Tag Archives: dry ice

Curriculum Testing – Determination of Most Useful Concentration of Sodium Carbonate Solution

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.

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.

Curriculum Testing – Viability of Using Dry Ice to Alter pH

Ran some basic tests to get an idea of how well (or poorly) the use of dry ice and universal indicator would be for this lesson.

Instant Ocean mix (per mfg’s recs): 0.036g/mL

Universal Indicator (per mfg’s recs): 15μL/mL

Played around a bit with different solution volumes, different dry ice amounts, and different Universal Indicator amounts.

Indicator Vol (mL) Solution Solution Vol (mL) Dry Ice (g) Time to Color Change (m) Notes
3 Tap H2O 200 1.5 <0.5
3 Tap H2O 200 0.5 >5 Doesn’t trigger full color change and not much bubbling (not very exciting)
5 Tap H2O 1000 12 <1
3 Instant Ocean 200 1.5 <0.5 Begins at higher pH than just tap water. Full color change is slower than just tap water, but still too quick for timing.
2 1M Na2CO3 200 5 >5 No color change and dry ice fully sublimated.
2 1M Tris Base 200 5 >5 No color change and dry ice fully sublimated.
2 Tap H2O + 20 drops 1M NaOH 200 5 2.75 ~Same color as Na2CO3 and Tris Base solutions to begin. Dry ice gone after ~5m and final pH color is ~6.0.



  • Universal Indicator amount doesn’t have an effect. It’s solely needed for ease-of-viewing color changes. Use whatever volume is desired to facilitate easy observations of color changes.
  • Larger solution volumes should be used in order to slow the rate of pH change, so that it’s easier to see differences in rates of change between different solutions.
  • 1M solutions of Na2CO3 and Tris Base have too much buffering capacity and will not exhibit a decrease in pH (i.e. color change) from simply using dry ice. May want to try out different dilutions.
  • Use of water + NaOH to match starting color of Na2CO3 and/or Tris Base is a good way to illustrate differences in buffering capacity to students.
  • Overall, dry ice will work as a tool to demonstrate effect(s) of CO2 on pH of solutions!

Some pictures (to add some zest to this entry):