7.00 Let the Titrations Begin

Transcription

1 7.00 Let the Titrations Begin Dr. Fred Omega Garces Analytical Chemistry 251 Miramar College, SDCCD 1 Let the Titration Technique

2 Titration A technique of chemical analysis to determine the amount of a substance in a sample. Common titration include A sample can be tested by titration. In a titration experiment, a known volume of a standard concentrated solution (the titrant) is used to analyze a sample (the analyte). One is usually an acid, the other a base. An indicator is added to the analyte to signal when the titration is complete. This is called the endpoint. When the moles of acid(h 3 O + ) and moles of base (OH - ) are equal in a titration experiment, the stoichiometric equivalent point is reached. This is called the equivalent point. Indicator changes endpoint moles titrant = moles equivalent point. 2 Let the Titration Technique

3 ... remember a titration problem is nothing more than a stoichiometry problem 3 Let the Titration Technique

4 Titration A technique of chemical analysis to determine the amount substance in a sample. Common titration include: Acid-Base Oxidation-Reduction Complex Formation Precipitation Reaction The requirement for these classic techniques are that the reactions are fast, have large K eq, and the analyte are consume and used up. Common Terms: Titrant Analyte Indicator Endpoint Equivalent point Titration Error Blank Titration Primary Standard Standardization Direct Titration Back Titration 4 Let the Titration Technique

5 Setting up a titration experiment. Titration Setup Titration is a common laboratory method of quantitative/chemical analysis which can be used to determine the concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant, of known concentration (a standard solution) and volume is used to react with a measured quantity of reactant (Analyte). Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint is the point at which the titration is stopped. This is classically a point at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in di- or tri- protic acids). In the classic strong acid-strong base titration the endpoint of a titration is when the ph of the reactant is just about equal to 7, and often when the solution permanently changes color due to an indicator. There are however many different types of titrations (see below). Many methods can be used to indicate the endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes color). In simple acid-base titrations a ph indicator may be used, such as phenolphthalein, which turns (and stays) pink when a certain ph (about 8.2) is reached or exceeded. Methyl orange can also be used, which is red in acids and yellow in alkalis. Not every titration requires an indicator. In some cases, either the reactants or the products are strongly colored and can serve as the "indicator". For example, an oxidation-reduction titration using potassium permanganate (pink/purple) as the titrant does not require an indicator. When the titrant is reduced, it turns colorless. After the equivalence point, there is excess titrant present. The equivalence point is identified from the first faint pink color that persists in the solution being titrated. Due to the logarithmic nature of the ph curve, the transitions are generally extremely sharp, and thus a single drop of titrant just before the endpoint can change the ph significantly leading to an immediate colour change in the indicator. That said, there is a slight difference between the change in indicator color and the actual equivalence point of the titration. This error is referred to as an indicator error, and it is indeterminate. (wikipedia.org) 5 Let the Titration Technique

6 Acid-Base Calculations Indicator changes endpoint. Indicator is chosen so that endpoint occurs at equivalent pt. The following is true at the equivalence point. moles H 3 O + = moles OH - M acid V acid = M base Vbase M acid V acid M base Vbase For monoprotic A & B For polyprotic A & B but N acid V acid = N base V base for monoprotic and polyprotic A & B eq H 3 O + L H 3 O + = eq OH - L OH - L Soln L Soln For Acid base calculation at Equivalence pt: Sometimes: M acid V acid = M base V base but always: N acid V acid = N base V base 6 Let the Titration Technique

7 Acid-Base Titration of Mixture A mixture containing K 2 (FW ) and KH (FW ) requires 15.00mL of 1.000M HCl. Find the mass of each component. Stoichiometry Problem: K HCl g 2 KCl + H 2 O + CO 2 KH + HCl g KCl + H 2 O + CO 2 moles K 2 + moles KH = moles H + = mole L ml = mmol moles K 2 = Mass (g) K 2 FW K 2 = x (g) K 2 g K 2 mol 15.00mmol = x (g) K x (g) KHCO + 3 g g K KH mol mol moles KH = Mass (g) KH = x (g) KH FW KH g KH mol x = g = Mass K g = Mass KHCO3 7 Let the Titration Technique

8 The Precipitation Titration In a precipitation titration, the reaction is monitored by weighing out the precipitate and knowing the stoichiometric ratio. A precipitation titration curve is monitored like an acid-base titration curve. MA D M + + A - : M+ is the titrant, K sp is used for rxn. px = - log [X], where [X] is the concentration of X. X = M+ Before Equiv Pt: [M+] = K sp / [A-], A - is in excess before equivalent pt and determines the px of the solution. The excess A- concentration is corrected for by dilution factor. At the Equiv Pt: K sp = [M + ] [A - ] = s 2, pm = -log s Past Eqivalent Pt. All M+ added has precipitated. There is now excess M+ which determines the pm. Remember the M+ must be corrected for by dilution factor. 8 Let the Titration Technique

9 The Precipitation Titration Curve Shape of titration curve Before Equiv Pt At Equiv Pt Past Equiv Pt. 9 Let the Titration Technique

10 Titration of a Mixture In a mixture, two ions are titrated and less soluble precipitates first. Consider the addition of AgNO 3 to KI and KCl, K sp (AgI) << K sp (AgCl) 10 Let the Titration Technique

11 Titration with a Spreadsheet Titration of I - with Ag + via Excel 11 Let the Titration Technique

12 End Point Detection: Indicator Some common acidbase indicators. The color changes occur over a range of ph values. Notice that a few indicators have two color changes over two different ph ranges. Mechanism for phenolphthalein indicator. At Low ph phenolphthalein is colorless and has a structure in which there is a five membered ring. In the presence of excess acid the five membered ring is broken and the resulting change in conformation gives rise to a compound which is pink. 12 Let the Titration Technique

13 End-Point Detection Volhard Titration: The technique is to measure Cl-. Cl- is precipitated by addition of Ag+ to Ag + (aq) + SCN - (aq) AgSCN (s). form AgCl. AgCl is precipitated out and worked Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq). up and the Ag+ is titrated with KSCN. With the formation of FeSCN 2+ indicating the end point of the titration. Fajans Titration: Uses adsorption indicator. When Ag+ is added to Cl-, AgCl forms and its surface attracts (adsorbs) Cl- ions. After equivalence point, the excess goes from Cl- to Ag+. The abrupt change from negative to positive occurs at the equivalence point. An adsorption indicator such as will change color when the charge is reverse. 13 Let the Titration Technique