Introduction
Figure 1: Red Cabbage and Natural Universal Indicator Colour Changes
I have always been interested in environmental conservation and colourful , and was one day transferred back to a particular day in the kitchen, as a young child, while observing my mother cook red cabbage (Brassica oleracea var. capitata f. rubra) coleslaw, which is naturally a rich purple in colour. It turned greyish-blue during the process, but regained its colour as soon as she added vinegar to the recipe – a chemical reaction happening right in front of my eyes – but I did not realize it back then.
After recalling these memories and reflecting upon the nature of science, I discovered that red cabbage is the most popular natural indicator through extensive research. Afterwards, I decided to implement it into my individual investigation for additional consultation of data and exploration of chemical concepts in further detail, because it greatly sparks my interest, and connects on a personal level.
In addition, I had used a universal indicator before, however, I haven’t explored more than two different concentrations of pH by experiment in the classroom, thus the idea of using both an eco-friendly indicator and exploring different types of household products’ pH was fascinating and very different from any other laboratory experiments conducted in the past; it was an opportunity to further investigate the properties of substances and their relationship to pH on both a microscopic and macroscopic level. The investigation will explore the concepts of acids and bases, as well as organic chemistry.
Background
Figure 2: Flavonoid condensed structural diagram and ball-and-stick model
The primary organic molecules that are responsible for the red cabbage indicator colour changes during pH titrations are variations of the flavonoid group, which can be characterized by one heterocyclic ring (C) and two phenyl rings (A and B). They are examples of weak organic acids that react with other compounds to form a colour change in the substance.
Depending on the pH of the substance, known simultaneously as the concentration of hydronium ions in solution, the physical properties (in this case, colour) will vary from crimson red to a pale violet; all across the spectrum. The red cabbage indicator contains anthocyanins, belonging to the flavonoid group of molecules, which play the largest role in determining the visible change of the reaction.
(i) How does the concentration of hydronium (H3O+) ions in each substance affect the volume of red cabbage indicator used?
(ii) Which chemical and physical factors of a substance account for the quantified discrepancies of indicator usage?
From the graph, the outlier, skim milk, is significant as it peaks at a point, much higher than the rest of the substances. The irregularity would be due to the chemical makeup and structure of milk, as it is the only substance conducted within the experiment that is a colloid. The properties of colloids include larger molecules, non-settling of particles, and semi-permeable separation of molecules in contrast to chemical solutions. In addition, skim milk, as opposed to whole milk, is a homogenous mixture, as the fat particles have been removed in the complex filtration process, but still containing the original organic compounds in its molecular structure before the process. If the outlier was not included in the graph, there would again not be a significant trend, suggesting that the concentration of hydronium ions do not directly affect the amount of indicator used.
Compared with a purer substance, such as vinegar (CH3COOH), which is simply acetic acid diluted with water, the reaction of anthocyanins would proceed at a slower rate because there are more variations of milk compounds to react with on a intermolecular level. In addition to that, milk is an opaque substance, which furthermore accounts for the discrepancies in red cabbage usage levels. This is significant as the other substances, before the colour changes, were either transparent (e.g. acetic acid) or translucent (e.g. toothpaste). These are examples of physical properties which affect the data as the reaction would continue under translucent conditions as opposed to transparent conditions. Figure 3: Lactose, Anthocyanin, and Cyanidin organic molecules
[Above, left: lactose (C12H22O11)] is the main organic compound in milk, which react with anthocyanins (above, right) in the red cabbage indicator, to undergo a colour change. The specific anthocyanin present is cyaniclin. The molecules are part of the flavonoid group of phytochemicals, Anthocyanins behave somewhat inversely in that the pigments “gain” an -OH at basic pH, but lose it at an acidic pH.
Anthocyanins are weak organic acids that change different colors, which depend upon the numbers of removable protons (p+) that remain attached to the molecule. For instance, a substance having more protons associated with its structure results in a shift towards the warm colours of the spectrum, and having less shifting towards the cooler colours. The colorimetric change noted with most indicators is due the gain or loss of a proton (H+), by the pigment. However, in the case of anthocyanins, the color change proceeds due to the gain or loss of a hydroxyl ion (OH-).
A change the concentration of hydronium ions (H3O+) in the water affects the probability that the cyanidin will keep or lose a portion of its H+, affecting the shape of the molecule. Thus, by altering the the acidity or alkalinity of the solution, the cyanidin changes its structure, which then changes the absorbed wavelengths.
The resulting color changes are caused by changes in electron (e-) movement or confinement in a double bond. More confinement results in more blue absorbed light, corresponding to a shorter wavelength, and with less confinement more red, corresponding to a longer wavelength. These differences determine whether the colour change is more concentrated of H+ ions or OH- ions.
When a hydrogen ion combines with the basic form of an indicator, it will confine two formerly mobile electrons to a single covalent bond with the hydrogen shifting the light that is absorbed toward the blue end of the spectrum. In the case of an acidic form, it will shift the light that is absorbed toward the red end of the spectrum.
