After evaluating whether or not dry ice would be effective to trigger a noticeable change in pH in a solution, I determined which concentration(s) of sodium carbonate (Na2CO3) would be most useful for demonstration and usage within the curriculum. Previously, I used a 1M Na2CO3 solution a the universal pH indicator showed no change in color. What I want is a color change, but one that takes place at a noticeably slower rate than the other solutions that are demonstrated/tested; this will show how sodium carbonate acts as a buffer to CO2-acidification.
Additionally, I tested the difference in rate of pH change between Instant Ocean and sodium chloride (NaCl). The reason for testing this is to use this as a demonstration that salt water (i.e. sea water, ocean water) isn’t just made up of salt. It’s likely that many students simply think of the ocean as salt water and have not considered that the makeup of sea water is much more complex.
Finally, I performed these tests in larger volumes than I did previously to verify that the larger volumes will slow the rate of pH change, thus increasing the time it takes for the universal pH indicator to change color, making it easier to see/monitor/time.
Instant Ocean mix (per mfg’s recs): 0.036g/mL (36g/L)
For the NaCl solution, I used the equivalent weight (36g) that was used to make up the Instant Ocean solution.
- Use of 0.001M Na2CO3 is passable, but due to the fact that it’s a diprotic base, the pH indicator didn’t progress lower than ~pH 6.0 in my limited tests. Adding additional dry ice (or using an even more dilute solution) are options to drive the pH lower.
- The comparison between salt water and Instant Ocean will work well as a demonstration to introduce the concept that sea water is more complex than just being salty.
- Using 1L volumes works well to slow the color changes of the universal pH indicator to improve the ability of the students to observe and measure the rate of color change.
The table below summarizes what I tested.
|SOLUTION||VOLUME (mL)||DRY ICE (g)||TIME||OBSERVATIONS|
|0.1M Na2CO3||1000||3.0||—||No color change. Dry ice gone.|
|0.01M Na2CO3||1000||3.3||—||No color change. Dry ice gone.|
|0.001M Na2CO3||1000||3.3||~20s||Dry ice gone, but final color indicated a pH ~6.0.|
|Instant Ocean||1000||3.3||3m||Initial color change noticeable within 10s; full color change after ~3m|
|NaCl||1000||3.0||instant||Immediate, complete color change.|
|Tap H2O||1000||3.3||3m||pH started @ ~7.5. Full color change took place.|