5.2.5: pH measurement

Lab #9: pH of liquids around you

The chemistry that happens in aqueous solution can change a lot with pH. The digestive system is an example. The pH in the stomach and small intestine is very different, so different types of reactions happen in those two connected organs.

Beginning question

What is the pH of solutions in the household, and what is the pH of a mixture of two liquids?

Choosing samples to test

The activity today is to explore the pH values of liquids you come into contact. WARNING: only test liquids made for human consumption (coffee, soda, milk, sparkling water, vinegar etc) or meant to be handled with bare hands (shampoo, liquid hand soap). You can also dissolve substances in water and test their pH (table salt, baking powder or soda). The liquids must have water as their solvent (so no oils, hand lotions etc).

Estimate the pH with pH paper

You have 4 strips of pH paper in your kit. You should cut them in half to be able to test more samples. Take a small volume of sample (i.e in a tea spoon) and dip edge of the pH paper in it. Discard the remainder of the sample once you are done. Read the pH on the strip by comparing it with the chart below:

Get a more accurate reading in the range of 4.5 to 9.0

The kit contains four pH strips for more accurate readings. Pick two of the samples for further analysis. They should have a pH between 4.5 and 9.0. Ideally, one of them will be acidic and the other basic. Measure the pH of the sample using the first and second pH strip (plastic backer, two colored square at the end). Dip the strips into the sample so that both colored squares get wet, wait 15 seconds and compare the color with the chart below (notice the orientation of the strip on the right, which has a pH of about 6.5):

Mix two samples and measure the pH

To explore what happens to the pH in a mixture, mix equal amounts of the two samples you picked. Looking at the pH values you measured of the two samples, make a prediction of the pH of the mixture and write it down. Then, measure the pH of the mixture with the third pH strip.

Dilute one sample and check the pH

Of the two samples you measured with the pH strip, take the one with the more extreme pH (further from pH = 7) and dilute with 9 parts of water. This will lower the concentration of solutes by a factor of 10. To obtain the dilution, you can measure using kitchen utensils (i.e. mix one teaspoon of the sample with nine teaspoon of water) or the syringe in our kit (i.e. mix 1 mL of the sample with 4 mL + 5 mL of water). Predict how the pH might change with dilution (write it down), and then measure the pH using the final pH strip.

Analysis

For the samples you measured, order them from most acidic to most basic. Research what the active ingredient responsible for the pH is, and whether it is a strong acid or base or a weak acid/base pair.

Lab #10 Buffers - a simulated experiment

This activity consists of exploring buffers made by adding either HCl or NaOH to a solution of ammonium acetate. How is a buffer different than an unbuffered solution of the same pH? We will simulate this activity rather than doing it hands-on because we don't have access to a facility where we can handle strong acids and bases.

The ammonium ion (a weak acid with pKa near 9) together with its conjugate base, ammonia, can act as a buffer according to this equilibrium

\[\ce{NH4+ <=> NH3 + H+}\]

Likewise, the acetate ion a weak base together with its conjugate acid, acetic acid, pKa near 5, can act as a buffer according to this equilibrium:

\[\ce{CH3COO- + H+ <=> CH3COOH}\]

Beginning question

What is the pH range of a buffer, and what happens to the pH when it is diluted or small amounts of strong acid are added?

Task 1:

Make some buffers: Combine ammonium ions (weak acid, c = 0.1 mol/L) with HCl (strong acid, c = 0.1 mol/L) for experiments 1, 2, and 3. Then, combine acetate ions (weak base, c = 0.1 mol/L) with NaOH (strong base, c = 0.1 mol/L) for experiments 4, 5, and 6. The volumes for each recipe are given below.

#	Label	Volumes to be mixed
1	1:1 recipe (HCl)	10 mL acetate, 5 mL HCl, add water to fill to 40 mL
2	10:1 recipe (HCl)	10 mL acetate, 1 mL HCl, add water to fill to 40 mL
3	1:10 recipe (HCl)	10 mL acetate, 9 ,m HCl, add water to fill to 40 mL
4	1:1 recipe (NaOH)	10 mL ammonium, 5 mL NaOH, add water to fill to 40 mL
5	10:1 recipe (NaOH)	10 mL ammonium, 1 mL NaOH add water to fill to 40 mL
6	1:10 recipe (NaOH)	10 mL ammonium, 9 mL NaOH, add water to fill to 40 mL

We will use the web-based simulator on this web site: https://ph.lattelog.com/melange. The use of the calculator is explained in this video:

pH_calculator from Karsten Theis on Vimeo.

The recipe gives volumes in milliliters, but the pH calculator only accepts volumes in liter. Remember that mL is a smaller unit than L, by a factor of a thousand. Also, the volume of water you have to add is not given directly. Instead, the volume of the water plus that of the other ingredients has to add up to 40 mL, so you have to calculate the volume of water to be added. For your convenience, there is a conversion tool below

Enter volume in mL

Calculate

Volume in L

We need to use a trick to add water because there is no direct way to do it in the pH calculator: Use H₃PO₃ with a concentration of zero (which is water). For example, to add 10 mL of water, set the volume of H₃PO₃ to 0.010 L and leave the concentration at 0.0. Make a table with the pH values, and discuss the range of pH values you get for recipes 1-3 vs 4-6. How does this relate to the pKa values of the species in the buffer?

Task 2:

Adding acid test: Roll a dice or choose a number between 1 and 6. For the recipe with that number, add 0.4 mL (= 0.0004 L) of HCl (c=0.1 mol/L) to the original recipe, and measure the pH again. How much did it change? To compare, check the pH of 0.4 mL (= 0.0004 L) of HCl (c=0.1 mol/L) added to 40.0 mL of water.

Task 3:

Dilution test: For the same recipe # as in task 2, add 360 mL of water (to the original recipe, diluting it by a factor of ten), and measure the pH. How much does dilution change the pH?