The discrepancies in colours of the different pH compositions of substances accounts for the number of electrons gained or lost during the process of anthocyanins reacting with compounds to formulate a visible change. It has very similar rainbow effects to that of a universal indicator, ranging from red to indigo.
When reacted with different molecules in substances of different ranges of pH, the colour changes undergo different reactions, which represent the differences in colour change. Interestingly, substances undergoing similar colour changes (i.e. vinegar and lemon juice, both with respective pHs of 2.2 and 2.0) also experience similar reactions, which explains the like physical changes.
A variety of reversible equilibrium reactions of different molecular structural diagrams is shown above, as pH is interchangeable, and not necessarily stable, for example, if bleach was added to pure water which displayed a rich blue colour, it would gradually change to purple, as the concentrations of OH- ions would be higher, accounting for the shift. The reactions are reversible because the concentrations are fluid and easily interchangeable, which thereafter account for the differences in colour.
The neutralization of the substances, which is a key step in the cleanup of the experiment, requires the eye-testing of colours to ensure that only neutral substances are discarded properly down the drain. Neutral substances would appear blue under the red cabbage indicator test, which is the ideal colour after balancing the concentrations of hydronium ions in the solutions, keeping safety measures paramount.
The organic molecular structures, which all represent weak acids, only incorporate elements of oxygen, hydrogen, and carbon, as they are considered the “building blocks” of nature. They are examples of phenol structures, which are benzene rings bonded to OH- ion(s). The addition or reduction of water and hydrogen/hydronium ions affect the colour, evident in macroscopic changes, and underlying microscopic pH and organic chemistry principles.
Every element and substance has its own unique emission spectrum and energy levels, which explains the differences in colour changes according to the graph shown above – substances with higher concentration of hydronium (H3O+) ions tend to shift toward the warm colour side of the spectrum (e.g. red and orange), whereas the ones with higher hydroxide ion (OH−) concentrations shift toward the cooler colours (e.g. blue and purple). In this case, substances that range in colour were tested from all over the spectrum (pH 1-12), to investigate the complexity of pH to the utmost holisticity, and better generalization of results.
The emission spectra is also affected by the concentration of a specific substance. Therefore, when analysing quantitative data, there may be discrepancies in the data (when conducting separate trials) such that there may have been slight differences in the substances (due to issues of impurity of the air or due to the differences in the indicator itself after being refrigerated for an extended period of time. Therefore, it is important to consider the aspects of the environment, as the experiment is not conducted under a closed system.
To conclude the two research questions posed in the beginning:
(i) The average amount of indicator used in experiment is 7.10 mL +/- 0.05 mL, and is not affected by the concentration of hydronium ions in solution, as the graph depicting the relationship between the amount of indicator (mL) used and the H3O+ concentration does not represent a recognizable relationship, even after consideration of the removal of the outlier (skim milk).
(ii) Quantified results suggest that the trend of indicator used according to the pH of individual substances do not depend upon pH, but rather the original state of the substances, depending on both physical factors such as transparency, and chemical properties such as purity.
Although the experiment was performed to the utmost accuracy, limitations would be inevitable, from both random errors and systematic errors, as shown by slight differences in measured trials of a The materials used during the experiment may have had slight impurities from beaker cleansing and exposure to the air, and the same effect could be said about the pH indicator itself after being refrigerated for approximately an hour, which would thereafter alter its physical properties, which would result in differences in data for the first set of trials and others conducted more closely towards the end. The pH probe may have accounted systematic errors as well – to resolve this issue in a further extension, different pH probes would be used to measure a single substance for paramount certainty of measurements.
According to the kinetic theory of molecules, the atoms move slower at a lower temperature, thus potentially reducing its reaction speed and effects. The addition of artificial dyes in certain substances such as apple juice may also affect the colour change, as the subjectivity of a properly titrated colour change may be more subtle in the case of impurities. Lastly, the colour change itself is subjective – the extent to which one substance is considered properly titrated varies from individual to individual as one would perceive the intensity of colour change uniquely through the lens of one’s own eyes, which could result in random error.
If the entire investigation were to be conducted a second time, it would involve the usage of substances ranging from pH of 1 to pH of 14, with additional care undertaken, as the substances at and near the extremes of the scale are dangerous if inhaled or touched. In addition, an implication of materials with smaller increments would aid in the precision of the experiment. Last but not least, instead of conducting the experiment three separate times, I would titrate each type of substance side by side to two identical cups to see their respective colour changes simultaneously, five times the individual substance, which should closely match under the ideal conditions.
My red cabbage indicator experiment, in application to the real world, would be a potential alternative to traditional universal indicators, which is actually composed of several different individual indicators, including (but not limited to): phenolphthalein, methyl red, and bromothymol blue. Its complex organic molecular makeup is ideal for titrating substances to measure pH, which is safe for the environment and biodegradable to promote conservation of resources. One head of red cabbage can produce over two litres of indicator, and its growth period is approximately six to seven months, which is ideal for factorial production conditions. In the future, commercialized indicators may incorporate more naturally-occurring substances such as red cabbage or like plants such as petunias or onions. Biochemistry would merge with pH for a future in laboratorial pH indicator developmental procedures!
