The need to develop effective alternative for synthetic indicators is the demand of present-day chemistry. The acid-base indicator properties of Rose (
Despite the presently available instrumental techniques for the chemical analyses of various samples, the conventional methods of analyses are still appropriate in most applications. A number of conventional analytical techniques presently in use today include the gravimetry and titrimetry. In titrimetry, the equivalence point is usually determined by the end point in the titration. The end point in traditional titrimetry is more often than not indicated by some substances added into the analyte solution, which change colour right away after the equivalence point has been attained. These substances are known as indicators [
Indicators are pigments or dyes that can be isolated from a variety of sources, including plants, fungi, and algae [
Acid-base indicators are substances that are revealed through the characteristic colour which changes the degree of acidity or alkalinity of solutions [
On the bases of these rationales of the hazardous effects of synthetic indicators, there has been an increasing interest in the search for alternative sources of indicators from natural sources of plant origin. These alternatives from plant origin are probably cheaper, readily available, easy to extract, less toxic to users, and environmentally friendly [
An indicator does not change colour from pure acidic to pure alkaline at specific hydrogen ion concentration, but rather colour change occurs over a range of hydrogen ion concentrations. This is the colour change interval expressed as the pH range. A natural indicator is a natural substance usually from plant origin that can be used to determine the pH of another substance [
The materials are Allamanda flowers, Rose flowers, Hibiscus flowers, distilled water, methanol, sodium hydroxide, and concentrated hydrochloric acid.
The apparatus consists of mortar and pestle, weighing balance, beakers, conical flask, burette, pipette, retort stand with clamp, white tile, wash bottle, spatula, stirrer, soxhlet extractor, and filter paper (whatman 40).
The three different samples collected were free from unwanted materials (pistil, stamen, and stalk). They were dried at room temperature. The weights of the samples were constantly taken to ensure that the samples are completely dried. The samples were grounded with mortar and pestle. Each of the samples was filtered and 20 grams was weighed for each and extracted with distilled water and methanol.
The three flowers, Rose, Hibiscus, and Allamanda, were extracted by two main methods, namely, soxhlet extraction method and cold method of extraction [
Exactly 20 grams of each sample of Rose, Hibiscus, and Allamanda was weighed into a paper and it was wrapped and placed inside a soxhlet apparatus. A condenser and a round bottom flask were fitted to the extractor; 250 mL of methanol was placed in the extractor and the temperature was set to 65°C, that is, the boiling point for methanol. The colouring matter of the sample was allowed to continue siphoning until the solvent becomes colourless. The sample was then removed from the extract to allow the extracting solvent to be recovered. The extract was poured into an evaporating dish and allowed to dry on the water bath. It was then placed inside the oven for further drying and then kept in desiccators to cool [
Exactly 20 grams of each sample of Rose, Allamanda, and Hibiscus was weighed and transferred into three separate beakers; 250 mL of distilled water was added into each sample and left overnight. On the following day, they were decanted into clean beakers and rinsed with 20 mL of water in order to clear out the colouring matter. It was then concentrated on a water bath [
The extracts were characterized with the use of UV/Visible spectroscopy to determine the wavelength of maximum absorption.
Samples of the extract obtained were added to different acids and bases to test if there will be any colour change. The acids used for these were H2SO4 and CH3COOH, while the bases used were NaOH and KOH.
Titrations were carried out using 0.1 M NaOH and 0.1 M H2SO4 for strong acid-strong base titration. 0.1 M solution of CH3COOH and 0.1 M NaOH were used for weak acid strong base titration. The accuracy of the end point for all experimental samples and trials was repeated three (3) times to check the precision and reliability.
The Jenway 6305 spectrophotometer was used; the cell to be used for the UV/Visible spectroscopy was washed thoroughly with distilled water. Distilled water was used to calibrate the instrument at the wavelength of 400 nm. Therefore 0.001 mL of each extract was diluted with 10 mL of distilled water and 5 mL of the extract was measured and placed in the cell. The absorbance of the extract was determined within the visible region (i.e., 400–750 nm) and the wavelength of maximum absorption (
The results obtained for the evaluation of the extracts are as presented in Tables
Percentage yield of the plant extract.
Plant | Extraction solvent | Mass of powder plant (g) | Mass of extract (dye) obtained (mg) | Percentage yield (%) |
---|---|---|---|---|
Rose | Water | 20 | 220 | 1.10 |
Methanol | 20 | 325 | 1.63 | |
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Allamanda | Water | 20 | 250 | 1.25 |
Methanol | 20 | 310 | 1.55 | |
|
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Hibiscus | Water | 20 | 290 | 1.45 |
Methanol | 20 | 340 | 1.70 |
Hibiscus extract (cold extract) (initial colour of extract, blood red).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Pink |
CH3COOH | + | Extract | ⇒ | Pink |
BASE | ||||
NaOH | + | Extract | ⇒ | Pale yellow |
KOH | + | Extract | ⇒ | Pale yellow |
pH = 5.24.
