The aqueous and alcoholic extract of <i>Urena lobata</i> flowers has been used in acid/alkali titrations over a wide range of concentration. The results obtained with the flower extracts have been compared with those obtained by using traditional chemical indicators like phenolphthalein and methyl orange. It has been established experimentally that the flower extract can be successfully used in place of phenolphthalein and methyl orange for acid/alkali titrations. The presence of anthocyanins is supposed to impart pH sensitive colour dependence to the natural indicator.
Ever since the beginning of chemical science, extensive use of chemicals in different types of processes, reactions, synthesis etc. has brought the humanity at a dangerous edge where the tolerance of environment pollution has reached its limit. Nowadays, trials are being made to accomplish all the chemical processes through green routes. This has given birth to a new branch of chemistry, the Green chemistry. A green route is environment- friendly and minimizes hazards from chemicals. It also cuts the cost of a process phenomenally and may contribute to the economic growth of a country. It is really difficult to limit the scope of green chemistry.
Volumetric estimation is one of the common procedures for quantitative evaluation of solutions. The process involves a pH sensitive indicator namely phenolphthalein, methyl orange etc. which cause environment pollution [
The flowers of Urena lobata were collected from Kotdwar region of Garhwal Himalayas in Uttarakhand and authenticated by Prof. A. K. Agarwal, Head Department of Botany, Government P. G. College, Uttarkashi. The flowers were washed thoroughly with distilled water and the petals were fragmented into small pieces and macerated for 36 hours in distilled water. Another portion of the fragmented petals was dried in shadow until the mass lost water layer. These were then soaked into pure alcohol. About 5.0 gram of the petals were soaked into 50 mL of a liquid. A soaked mixture was then vortexed for 5 minutes at room temperature (about 25˚C) and then filtered through a Whatman No.1 filter paper into a clean dry culture tube capped with Teflon cap and stored away from sunlight. The aqueous and alcoholic solution so obtained were named ULFE, Urena lobata flowers extract, and used as indicator solutions in the present study.
All the chemicals used namely sulphuric acid, Hydrochloric acid, Acetic acid, sodium hydroxide, methyl orange, phenolphthalein, Oxalic acid and ammonium hydroxide were analytical grade BDH chemicals. The alcohol was BDH chemical. All the chemicals were used without further purification. The distilled water used in all these studies was doubly distilled.
All the glassware namely Burette, pipette, beaker, conical flask etc. were Corning glass apparatus.
The standard solutions of sodium hydroxide, sulphuric acid, hydrochloric acid, ammonium hydroxide are difficult to prepare and special care was taken to prepare these. In order to prepare any such solution, say sodium hydroxide, first about 10 times more concentrated solution was prepared. This was standardized by titrating it with a standard oxalic acid solution using two drops of phenolphthalein as indicator. This was named as mother solution, Further, a quantitative aliquot of the mother solution was diluted to prepare, say 0.1 M NaOH solution. This was again titrated against standard oxalic acid solution and its strength confirmed. Similarly, mother solution of sulphuric acid, hydrochloric acid and ammonium hydroxide were prepared and diluted to prepare a said solution by way of standardization using appropriate indicator. It is emphasized that unless these solutions are accurate, it would not be possible to observe the correct titre value.
Titration was performed of a strong base (NaOH) against strong acids (H2SO4 & HCl) and weak acids (C2O4H2∙2H2O & CH3COOH); A weak base (NH4OH) against strong acids (H2SO4 & HCl) and weak acids (C2O4H2∙2H2O & CH3COOH). A particular titration was repeated with every indicator namely, aqueous Urena lobata flower extract (ULFE)aq, alcoholic Urena lobata flower extract (ULFE)al, methyl orange, and phenolphthalein. The titre values were determined according to the standard method [
The ULFE indicator possesses a light pink color both in aqueous and in alcoholic preparation. It retains its light pink colour in acidic medium but the colour turns light green in alkaline medium. The change of colour makes the basis of this indicator. The process of colour change is found reversible. That is, if the medium of the solution containing the ULFE indicator is changed from alkaline to acidic, the colour changes from light green to light pink. The colour change is so distinct that it can be recognized visually without any aid of colorimeter or spectrophotometer.
The scheme of titration has been shown in
Titrant | Titrate | Indicator | Colour change at equivalence point | pH range of indicator |
---|---|---|---|---|
NaOH | H2SO4 | Phenolphthalein | Colourless to light pink | 5 - 9 |
HCl | Phenolphthalein | Colourless to light pink | ||
(ULFE)aq | Pink to light green | |||
(ULFE)al | Pink to light green | |||
NaOH | C2H2O4.2H2O | Phenolphthalein | Colourless to light pink | 7 - 10 |
CH3COOH | Phenolphthalein | Colourless to light pink | ||
(ULFE)aq | Pink to light green | |||
(ULFE)al | Pink to light green | |||
H2SO4 | NH4OH | Methyl orange | Orange to yellow | 5 - 3 |
HCl | NH4OH | Methyl orange | Orange to yellow | |
(ULFE)aq | Light green to pink | |||
(ULFE)al | Light green to pink |
equivalence point, a slight excess of strong base may rise pH to 11 or more. Thus phenolphthalein is a suitable indicator for these titrations too. A careful comparison of the curves is shown in
Lastly, the validity of ULFE indicators was checked for titrations of weak acids, (C2O4H2∙2H2O & CH3COOH) against weak base, NH4OH. In this case, the final pH values are in the range 4 - 5 and 8 - 10 and the equivalence point may be in the range 6.5 - 7.5. Some indicators have been used by earlier workers [
It is proposed that the presence of flavonoid pigments known as anthocyanins are responsible for imparting colour to the ULFE solutions. The anthocyanins are water soluble, pH sensitive strong colour [
Present studies established that aqueous extract of Urena lobata flowers is a versatile natural indicator for acid/ alkali titrations. It is a good substitute of phenolphthalein for strong base/strong acid, strong base/weak acid titrations. It is equally good substitute of methyl orange for weak base/strong acid titrations. The anthocyanins present in Urena lobata flowers seem responsible for showing pH sensitive colour change. The ULFE indicator, when disposed into water, does not pollute it and is biodegradable. Also, the ULFE dye decays aerobically and non-aerobically and thus automatically removed from the atmosphere. Hence Urena lobata flower extract definitely provides a green route to the volumetric estimations.