Purple Potato pH Palette

The mystery of the purple-turned-blue potato scraps… (aka, fun with pH & colors!)

Initial observation:

A little piece of purple potato at the bottom of the Primary Testing Facility’s kitchen sink turned bright blue after some trivial time in contact with the washing water at the bottom of the sink. Don’t worry, the author now has purple-potato induced alliteration out of their system. It took quite a while to figure out what the mysterious blue material was, because, while the color changing feats of red cabbage were known, it was also known that red cabbage hadn’t graced the test facility’s kitchen for an embarrassingly long time.

Question:

Can purple potatoes be used as a pH indicator similarly to red cabbage?

Expectation:

The purple color in these potatoes is caused by anthocyanin, which is the same pigment in red cabbage, blueberries, and pretty much any fruit/vegetable that is red, blue or purple. Based on that factoid and the observed behavior in the sink, it is believed that the potato will experience a color change in the presence of waters of different acidity. The magnitude, direction, and speed of color change is unknown.

Equipment & Materials:

  • 1 small purple-fleshed potato. In our case, cooked. (If you are adventurous, try growing some!)
  • 3 glasses, or other water holding containers
  • vinegar
  • baking soda
  • measuring cups & spoons
  • timer
  • optional: pH testing strips
A small, cooked (roasted), purple potato that has been sliced.

A small, cooked (roasted), purple potato that has been sliced.

Procedure:

  1. Pour an equal amount of water into each glass (I used 1/4 cup per glass)
  2. add 1 teaspoon of vinegar to one glass
  3. add 1 teaspoon of baking soda to a different glass (stir well, some will dissolve, and some will settle to the bottom of the glass)
  4. leave the third glass as plain tap water
  5. somehow label or mark at least two of your glasses, so you know which glass is which later on
  6. Cut three slices of purple potato, place one slice in each glass
  7. Take a before photo
  8. start a timer, check back on the potatoes periodically

Data:

Baking powder solution: 18.3 g slice of potato (marker = white stick)
Vinegar solution: 16.9 g slice of potato (marker = blue stick)
Tap water: 14.5 g slice of potato (no marker)

5 minutes: there is obvious leaching of color into tap water and baking soda solutions. The baking soda sample is decidedly bluer looking than the tap water sample, which is more purple. The acidic vinegar solution does not seem to be drawing out color at this stage.

Potato slices at start of experiment (left) and after 5 minutes (right). In both views, glasses are baking soda, vinegar, tap water from left to right.

Potato slices at start of experiment (left) and after 5 minutes (right). In both views, glasses are baking soda, vinegar, tap water from left to right. White film in left-most glass of each photo is undissolved baking soda.

30 minutes: the color being extracted  by the baking soda and tap water samples seems to be sitting in distinct layers of the glass. In the baking soda sample the colored water is at the top of the water column, but in the tap water, the colored layer is at the bottom of the column. The vinegar sample has a light, diffuse pink coloration.

60 minutes: similar to above.

Potato slices after 30 minutes (left) and 1 hour (right). In both views, glasses are baking soda, vinegar, tap water from left to right.

Potato slices after 30 minutes (left) and 1 hour (right). In both views, glasses are baking soda, vinegar, tap water from left to right. Note that coloration is in present in distinct layers either at the top, or bottom, of the glasses.

2 hours: It is more obvious that the coloration in the vinegar sample is creating a layer near the bottom of the glass, similarly to the tap water sample. However, the coloration in the acidic water is a pale pink, so the layer is harder to see.  The layers in the other samples do not seem to be growing significantly in thickness, however the color is becoming darker.

5 hours: The blue coloration in the baking soda glass is very dark compared to the other colors. It is almost opaque, making it difficult to see the potato slice through the colored layer.

Potato slices after 5 hours. In both side (left) and above (right) views, glasses are baking soda, vinegar, tap water from left to right.

Potato slices after 5 hours. In both side (left) and above (right) views, glasses are baking soda, vinegar, tap water from left to right. Colored layers are thicker and darker.

15 hours (the next morning): The color has spread and now occupies the full water column in each sample glass.

