The pH of various solutions will be measured using a variety of indicators and a pH probe.
Measurement of pH CHEM 111BF – Experiment #5 Objective: The pH of various solutions will be measured using a variety of indicators and a pH probe. The acidity of some solutions will be compared, and others will be examined with respect to the further addition of acid and base. Introduction: As seen in Reaction (1), the self-ionization of water results in the formation of the hydronium ion (H3O+) and the hydroxide ion (OH−): (1) H2O(l) + H2O(l) ⇌ H3O+(aq) + OH−(aq) The acidity (or basicity) of a solution depends on the relative amounts of the hydronium and hydroxide ions. Whereas a neutral solution will contain equal concentrations of the hydronium and hydroxide ion, unequal concentrations of these two ions will produce either acidic or basic solutions. The acidity of a solution can be represented by its pH, a value which is calculated from the concentration of the hydronium ion: pH = − log[H3O+] For aqueous solutions at 25 oC: If… [H3O+] > [OH−] Then… pH < 7.00 And the… Solution is Acidic [H3O+] = [OH−] pH = 7.00 Solution is Neutral [H3O+] < [OH−] pH > 7.00 Solution is Basic A variety of techniques exist for measuring the pH of a solution. A pH meter (or probe) measures the potential difference between two electrodes. The pH meter allows for a direct measurement of the solution acidity (or pH), as the potential difference between these electrodes is dependent on the concentration of the hydronium ion. Aside from using a pH meter, the solution pH can be determined using an indicator. Indicators are organic compounds that undergo structural changes with adjustments to the pH, resulting in changes to the color of the solution. For example, a solution containing the indicator thymol blue will be red for pH values below 1.2, yellow for pH values between 2.8 and 8.0, and blue for pH values greater than 9.2. As the pH of a solution containing thymol blue is increased from acidic conditions (less than a pH of 1) to basic conditions (greater than a pH of 9), the solution is expected to change color from red to yellow to blue. Table #1 (on the next page) provides a list of indicators which are commonly found in the laboratory, along with the expected color changes for various pH ranges. 1 Table #1: Indicator thymol blue methyl orange methyl red bromothymol blue phenol red thymol blue phenolphthalein Chemicals and Equipment: 0.01 M hydrochloric acid, HCl 0.10 M sodium hydroxide, NaOH 0.10 M formic acid, HCO2H 0.10 M sodium formate, NaHCO2 0.10 M ammonium chloride, NH4Cl 0.10 M formic acid/formate buffer pH paper Litmus paper (red and blue) Household Chemicals: Glass cleaner 1% Baking soda, NaHCO3 5% Soap solution pH Range 1.2 – 2.8 3.1 – 4.4 4.4 – 6.2 6.0 – 7.6 6.8 – 8.2 8.0 – 9.2 8.0 – 9.8 Solution Color Change red red red yellow yellow yellow colorless to to to to to to to yellow yellow yellow blue red blue pink 2.5-mL Beral transfer pipets (6) 24-well plate Small beakers and flasks LabQuest2 pH probe Lemon juice Milk 7-Up Safety: The chemicals used in this experiment are corrosive, and both skin and eye irritants. Use care in handling the solutions used in this experiment. Safety goggles should always be worn while in the laboratory. Rinse your skin thoroughly in case of a spill and wash your hands before leaving lab. Dispose of all chemicals in the provided waste container. Procedure: You will be working alone for this experiment and will be responsible for producing your own interpretations of the data. Using a 50-mL beaker, obtain approximately 25 mL of each solution, which can then be transferred to the 24-well plate using a disposable 2.5-mL Beral transfer pipet. As the volumes being delivered to the 24-well plate do not need to be accurately measured, delivery with the Beral transfer pipet will be adequate. Instructions for the use of the pH probe will be provided in the laboratory. 2 V03242022 Part I: pH Measurement of Household Chemicals Prepare the 24-well plate by adding about 1.5 mL of each household chemical to each of four wells as indicated in Figure #1: Figure #1: 24-well plate – Household Chemicals 1 I 2 HE 3 4 5 6 A Column 1: Glass Cleaner Column 2: 1% Baking Soda Solution Column 3: 5% Soap Solution B Column 4: Lemon Juice C Column 5: Milk D Column 6: 7-Up Once the reagents have been added, test the pH of the wells in Rows A – C by dipping the respective paper test-strips into each well for the specified rows, recording your observations into the laboratory notebook: Row A: pH paper intoblue Row B: Red litmus paper testforbasisturn Row C: Blue litmus paper testforacids Using the pH probe, measure the pH the of each solution along Row D. Record the pH values into your laboratory notebook. Be sure to properly rinse and “blot dry” the electrode between all measurements. Finally, add one drop of indicator solution to each well in the specified row, observe and record any color change: Row A: Thymol blue Row B: Methyl orange Row C: Methyl red Row D: Bromothymol blue Discard the contents of the 24-well plate into the waste container. Wash and rinse the 24-well plate. 