titration is an indicator, an organic compound added to the solution to see the equivalence point by a change of color. In doing so, a pH meter is used to monitor the measurement of pH of the titration process at hand. The pH meter measures the [H+] concentration of the solution, so if the titration goes from base to acid, the pH must be computed as pH=14-pOH, where pOH is the initial reading of pH. By using a pH meter in an acid-base titration to monitor the pH variations of the titration process, we call this process a potentiometric titration, wherein it finds the equivalence point not through an indicator, but through a graph that signifies the relationship between the recorded amount of titrant used, and the measured pH of the system.

II. MethodologyObtain the unknown sample for your group. From your laboratory instructor, obtain the mass of the unknown sample and its molar concentration.

Potentionmetric Titration of Unknown Acidburet was filled up to the 0.0-mL mark with the standard 0.05 N NaOH. The initial pH of the unknown acid solution was recorded. The unknown acid solution was titrated potentiometrically with the standard NaOH solution using 1.50-mL aliquots of titrant at a time. Steer the Solution after every titration. The 1.50-mL aliquots of titrant was repeated 4 times, next 5mL aliquots for 8 times, then 10mL aliquots 6 times, and lastly 20ml aliquots for 2 times. The pH level was recorded after every titration.

III. Results and DiscussionUnknown Acid SampleMass of beaker67.4302g

Mass of beaker + acid88.3175g

Mass of acid20.8873g

Molarity0.2M

pH1.4

Potentionmetric Titration of Unknown AcidTitrant Volume, mL (1.5mL aliquot)pH

1.51.4

31.4

4.51.5

61.5

Titrant Volume, mL (5mL aliquot)

111.5

161.6

211.7

261.7

311.8

361.9

412.0

462.0

Titrant Volume, mL (10mL aliquot)

562.4

662.5

763.1

865.8

966.4

1066.9

Titrant Volume, mL (20mL aliquot)

1267.4

1468.2

Graph pH vs. Volume of titrant

pH

Titrant Volume, mL

Based on the graph will be the determination of the ionization constant Ka of the unknown acid. This will come in two ways. First would be the calculation of the ionization constant using titration data. By getting the volume of the titrant at the equivalence point, on the half -equivalence point, and the pH value of 50% neutralization, we can compute for the pH of the system by using the equation: pH50% neutral = pKa and pH = pKa +(A- / HA). After this, the other method would be the calculation of the ionization constant based on initial pH. By having the assumed concentration of the unknown acid to be equal to the concentration of the titrant and its initial pH, the pH will be computed by the following equation: pH = [H+]2 / (CHA - [H+]). By comparing both values to some known acids ionization constants, the identity of the unknown acid will be clarified.

IV. Summary and conclusionThis experiment concluded the concepts involving potentiometric titration. By specifically using a calibrated pH meter using buffer solutions of standardized concentrations, the pH of the solution is constantly measured as to adding an exact amount of the titrant that is measured accordingly to its equivalent aliquot. This is done until the pH of the system is constant and more or less basic (since we are handling an analyte that is classified as an unknown acid, which is being titrated by a strong base). After plotting the values of pH against volume of the titrant, we form a graph that will determine the equivalence point of the reaction. By using the values of the volume @50% neutralization, the equivalence point and the initial value of the pH, we can compute for the ionization constant of the unknown acid, as well as identifying the molecular formula of the acid and identifying the acid itself. Errors should be avoided especially on the consistency of putting an exact amount of the titrant, as this will in small amounts, will collectively deter the position as to where the equivalence point should be, therefore getting an incorrect ionization constant in the following changes, and it further deteriorates the consistency of the experiment itself.

V. ReferencesChristian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc.

Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New Jersey: Pearson Prentice Hall.

Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8th ed.). Singapore: Thomson Learning.