Potato slices after 15 hours. In both side (left) and above (right) views, glasses are baking soda, vinegar, tap water from left to right.

Potato slices after 15 hours. In both side (left) and above (right) views, glasses are baking soda, vinegar, tap water from left to right. Note that color occupies entire water column of each glass now.

25 hours (after getting home): The baking soda sample has taken on a green-ish hue, but the other two have remained pink (vinegar) and purple (tap water).  Based on some (highly outdated) aquarium test kits found in the Primary Testing Facility’s indoor aquatic habitat area, the baking soda solution had a pH over 8.4, the vinegar solution had a pH under 6.2, and the tap water was between 6.2 and 6.4. Being terribly out of date, (which is part of the reason why the testing facility has tiger barbs instead of picky fish), so the accuracy of the pH readings is questionable. However, there is a real difference in the pH of the fluids, even if the exact pH of each fluid cannot be measured with the available equipment (sorry, left the pH meter in the office).

Potatoes at end of experiment, after 25 hours.  Note different color of potato and fluid. Samples are baking soda solution (left), vinegar solution (middle), and tap water (right).

Potatoes at end of experiment, after 25 hours. Note different color of potato and fluid. Samples are baking soda solution (left), vinegar solution (middle), and tap water (right).

Ending Potato Masses:

Baking powder: final mass = 18.6 g, (+ 1.6% mass gain)
Vinegar solution: final mass =17.6 g (+ 4.1% mass gain)
Tap water: final mass = 15.5 g (+ 6.9% mass gain)

All potato samples gained mass, though the % change varied and did not seem to correlate with color intensity of the water.

Surprise Results (fun with density!): Distinct layers of colored water at different levels!. It is hypothesized that the blue layer (baking soda)  was located at the top of the glass due to the presence on undissolved baking soda at the bottom. This would likely create a density gradient, with denser water at the bottom. Color and fluid leached from the potato is expected to be fresher/less dense, causing it to be at the top. In the other glasses, the color bands likely started at the bottom due to the fluid from the potato having a higher relative density. Vinegar is slightly denser than plain water (1.05 g/cc versus 1.0 g/cc), but starch is  significantly denser at 1.50 g/cc. Given that boiling cut potatoes produces starchy water, extraction of starch and color seems likely, and would explain the pink and purple layers being at the bottom, but the blue layer starting at the top. The density of sodium bicarbonate (baking soda) is a whopping 2.20 g/cc, meaning that the starchy water would be less dense, and float on top in that sample.

Conclusion: The normal tap water produced a purple potato & water, acidic water (with vinegar) resulted in obvious color leaching from the potato slice itself and the resulting water turned pink, and the basic water (with baking soda) produced a teal potato slice, with water that was initially deep blue but later transitioned to a greener hue between 15-12 hours. Given that the tap water resulted in not much color change of the potato itself, another culprit other than the tap water is to blame for the blue spud debris in the sink. Dish soap is the most likely candidate, as we do, now and again, wash our dishes. A little bit of Dawn in a glass of water registers as a pH of 7.8, making it decidedly more basic than our standard tap water. It is concluded that the anthocyanin content of the purple potatoes makes them a pH indicator similarly to red cabbage (although, the potatoes are likely not juice-able?), and that using dish soap raised the pH of the water causing what started as a purple potato to turn blue.

Potatoes at conclusion of experiment, sliced in half, with pH strips, and blotted paper-towel. Samples are baking soda, vinegar, and tap water, from left to right.

Potatoes at conclusion of experiment, sliced in half, with pH strips, and blotted paper-towel. Samples are baking soda, vinegar, and tap water, from left to right.

Future Questions:

Do raw purple potatoes behave differently? Can you juice a purple potato to make a liquid indicator, or would you just get purple potato pulp? (sorry, apparently the attraction to alliteration wasn’t completely out!) Can you tweak the pH to modify the color of mashed purple potatoes?

Would a one-eyed, one-horned, flying, purple tater eater be related to the 1958 billboard pop chart topper?

Related Works:

Hamouz et al. (2011). “Differences in anthocyanin content and antioxidant activity of potato tubers with different flesh colour“. PLANT SOIL ENVIRON., 57 (10): 478–485

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