3 V03242022 Part II: pH Measurement of Acid and Base Solutions Prepare the 24-well plate by adding about 1.5 mL of each solution to each of four wells as indicated in Figure #2: Figure #2: 24-well plate – Acid/Base Solutions 1 2 3 4 A 5 Column 1: 0.10 M hydrochloric acid Column 2: 0.10 M sodium hydroxide Column 3: 0.10 M ammonium chloride B Column 4: 0.10 M sodium formate C Column 5: Deionized water D Once the reagents have been added, test the pH of the wells in Rows A – C by dipping the respective paper test-strips into each well for the specified rows, recording your observations into the laboratory notebook: Row A: pH paper Row B: Red litmus paper Row C: Blue litmus paper Using the pH probe, measure the pH the of each solution along Row D. Record the pH values into your laboratory notebook. Be sure to properly rinse and “blot dry” the electrode between all measurements. Finally, add one drop of indicator solution to each well in the specified row, observe and record any color change: Row A: Thymol blue Row B: Methyl orange Row C: Methyl red Row D: Bromothymol blue Discard the contents of the 24-well plate into the waste container. Wash and rinse the 24-well plate. 4 V03242022 Part III: Resistance to pH Change Prepare the 24-well plate by adding about 1.5 mL of each solution to each of three wells as indicated in Figure #3: Figure #3: 24-well plate – pH Change 1 2 3 4 Column 1: Deionized water A Column 2: 0.10 M formic acid Column 3: 0.10 M sodium formate B Column 4: 0.10 M formate buffer C Using the pH probe, measure the pH of each solution along Row A. Record the pH values into your laboratory notebook. Be sure to properly rinse and “blot dry” the electrode between all measurements. Add one drop of 0.10 M hydrochloric acid to each well along Row B. Using the pH probe, measure the pH of each solution along Row B. Record the pH values into your laboratory notebook. Be sure to properly rinse and “blot dry” the electrode between all measurements. Add one drop of 0.10 M sodium hydroxide to each well along Row C. Using the pH probe, measure the pH of each solution along Row C. Record the pH values into your laboratory notebook. Be sure to properly rinse and “blot dry” the electrode between all measurements. Data: The data section will include the information obtained in Parts I, II and III. All of the recorded observations (colors) and pH values should appear in tables. Suggested column headers for the tables are shown below: Part I: pH Measurement of Household Solutions Part I, Table #1: Indicator Glass Cleaner 1% Baking Soda Solution 5% Soap Solution Lemon Juice Milk 7-Up Thymol blue ⁞ 5 V03242022 Part I, Table #2: Test Strip Glass Cleaner 1% Baking Soda Solution 5% Soap Solution Lemon Juice Milk 7-Up Glass Cleaner 1% Baking Soda Solution 5% Soap Solution Lemon Juice Milk 7-Up pH paper ⁞ Part I, Table #3: pH Value Part II: pH Measurement of Acid and Base Solutions Part II, Table #1: Indicator HCl NaOH NH4Cl NaHCO2 H2O NaOH NH4Cl NaHCO2 H2O NaOH NH4Cl NaHCO2 H2O Thymol blue ⁞ Part II, Table #2: Test Strip HCl pH paper ⁞ Part II, Table #3: HCl pH Value Part III: Resistance to pH Change Part III, Table #1 H2O HCO2H NaHCO2 Buffer Initial pH Value Row A Final pH Value Row B Final pH Value Row C 6 V03242022 Calculations and Results: This section should contain three tables: (1) Measured pH values from Part I (2) Measured and Calculated (from Pre-Lab) pH values from Part II (3) Change in pH values from Part III The conclusion of this section should be a brief narrative which identifies the major observations from the experiment. Discussion: This section should contain a detailed discussion of the data and results with conclusions drawn wherever appropriate. ü Discuss the ability to measure pH using o Indicator solutions o pH paper o Litmus paper o pH probe ü Compare and comment on the pH of 0.10 M hydrochloric acid and 0.10 M ammonium chloride ü Compare and comment on the pH of 0.10 M sodium hydroxide and 0.10 M sodium formate ü Compare the ability of different solutions to resist pH change (Part III) Add any other trends and/or insights that may have been examined/gained from the experiment. Error Analysis: State the sources of errors which were inherent to the experiment (those beyond your control), and explain how they may have affected the final results. Specifically, explain why the calculated pH values might not agree with the measured pH values. 7 V03242022 (This page is intentionally blank.) 8 V03242022 Pre-Laboratory Assignment #5 Measurement of pH Name:______________________________ 1. Calculate the pH of the following solutions: (Ka HCO2H is 1.8 x 10–4 and Kb NH3 is 1.8 x 10–5) a. 0.10 M HCl acid b. 0.10 M NaOH base c. 0.10 M HCO2H Hal strong formic acid weak HCO H2O I 010AM Cl Hot Nat NaOH strong acid H2o OH 430 02H 0 0 E sodium former d. 0.10 M NaHCO2 Nayco Nation t H 0219 Nat 420 7 H102 140 e. 0.10 M NH4Cl 9 V03242022 Question #1 Cont’d f. 0.10 M HCO2H / 0.10 M NaHCO2 buffer 2. Briefly describe why the pH of 0.10 M hydrochloric acid is expected to be lower than the pH of 0.10 M formic acid. 3. Briefly describe why the pH of 0.10 M ammonium chloride is expected to be lower than the pH of 0.10 M sodium chloride. 4. Briefly describe why a solution that is 0.10 M in formic acid (HCO2H) and 0.10 M in sodium formate (NaHCO2) is able to resist pH change upon the addition of a strong acid. 10 V03242022
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