Measuring pH Indicators Titrations Pg 579 - 584
Measuring pH Three ways to measure pH pH paper pH meter turns a specific color for each pH value pH meter measures the voltage difference between a reference solution in the probe and the solution it’s immersed in and translates it into a pH reading.
Measuring pH Indicators HIn + H2O H3O+ + In- yellow red weak acids/bases that dissociates in a certain pH range, changing colors as it does so. HIn + H2O H3O+ + In- yellow red HIn = indicator in the acidic form (H = acidic, In = indicator) In- = indicator in the basic form
Indicators HIn + H2O H3O+ + In- yellow red What color will it be when you add acid? yellow What color will it be when you add a base? red
Shows various indicator color ranges – pg. 580 in your text Which indicator would you use to show that a reaction solution has changed from pH 10-11? Alizarin Yellow R
Indicators The change in the two forms occurs in a range of about 2 pH units for most indicators. In this range it is a mix of the two colors. Below this range it has the acidic color. Above this range it has a basic color.
Limitations of Indicators Solution should be colorless Need the ability to detect slight color changes Can only give approximate information on pH Is dependent on temperature To test the pH of a liquid over a wide range a combination of indicators are needed. (Universal Indicator)
Titrations Titration – a method in which a standard solution is used to determine the concentration of another solution. Standard solution – one for which the concentration is known.
Titration You would know the concentration of one of the solutions – either the solution in the buret or the solution in the flask.
Titration and Neutralization When a strong acid and a strong base are mixed, the H+ and OH- react to form H2O H+ (aq) + OH- (aq) H2O (aq) This is called a neutralization reaction Eventually, a neutral solution will result! To analyze the acid or base content of a solution, chemists use titrations
Titration Titration uses a buret to slowly add one solution (called the titrant) to the other (called the analyte) One of the solutions concentrations is known Compare the number of moles added to reach the endpoint The endpoint is known as the equivalence point The endpoint would be when the solution has a pH of 7
Titration Tritant Analyte
Titration and Neutralization What has to be true about the concentration of the ions present when the neutralization is complete? The moles of OH- added must be exactly the same as the moles of H+. How could we see the endpoint with an indicator? Use an indicator that changes colors when it changes from an acid to a base! [OH-] add = [H+] cuz the solution ends-up being neutral!!
Titration Endpoint or Equivalence point: When enough titrant has been added to react exactly with the substance in solution that is being titrated. The portion of the curve with the steepest slope. Occurs when solution stays colored for 15 sec with swirling
Titration What happens to the graph as the titration nears its equivalence point? Steep slope shows a rapid change in pH What happens to the pH if you continue to add base? The pH will increases
Titration Calculations How many mL of 0.200 M KOH will neutralize 15.0 mL of 0.400M HCl? Write a balanced chemical equation. HCl + KOH KCl + HOH Develop a conversion strategy L HCl mol HCl mol KOH L KOH Since M = mol/L we can use it as a conversion factor to change from liters to moles!!!
Titration What would happen if we left everything the same, but used H2SO4 instead? It would take twice as much KOH because we have two H+ in every acid that need to be neutralized. The amount of H+ and OH- must be equal if complete neutralization is to occur.
Titration Write a balanced chemical equation. H2SO4 + 2KOH K2SO4 + 2HOH How do we use the mole ratio when we are looking for amount of base? mol H2SO4 x mol ratio = mol KOH Ma. Va . b/a = Mb . Vb b/a is the mole ratio of base to acid (put on the acid side) Can use (Ma)(Va)(b/a) = (Mb)(Vb) OR stoich way…it doesn’t matter!! You get the same answer EVERY time!!