132 Laboratory 13: Floral pigments as litmus tests for acids and bases

Chemistry from the flower garden into the kitchen

Questions

What floral pigments make the best detectors of acids and bases? What substances are acids, bases, or neutral?

This laboratory explores whether flower petal pigment solutions are good indicators of acids and bases. The flower petal pigment solutions are produced by boiling flower petals in water. If a flower petal solution is found that is a good detector of both acids and bases, then that solution will be used to determine whether unknown substances are an acid, a base, or neutral. Flower pigment solutions are also called floral pigments in this laboratory.

In the English language acids are said to be sour or tart. Bases are often said to be bitter.

Demonstrate litmus paper and its reaction to the known acid and base. Note that litmus paper is not always available. In lieu of litmus paper paper, one can produce litmus solutions.

Phases

There are three phases to this laboratory.

  1. In the first phase you will gather flowers to use as tests for determining whether a substance is an acid or a base. Pick common flowers. Do not pick flowers that belong to someone. Do not pick the orchids in front of the library or other rare flowers. You might also try colorful leaves, again, avoiding rare plants.
  2. In the second phase you will determine which flower produces the best floral fluid for determining whether something is an acid, a base, or neutral. After boiling the petals, the fluid will be tested with a known acid and a known base. You are looking for a flower fluid which shows a distinct and different color change for the known acid and the known base. In this lab lime juice is usually our known acid and baking soda is usually our known base.
  3. In phase three you will use that one "best" flower fluid to test liquids to determine if they are an acid, a base, or neutral. When working with test tubes, be sure to rinse them well between tests. Be careful, some unknowns are poisonous, others can burn your skin.

Equipment

Phase I: Collecting

Collect the flowers and leaves, boil them.

Phase II: Finding a litmus solution

Test each floral solution for color changes, including changing to clear or clear changing to a color.

Make a table [d][t] of your results for the various flowers noting the color change, if any, caused by the known acid or the known base. The table should include:

Tables are intentionally omitted. At this point in the term students are being marked on their ability to design meaningful tables without direct guidance.

Phase III: Testing household substances

Use flower petal solutions that change color in a consistent way for acids or bases. If you find one that changes color for both acids and bases, use that one. If you find that one changes colors for acids and another changes color for bases, use both of those two floral pigment solutions. These floral solutions can also be called "litmus" solutions.

Use the solution to analyze [a] whether the following household substances are acids or bases (different compounds might be made available on lab day):

Make a table [ta] of your results for the household substances noting the color change and whether the substance is an acid, a base, or neutral.

Conclusion [c]

[c] Conclusion. Discuss what flowers showed the most distinct color change for the known acid. Discuss what flowers showed the most distinct color change for the known base. Discuss any difficulties you encountered.

Notes for spring 2008: The chromatograph appears to have failed, despite prior work suggesting it could be done. The only difference was the use of a rock pounder in the functional chromatograph and the use of a mortar and pestle in class. Not enough crushing? The upshot is that pens or MM's would work better and this section of the lab was effectively not done.

Need more substances to test next term: ammonia, cream of tartar, acetone, drano...Could use more droppers.

Post-lab marking notes: Students remain unclear on acids versus bases. That concept in and of itself appears to be sufficient for the laboratory. Drop the chromatograph. Label the "known" substances more clearly. "Lime. Citrus fruit. Sour. Known acidic solution. Baking soda. Bitter. Known basic solution." Test more substances. Dilute bleach to see if oxidizing effect can be reduced. Test neutral fluids such as rubbing alcohol, vegetable oil. Dilute drain cleaner. Look for and acquire other substances such as cream of tartar, baking powder, coralline lime powder.

Dark red leafed coleus works almost better than Hibiscus tiliaceus. Early morning H. tiliaceus does not produce as much pigment as afternoon H. tiliaceus. Cream of tartar was impossible to find spring and fall 2008, and both limes and baking soda were scarce on island the day before the lab in spring 2008.

Spring 2008 lab report follow-up: Students thought the floral fluids were acid or base and that we were testing them to see which were acidic and which were basic. Misunderstood the whole role of a "known" as used to find a best possible litmus fluid.

This lab needs further refinement, the language below and comment outs, prior to fall 2008 run. The three phase piece was added for 11:00 and really helped. The coleus made an excellent indicator. Maybe phase one should be tossed and coleus grown prolifically on campus?

Fall 2008: trying a more distinct three table layout in sequence from the door. Students have difficulty with colors, print the x11 color chart on a color printer for reference in class. Worked well. Hibiscus rosa-sinensis was the concensus choice, although one group did well with a high concentration Spathoglottis plicata. Coleus was not as clearly indicative of bases. Two different coleus performed differently, the dark purple leafed varieties may make better indicators.

I intentionally did not want to put in fill-in the blank table. At this point in the term I want to see what the students will do if left to their own devices. That said, the ul list was added post-hoc fall 2008 as the tables still really confusing. The whole idea of finding a known predictable test substance and then using that new found standard is completely new to the students.

Fall 2009: Despite the ul list and the three table layout, many student remained deeply confused about the conceptual underpinnings of the laboratory. The concept of a litmus fluid eluded them. I failed to show them litmus paper, this may have been a critical omission. The school schedule was also suboptimal: Monday and Wednesday were holidays, so the lab had no introductory material.

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