• 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 acid, >7 base
– Universal indicator
• multi-colored
– Indicators
• Phenolphthalein
– Natural indicators like red cabbage, radishes

• 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 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

• 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

ACID-BASE REACTIONS
TITRATIONS
• 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

Setup for Titrating an Acid with a Base

HOW TO SOLVE A TITRATION-TYPE
PROBLEM
• 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!

• 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 35.00 grams of calcium hydroxide?

• 35.62 mL of NaOH is neutralized with
25.2 mL of 0.0998 M HCl by titration to an equivalence point. What is the concentration of the NaOH?

• 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**