Hibiscus extracts (methanol extract) (initial colour of extract: blood red).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Pink |
CH3COOH | + | Extract | ⇒ | Pink |
BASE | ||||
NaOH | + | Extract | ⇒ | Pale yellow |
KOH | + | Extract | ⇒ | Pale yellow |
pH = 6.52.
Allamanda extracts (cold extract) (initial colour of extract, brown).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Brown |
CH3COOH | + | Extract | ⇒ | Brown |
BASE | ||||
NaOH | + | Extract | ⇒ | Yellow |
KOH | + | Extract | ⇒ | Yellow |
pH = 5.35.
Allamanda extracts (methanol extract) (initial colour of extract, brown).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Golden brown |
CH3COOH | + | Extract | ⇒ | Golden brown |
BASE | ||||
NaOH | + | Extract | ⇒ | Yellow |
KOH | + | Extract | ⇒ | Yellow |
pH = 5.45.
Rose extract (cold extract) (initial color of extract, red).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Pink |
CH3COOH | + | Extract | ⇒ | Pink |
BASE | ||||
NaOH | + | Extract | ⇒ | Yellow |
KOH | + | Extract | ⇒ | Yellow |
pH = 5.50.
Rose extract (methanol extract) (initial color of extract, red).
Solvent | + | Extract | = | Colour change |
---|---|---|---|---|
ACID | ||||
H2SO4 | + | Extract | ⇒ | Pink |
CH3COOH | + | Extract | ⇒ | Pink |
BASE | ||||
NaOH | + | Extract | ⇒ | Pale yellow |
KOH | + | Extract | ⇒ | Pale yellow |
pH = 5.60.
Acid-base titration with extract as indicator.
Indicator | Strong acid (H2SO4) | Weak acid (CH3COOH) |
---|---|---|
Strong base (NaOH) | Strong base (NaOH) | |
cm3 | cm3 | |
Phenolphthalein | 12.04 ± 0.15 | 41.80 ± 0.30 |
Methyl red | 12.04 ± 0.05 | 44.20 ± 0.15 |
Bromophenol blue | 12.06 ± 0.10 | 60.50 ± 0.05 |
Hibiscus (cold extract) | 11.70 ± 0.06 | 25.00 ± 0.15 |
Hibiscus (methanol extract) | 11.50 ± 0.25 | 25.50 ± 0.60 |
Allamanda (cold extract) | 12.60 ± 0.20 | 42.60 ± 0.35 |
Allamanda (methanol extract) | 12.00 ± 0.15 | 36.00 ± 0.50 |
Rose (cold extract) | 13.00 ± 0.60 | 20.50 ± 0.45 |
Rose (methanol extract) | 13.01 ± 0.40 | 22.60 ± 0.25 |
Values are mean ± SD of 3 readings.
From the two methods used in the extraction of the flowers, solvent extraction was so far the best method because more dye was obtained especially for hibiscus which is sticky and slippery in water (Table
Furthermore, the titration of weak acid and strong base has an equivalence point lower than pH 7 and the extracts can also fit such a titration. Allamanda methanol extract when titrated against a strong acid had similar titre value as phenolphthalein and methyl red and therefore can serve as alternative for this indicator. This was similar to the reports of Eze and Ogbuefi [
Ultraviolet/Visible Spectroscopy was carried out on each extract of Rose, Allamanda, and Hibiscus and the absorbance was plotted against wavelength. From Figures
Allamanda distilled water extract.
Allamanda methanol extract.
Hibiscus distilled water extract.
Hibiscus methanol extract.
Rose distilled water extract.
Rose methanol extract.
The results obtained from the present study reveal that the analytical potential of the dye extracts is very promising as seen in its application in acid-base titrimetry where it was discovered to perform best in strong acid-strong base titration compared to weak acid strong base with a sharp and clear colour change from yellow to brown for Rose extract, from yellow to colourless for Allamanda extract, and from pale yellow to colourless for Hibiscus extract. The two extracts gave clear colour change with acids and bases and the colour change was maintained with different acids and bases. The sharp contrast between their colours in acid and base made the pigment suitable for use as acid-base indicators. Also for the fact that these flowers are readily available and the extraction procedure is simple, with excellent performance, precise and accurate results would make a suitable substitute of presently available synthetic indicators.
In a nutshell, tannery 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 indicator from flowers as substitutes or possible replacement for standard indicators.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Stanley I. R. Okoduwa got the concept and design of the study critically revised important intellectual content. Lovina O. Mbora carried out the chemical analysis and drafted the very first version of the paper. Matthew E. Adu participated in the acquisition of data, drafting of the paper, and its subsequent revision. Ameh A. Adeyi carried out the statistical analysis and the interpretation of the results. The present final version was written partly by Stanley I. R. Okoduwa. All authors gave a final approval of the revised version to be published.