Agenda Collect Pre-lab #3 Lab quiz #2 Doing Lab #3: pH and buffers today 1 st four Microworlds entries due at END of lab today

Lab #3: pH and Buffers

pH ACIDS Releases or causes the release of H + (hydrogen ions) into solution pH lower than 7 Hydrochloric acid (HCl) Sulfuric acid (H 2 SO 4 ) Acetic Acid (vinegar) Carbonic acid BASES Removes H + (hydrogen ions) from solution pH higher than 7 Sodium Hydroxide (NaOH) Magnesium Hydroxide (Mg(OH) 2 ) Potassium Hydroxide (KOH)

pH Each step of the pH scale = 10-fold decrease in H + concentration What fold difference in H+ concentration is there between pH: 7 and 9? 11 and 4?

Safety Precautions Wear GOGGLES at all times when there are people at your table working with hazardous chemicals

A: pH Indicators Change color depending on the pH. Phenol red turns RED when solution is basic and turns YELLOW when solution is acidic

A: pH Indicators - anthocyanins Red cabbage has anthocyanins Boiling will extract the anthocyanins which can then be used as a pH indicator Ex.1, create a set of standards at pH 3, 5, 7, 9 (Skip 11)

A: pH Indicators - anthocyanins

B: Comparing pH of Beverages and Medicines Ex.2: Determine the pH value of various solutions Pay attention to how the parts of this experiment fit the Scientific Method But be quick, divide and conquer!

10 Buffers Control pH Resist changes in pH Does NOT make a solution NEUTRAL Allows biological molecules to maintain their structure, shape, characteristics Buffers are important for life

Buffered vs non-buffered solution 11 Add HCl to water (low buffering capacity) Add HCl to buffer

Bicarbonate Buffer System Carbon dioxide in blood forms carbonic acid, which separates into H + and bicarbonate 12 Excess OH- ions combine with H+ to form H 2 O Excess H+ ions combine with bicarbonate to form carbonic acid

Bicarbonate Buffer System Organs assist in maintaining blood pH homeostasis Lungs remove excess CO 2 Kidneys remove excess HCO 3 - Other buffers in the body 13

14 Buffers in the environment Some SOILS are an excellent buffer – LIMESTONE

Ex 3: Using pH meter to determine buffering capacity Why would we use a pH meter instead of red cabbage extract? BUFFER: resists pH changes when a small amount of acid or base is added Must have an acid component to add in H+ and a base component to take out H+

Ex 3: Using pH meter to determine buffering capacity How can you tell the pink line is a good buffer while the blue line is not?

Ex 3: Using pH meter to determine buffering capacity At what pH(s) is this buffer useful?

Ex 3: Using pH meter to determine buffering capacity At what pH(s) is this buffer useful?

Ex 3: Using the pH meter Ask me how to dilute your mystery buffers. Only one can be used as is Dilute two ½ and one ¼ IMPORTANT NOTES: pH electrodes are fragile, handle gently After being in ANY solution, rinse the electrode with distilled water, & keep it wet! Immerse the electrode fully and swirl to get accurate reading

Ex 3: Using the pH meter Add 20mL of your mystery buffer (some dilute) Add 1mL HCl, swirl, & record pH (1ml ≈ 2 dropper fulls) Add 1mL HCl (2mL total), swirl, record pH on Fig. 3 Continue adding 1mL and recording pH until you’ve added 10mL total Rinse electrode WELL REPEAT experiment with 1mL increments of NaOH Record in table, zero is in middle of x-axis How to divide & conquer: two groups choose the same mystery buffer, one adds HCl, the other NaOH, now share your data!

Ex 4: Designing an Experiment Design an original experiment to investigate an aspect of pH or buffering capacity Use red cabbage to measure pH Use pH meter to measuring buffering capacity Be quick on other exercises to have time to do a good job here! Scientific Process – turn in one per group – Question – Hypothesis – Dependent variable – Independent variable – Control group – Brief Explanation – Predictions – Results/conclusions – ADD IN: control variables (constants)

Note Changes to Exercise 3 Make dilutions of mystery buffers as indicated earlier or ask me. Clean all glassware & put it away! Return the trays and all materials used to the place you obtained them from in a better condition than you got them in! These dilutions should help you get a graph similar to the one below, demonstrating the buffering capacity.