A pH indicator is just a weak acid-weak base with differently coloured acid and conjugate base forms. The blue and red pigments of flowers were isolated and extensively studied by Willstatter in Natural indicators such as litmus to indicate specific pH levels have been developed. The substances in the plant products such as tea, red cabbage, or grapes react with acids or bases resulting in changes at the molecular level which causes their colour to be different at different pH.
Red cabbage juice has been used as a natural pH indicator [ 7 ]. This indicator contains anthocyanin, which has pigment that reacts in a different way to acids and bases [ 7 ]. Each compound which can act as indicator has specific p K a value, and it is an important physical parameter to indicate the acidity of molecules. The objective of this work was to identify the eco-friendly natural indicators and determine their pKa values. The water extracts of three plants Bougainvillea glabra, Impatiens balsamina, and Bauhinia purpurea were used for this study.
Bougainvillea glabra is an evergreen, climbing shrub with thorny stems. Tiny white flowers usually appear in clusters surrounded by colourful papery bracts. These bracts contain eleven types of bougainvillein-v pigments betacyanin [ 8 ]. Impatiens balsamina garden balsam is native to places of south Asia such as India, Sri Lanka, Bangladesh, and Myanmar. The leaves are spirally arranged. The flowers are pink, red, mauve, lilac, or white.
Petals contain anthocyanidin and pelargonidin as colouring pigments [ 9 ]. Bauhinia purpurea , the purple orchid tree, is an exotic tropical tree that blooms over a long period of time. The beautiful and orchid-like flowers of Bauhinia purpurea are native to India.
The petals of Bauhinia contain chalcone and butein as colouring pigments [ 10 ]. The petals of each flower were collected, rinsed with distilled water, and pressed between pads of the absorbent paper to eliminate the surface water. The extracts were kept in dry and dark place.
Three types of titrations such as strong acid-strong base, strong acid-weak base, and weak acid-strong base were carried out using flower extracts as indicators, and their accuracy were compared with commercially available synthetic indicators such as methyl orange, methyl red, and phenolphthalein. Each titration was carried out in triplicate. The flower extract 2. Each solution was diluted to the mark with distilled water. This procedure was repeated for all the three flower extracts [ 11 ].
Calibration of instrument: I pH probe was connected to the meter, and locking nut was fitted properly. The meter was turned on. Flower extract 2. To the second volumetric flask labelled as B , flower extract 2. This was repeated for all the three flower extracts.
For this investigation, Biobase BK-D spectrophotometer has been used. Here, the blank solution includes all the chemicals and distilled water except the flower extract [ 11 — 14 ].
The above procedure was done for all the three flower extracts, and their experiment findings were compared with methyl orange indicator. The indicators are weak base or weak acid which exhibit different colours in different pH.
The equilibrium reaction of the indicator is shown in the following equation [ 12 ]:. The equation can be written as. In low pH solution, all of the indicators are in the form. As the result, in highly acid solution, and. As the result, in highly basic solution, and. Finally, the ratio can be determined by dividing the ratio of equations 5 — 8 by the ratio of equations 6 — 9 [ 11 , 12 ]:.
All flower extracts have shown different colours in acidic and basic solutions and have different highest peaks lambda max in strong acidic and strong basic medium Table 2. The end points detected for acid-base titrations using the flower extracts as indicators were found to be very similar to those of standard indicators used in laboratories Table 3. The p K a values for all the flower extracts and methyl orange for comparison were calculated directly from the intercept of the graphs, as shown in Figure 1.
The R 2 values of these three flower extracts observed to be higher 0. The p K a values calculated and the pH range for the corresponding indicators are listed in Table 4. The variation in the required volumes of titrant when flower indicators is used for strong acid-strong base titration is from 0.
For strong acid-weak base titration, the variation in the required volume of titrant was from 0. In the case of weak acid-strong base, variations for synthetic indicators were very high as the methyl orange indicator is not suitable for weak acid-strong base titrations.
However, methyl red, Bauhinia purpurea , and Impatiens balsamina indicators show a value very close to the standard indicator, phenolphthalein. The pH range of Bougainvillea glabra and Bauhinia purpurea is very close to the pH range of phenolphthalein 8.
Methyl orange failed to detect sharp end points for weak acid-strong base while flower extracts of Bougainvillea glabra and Bauhinia purpurea detect the end points more precisely than synthetic indicators. These natural flower indicators are found to be very important in modern laboratories when the automatic titrators fail to titrate some reactive liquids. Hydrocarbon may react with the plastic materials used in digital titrators, and therefore, these modern digital meters cannot be used.
Colorimetric titration takes advantage of the visual changes of a chemical compound when its environment shifts from acidic to alkaline. In the other words, the colour of this indicator will change at the pH corresponding to the inflection point.
Methyl red is used as the indicator, changing its colour from magenta to yellow at the pH corresponding to the inflection point. In ASTM D, similar to D, hydrochloric acid is used as the titrant; a mixture of toluene and isopropyl alcohol containing a small amount of water is used as the solvent system, and p -naphtholbenzein is used as the colour indicator, which is orange in acid and green-brown in base [ 15 ]. These synthetic indicators could be replaced by the natural flower indicators even in modern laboratories.
The results obtained from the present study reveal that the analytical potential of the flower extracts is very promising as seen in its application in acid-base titrimetry. It was found that these extracts perform best in strong acid-strong base titration compared to weak acid-strong base with a sharp and clear colour change from red to brownish yellow for the Bougainvillea glabra extract, from red to yellow for the Bauhinia purpurea extract, and from red to brownish yellow for the Impatiens balsamina extract.
The availability and the simple extraction procedure with excellent performance and accurate results would make these natural flower indicators suitable substitutes for synthetic indicators used in many laboratories and research institutes. In a nutshell, industries, research laboratories, schools, and chemical companies that make use of indicators for the determination of acidity, alkalinity, humidity, extent of reactions, and so forth would find the preliminary results from this study valuable in producing efficient indicators from flowers as substitutes or possible replacement for standard indicators.
The American Society for Testing and Materials ASTM is using colorimetric titration with synthetic indicators as the standard method for providing a means to quantify the immunity of a lubricant against the damaging effects of acidic constituents. These synthetic indicators could be replaced by the natural flower indicators in the modern laboratories.
However, the disadvantage of the flower extract is that they need to be prepared freshly since they are susceptible to fungal growth after three days. Kapilraj et al. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Journal overview. Special Issues. Ka is defined by the following equation. The square brackets indicate the concentration of respective components. Based on this equation, Ka expresses how easily the acid releases a proton in other words, its strength as an acid. Carboxylic acids containing -COOH , such as acetic and lactic acids, normally have a Ka constant of about 10 -3 to 10 Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive.
Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows. In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3. Another important point is the relationship between pH and the pKa of an acid. This relationship is described by the following equation. If the pH changes by 1 near the pKa value, the dissociation status of the acid changes by an extremely large amount.
In the case of acetic acid, for example, if the solution's pH changes near 4.
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