What did I just photograph? It’s a “volcanic” island of baking soda in a sea of tart cherry juice. In a nutshell, there are acidic ions within tart cherry juice and they’re reacting with the baking soda’s bicarbonate ion, creating water and bubbles of carbon dioxide. But there’s more. Excess bicarbonate also changes the structure of the cherry’s anthocyanin-pigments. These natural indicators are pH-sensitive; they are red at low pHs and purple under slightly alkaline conditions. The movement of water molecules has not had a chance to spread the bicarbonate ions beyond the centre, and the rest of the “sea” remains red.
Found in young shoots, flowers and autumn leaves, an anthocyanin is a molecule with a variety of physiological and ecological roles. An anthocyanin consists of an anthocyanidin and a sugar component. Cherries along with apples, and strawberries and a few other fruits all contain a type of anthocyanidin called cyanidin. Delphinidin is another version found in flowering plants as diverse as violets, larspurs and certain grapes. And there are many more compounds, but delphinidin and cyanidin are two of the six most common ones. From Chemspider.com, here is the structure of cyanidin in 3D and 2D:
All anthocyanidins with cyanidin have the cyanidin salt-pigment(the negative ion attached is not shown), but the sugar on the cyanidin can vary. The table below reveals the ones found so far in a couple of cherry species; the numbers in the table represent concentrations in parts per million. Note that varieties of the two species of cherries analysed have at least two different sugars attached to the cyanidin, and both tart cherries and sweet cherries can also have peonidin (pn). Some sweet cherries can have a third anthocyanidin known as pelargonidin (pg).
The only difference between cyanidin, peonidin and pelargonidin is that attached to the carbon at the 3′ position, we find OH , OCH3 and H, respectively. If the ratios were reversed, it would impact the colour of cherries (pelargodinin for example is purple, while peonidin is purplish red). But as we can see from the table, all cherries have far more cyanidins than any other pigment, so both the different reddish hues of cherries and the changes due to pH result mostly from cyanidin.
Bu why is the cyanidin compound in cherries red at pH < 3, violet at pH 7-8, blue around pH 11 and has other colours at higher pHs?
At a pH above 7, we get an excess of hydroxide ions relative to the small amount of hydronium ions contributed by water. The former remove an H+ ion from the OH group at position 4′ in the structure of cyanidin (see step 1 in adjacent mechanism) responsible for a red colour. This forms water (HOH) and starts a cascade of moving pairs of electrons. First, one of the oxygen’s unbonded pairs attaches itself to the hexagonal ring to form a double bond (step 2). This displaces a double bond away from the first carbon to an adjacent carbon(step 3). That in turn shifts the double bond to the two hexagonal rings (step 4), finally eliminating a double bond from the heterocyclic ring and also getting rid of the oxygen’s positive charge in step 5 . For each of the red and blue structures we are showing just just one of the possible resonance structures, but the important thing is that the electron movement creates a new conjugated system (alternating double bonds). As a result the gap between π-bonding and π-antibonding molecular orbitals changes. Different frequencies of visible light are now absorbed, leading to the reflection of purple frequencies, hence the colour of the island in our picture. If another H+ ( a proton) is stripped from the violet-reflecting structure by higher concentrations of hydroxide found at a higher pH, we free up electrons from another oxygen, again changing the conjugated system. A bluish colour is the result.
If instead of adding baking soda to the cherry juice, we try spraying a little oven cleaner which contains sodium hydroxide, we will see green instead of purple (pictured above, on the left). Higher concentrations of hydroxide bring about more structural changes, resulting in a different colour. What’s more interesting is that if we add NaOH directly to a purple solution created with baking soda, we will witness a buffer at work and see different hues of purple. Stubbornly they do not shift to green very easily. (see my above picture on the right)
How is the buffer created? First there was excess bicarbonate ion(HCO3-) lingering after the neutralisation of malic acid from the cherry juice. Then we got carbonate created by the action of HCO3- on NaOH. HCO3- is amphoteric. In an acidic environment it acts as a base, but in NaOH’s presence HCO3- acts as an acid.
NaOH + HCO3–(aq) → H2O(l) + CO32-(aq)
As long as the bicarbonate is still in excess, which is likely given that we added it in solid form, we have the following buffer of a weak base (carbonate) and its conjugate acid ( bicarbonate).
HCO3–(aq) + H2O(l) H3O+(aq) + CO32-(aq)
Like all buffer systems, the bicarbonate-carbonate system can withstand small amounts of strong acid or strong base, as long as the pair of key buffer species is not consumed. For example, adding another 1.0 ml of 0.1 M NaOH to a buffer containing 0.0036 moles of bicarbonate and 0.0001 moles of carbonate will only move the pH from 8.8 to 9.2. This can be easily verified using stoichiometry and with HCO3- ‘s acid dissociation constant (KA) of 4.8 × 10-11. And a pH of 9.2 is not high enough to yield a blue colour, let alone green.
Interestingly, a similar buffer system is present in our blood and protects us from fluctuations in blood pH, which would adversely affect the functioning enzymes among other disruptions. The difference is that the bicarbonate buffer we discussed is based on bicarbonate ion and carbonate, whereas our blood uses a buffer at a lower pH by having a mix of bicarbonate and carbonic acid. In geochemistry both buffering systems are found in lakes near limestone and in all oceans. These are important in protecting ecosystems from dramatic pH-swings due to volcanic or industrial emissions.
Recent Advances in Anthocyanin Analysis and Characterization
Adavnced Acid Base Equilibria http://www.cengage.com/chemistry/discipline_content/owl/support/chapters/owlbook_ch17.pdf
Sugars, organic acids, phenolic composition and antioxidant activity of sweet cherry
Buffering with Limestone http://butane.chem.uiuc.edu/pshapley/GenChem1/L26/web-L26.pdf