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# Acid and Base Concentrations

#### Lab Prep

Acid and Base Concentrations

## Learning Objectives

At the end of this lab, students should know...

#### What are Arrhenius acids and bases?

An Arrhenius acid is an acid that dissociates and produces hydrogen ions when dissolved in water, whereas an Arrhenius base produces hydroxide ions when dissolved in water.

#### What is pH, and how is it calculated?

pH is a measure of the degree of acidity or alkalinity of a solution. It is calculated as the negative log base 10 of the concentration of hydrogen or hydronium ions in the solution.

#### How is acid and base strength determined?

The strength of an acid or base is determined by its ability to dissociate into its conjugate ions in water. Strong acids and bases fully dissociate in water, whereas weak acids and bases do not. Acid strength is calculated using the dissociation constant, Ka, but a more commonly referenced is the negative log base 10 of the Ka known as pKa. The lower the pKa value, the stronger the acid.

#### What is titration, and what is it used for?

Titration is a technique used to determine the concentration of a solution. To perform a titration, a solution with a known concentration, called a standard solution, is used to determine the unknown concentration of a solution by slowly adding the known to the unknown. For acid-base titrations, a standardized solution of base is slowly added to an acid with unknown concentration (or the acid is added to the base) until the unknown solution is neutralized by the formation of a salt and water, yielding a neutral pH.

#### How is the unknown concentration of acid determined from an acid-base titration?

As base is added to the unknown concentration of acid, the pH is monitored using a pH indicator dye. When the pH changes from acidic to basic, the color of the pH indicator changes. Just before this point, when the solution is neutral, the concentration of hydrogen ions equals the concentration of added hydroxide ions. With the known concentration of base, the concentration of hydroxide ions is also known and used to calculate the concentration of hydrogen ions in the acid, and by extension, the concentration of acid.

## List of Materials

• 50-mL glass burette
5
• 60-mL plastic burette with two-valve system
5
15
5
5
• 100-mL beaker
5
• 250-mL beaker
5
• 400-mL beaker
5
• Glass stirring rod
5
• Ring stand
5
• Medium 2-prong clamp
5
• Burette clamp
5
• 1-mL pipette
5
• 1-mL pipette tips
75
• 500-mL polyethylene bottle with a cap
5
• Drop counter
5
• pH sensor probe attachment
5
• pH 7.00 calibration buffer
Dependent on lab size
• pH 10.00 calibration buffer
Dependent on lab size
• Handheld data acquisition device
5
• Flash drive
5
• Stirplate
5
• Stir bar
5
• NaOH pellets
30 g
• 14.8 M Phosphoric acid
1 mL
• Phenolphthalein
0.1 g
• Ethanol
5 mL
• KHP
20 g
• Powder funnel
3
• 150-mL beaker
1
1
1
• Paraffin film
1 roll
• 125-mL polyethylene bottle
2
• 1-mL volumetric pipette
1
1
• 5-mL pipette
2
1
• Spatula
2
• 100-mL beaker
1
• Plastic pipette/eyedropper
1
• Dropper bottle
1
• 200-mL glass jar with lid
1
• Oven
1
• Pipetter
1
• Weigh boats (at least 10)
Dependent on lab size
• Balance (at least 1)
Dependent on lab size
• Deionized water
Dependent on lab size

## Lab Prep

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Preparation of Reagents

Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

• Prepare 25 mL of 50% w/w NaOH solution. Weigh 25 g of NaOH pellets and add them to a 125-mL Erlenmeyer flask.
• Measure 25 mL of deionized water and add it to the flask.
• Add a magnetic stir bar and mix the solution on a stir plate until the solution appears homogeneous. After mixing, cover the opening of the flask with paraffin film. Note: The solution produces heat, so allow it to cool before handling.
• Once cooled, use a powder funnel to pour the solution into a polyethylene bottle labeled ‘50% w/w NaOH’. Store the solution in a corrosive storage cabinet until it is needed.
• Prepare 25 mL of 0.5 M phosphoric acid. Use a 1-mL volumetric pipette to measure 0.85 mL of stock 14.8 M phosphoric acid and add it to a 25-mL volumetric flask.
• Fill the flask with deionized water to the volume marking. Add a magnetic stir bar and mix the solution until it is homogeneous.
• Use a powder funnel to transfer the solution to a polyethylene bottle labeled ‘0.5 M H3PO4’. Store the solution in a corrosive storage cabinet until it is needed.
• Prepare 5 mL of 1% phenolphthalein. Use a volumetric pipette to transfer 2.5 mL of ethanol into an Erlenmeyer flask.
• Weigh 0.05 g of phenolphthalein and add it to the ethanol. Rinse the weigh boat with ethanol to transfer residual phenolphthalein.
• Add a stir bar and mix the solution until the phenolphthalein dissolves and the solution appears homogeneous.
• Add another 2.5 mL of deionized water to the flask and stir. Once the solution is well-mixed, transfer it to a labeled dropper bottle.
• Prepare dry potassium hydrogen phthalate (KHP). Weigh approximately 20 g of stock KHP into a glass container with a lid.
• Place the uncovered glass container in an oven at 125 °F to dry the KHP overnight.
• When the KHP is dry, secure the lid and set it by the balance to cool.
• Make sure there is a clean and dry spatula, several weigh boats, and a 150-mL beaker labeled, ‘Excess’.
2. Preparation of Laboratory
• Set out the following glassware and equipment at each lab station (we suggest that students work in pairs):  1    50-mL glass burette 1    60-mL plastic burette with 2-valve system 3    250-mL Erlenmeyer flasks 1    10-mL graduated cylinder 1    50-mL graduated cylinder 1    100-mL beaker 1    250-mL beaker 1    400-mL beaker 1    Glass stirring rod 1    Ring stand 1    Medium 2-prong clamp 1    Burette clamp 1    1-mL pipette 15    1-mL pipette tips 1    500-mL polyethylene bottle with cap 1    Drop counter 1    pH sensor probe 1    Stirplate 1    Stir bar 1    Data acquisition system 1    Flash drive

#### Tags

JoVE Lab Chem Lab: 38 Prep

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