There are several ways to test pH – Blue litmus paper red = acid – Red litmus paper blue = basic – pH paper multi-colored – pH meter 7 is neutral, 7 base – Universal indicator multi-colored – Indicators Phenolphthalein – Natural indicators like red cabbage, radishes pH TESTING

pH Indicators Indicators are dyes that can be added that will change color in the presence of an acid or base Some indicators only work in a specific range of pH Once the drops are added, the sample is ruined Some dyes are natural, like radish skin or red cabbage

Paper Testing Paper tests like litmus paper and pH paper – Put a stirring rod into the solution and stir – Take the stirring rod out – Place a drop of the solution from the end of the stirring rod onto a piece of the paper – Read and record the color change – Note what the color indicates – You should only use a small portion of the paper You can use one piece of paper for several tests

pH Meter Remember that acids and bases are electrolytes – Strong acids/bases are STRONG electrolytes – Weak acids/bases are WEAK electrolytes Tests the voltage of the electrolyte Converts the voltage to pH Very cheap, accurate Must be calibrated with a buffer solution

To analyze the acid or base content of a solution, chemists often perform a TITRATION – A titration involves the delivery of a measured volume of a solution of known concentration (TITRANT) from a buret into the solution being analyzed (ANALYTE) – Titrant is added slowly to the analyte until exactly enough has been added to just react with all of the analyte EQUIVALENCE POINT – [H+] = [OH-] – Indicator or pH meter is used to determine equivalence point – Neutralization is said to occur when the acid and base have same numbers of moles present in the combined solution ACID-BASE REACTIONS TITRATIONS

Setup for Titrating an Acid with a Base

Based on stoichiometry – Write a balanced chemical equation – Calculate the number of moles of the standard solution present in the solution Standard solution is the solution in the buret and has a known concentration Will always be given a volume and concentration of the standard solution – Use the molar relationship from the equation (STOICHIOMETRY) to convert from moles of standard solution into moles of unknown solution – Calculate the unknown quantity (molarity or volume) of the unknown solution by using the relationship M = n/V using the number of moles found in step 3 – Sometimes, you will need to convert from moles to other quantities such as volume of a gas (L) and grams Remember your stoichiometry road map! HOW TO SOLVE A TITRATION-TYPE PROBLEM

Boric acid (H3BO3) is a solid acid that is good for controlling ants. It also neutralizes bases. How many grams of boric acid would be needed to neutralize grams of calcium hydroxide? NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 1

35.62 mL of NaOH is neutralized with 25.2 mL of M HCl by titration to an equivalence point. What is the concentration of the NaOH? NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 2

25 drops of citric acid are titrated with.112 M KOH until the indicator changes from yellow to blue. 48 drops of KOH are added before a color change occurs. What is the concentration of the citric acid? What is the pH of the citric acid? **20 drops = 1 mL** NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 3