Comparison of efficiency of natural & artificial indicators

I always have an inclination for performing an experiment and collect data as that would really allow me to grow my self-management skill. So, the challenge was to arrive at a suitable topic for this. During the times of COVID, I have seen various ayurvedic (an ancient therapeutic science in India) and naturopathic treatments to build up immunity. Including that in my own family, I have seen people pondering over the fact that whether to go for the allopathic medicines or choose the natural alternatives. Incidentally, at the same time, I was introduced to the concept of – ‘Green Chemistry’ which in simple words speaks of greener and natural substitutes of synthetic chemicals. The concept that immediately clicked into my mind is to compare and contrast the effectiveness of natural substances over synthetic alternatives. But which category of substances should I choose? The choice was difficult as it has to be something which is easily available and the analytical procedure to compare the efficiency must also be not very complicated. Retracing back to my IGCSE Chemistry classes, I can remember of the natural indicators that can be used as an alternative for the synthetic indicators like methyl orange, phenolphthalein and methyl orange which are mostly used. Analysis of materials and quantification is an important aspect of chemistry. Continuous researches have offered more than one alternative for the determination of a particular parameter. For example, to determine the acidity of ocean there are so many methods like – using pH sensors, measuring levels of carbon-dioxide and so on. This inspired me to understand and evaluate how the choice of materials involved in an analytical procedure may impact the result obtained from it. Thus, I thought of conducting the titration between the same acid and base using various indicators to see which one would be more reliable and how the reliability can be ascertained. Thus, I arrived at the research question stated below.

How does the efficiency of natural indicators (turmeric, tomatoes, beetroot, cherries and Chinarose) measured in terms of pH range and pH at equivalence point differ from that of a synthetic acid base indicator-phenolphthalein, determined using a pH curve?

An acid base indicator is an organic acid or base where the conjugate acid base pair differ in colors. For example, if HX is taken as a weak acid that can acts as an acid base indicator and MOH as a weak base that acts as an acid base indicator, the equation of dissociation can be presented as:

 

\(HX (aq) + H_2O (l)\leftarrow--\rightarrow H_3O+ (aq) + X- (aq)\)          (equation -1)

 

 

\(B (aq) + H_2O (l)\leftarrow--\rightarrow BH+ (aq) + OH- (aq)\)\(\)           (equation - 2)

 

The color displayed by the conjugate acid and base differs. Thus, the color of the solution actually depends which one of the conjugate form–the acid or the conjugate base predominates. The change of pH of the medium dictates the position of the equilibrium and thus the color of the solution as well.

 

According to the equation (1)

 

Acid dissociation constant  - \( ka =\frac{[H^+][X^-]}{[HX]}\)

 

At half-equivalence point, the concentrations of the conjugate base and the undissociated acid is same. Therefore, [X-] = [HX]

 

k_a = [H⁺]

 

Taking negative logarithm on both sides,

 

− log k_a = − log[H⁺]

 

pk_a = pH

 

The calculation above shows that at a pH = pKa of the indicator, the indicator works the best.

 

\(ka = \frac{[H^+][X^-]}{[HX]}\)

 

 \(H+ =\frac{k_a\ ×\ [HX]}{[X^-]}\)

 

\(− log[H+] = − log ka + log \frac{[X^-]}{[HX]}\)

 

\(pH = pka + log\frac{[X^-]}{[HX]}\)

 

The color change of an indicator is prominent under two conditions

The concentration of the conjugate base is 10 times the concentration of the weak acid.

 

[X^-] = 10 [HX]

 

\(pH = pka + log\frac{[X^-]}{[HX]}= pH = pka + log\frac{[10HX]}{[HX]}= pka + log 10 = pka + 1\)\(\)

 

The concentration of the weak acid is 10 times the concentration of the conjugate base

 

[HX] = 10 [X^-]

 

\(pH = pka + log\frac{[X^-]}{[HX]} = pH = pka+ log\frac{[X^-]}{[10X^-]} = pka + log\frac{1}{10} = pka − 1\)\(\)

 

Thus, the pH range of an indicator is pk_a + 1 to pk_a - 1. It means that for an indicator to be suitably used in an acid base titration, the pH at the equivalence point must lie within ±1 of the pK_a value of that weak acid used as an indicator and pK_b for a weak base used as an indicator. For example, for methyl orange, a weak acid used as an indicator, the pk_a is 3.46. Thus, it is appropriate for an acid-base titration where the pH at the equivalence point lies within (3.46 + 1.00)= 4.46 to (3.46 – 1.00) = 2.46

The way, the weak acid HX changes it’s color depending on the presence of excess H+ ions in an acidic medium or excess of OH- ions in the basic medium can be explained using the Le-Chateleir’s principle and the equilibrium shown below

 

\(HX (aq) + H_2O(l)\leftarrow-\rightarrow H_3O+ (aq) + X- (aq)\)          equation-3 

 

In an acidic medium, there is excess of Hydronium ions (H₃O⁺). This causes the equilibrium in equation-3 to shift more towards the reactant and thus the color of the acid (HX) predominates. Thus, indicator HX will show Color-A in acidic medium.

 

In a basic medium, there is excess of hydroxyl ion in the medium. This combines with the H₃O⁺ which is liberated from the dissociation of the weak acid HX and form H₂O as shown in equation-4. As a result, the equilibrium concentration of H₃O⁺ decreases.

 

\(OH- (aq) + H_3O+ (l) ------\rightarrow2 H_2O  (l)\)          equation-4.

 

This reduces the concentration on the product side and thus the equilibrium shifts towards the product according to the Le-Chateleir’s principle. Thus, the conjugate base X- will predominate and Color-B will be shown.

A pH curve is obtained during an acid base titration where the pH of the analyte is measured as a function of the volume of the titrant added. It is presented graphically in the form of a scattered plot with pH along the y axes and the volume of titrant added along the x axes. A pH curve can be used to procure a lot of analytical information like volume of titrant required at the equivalence point to calculate the concentration of the analyte, pk_a of the acid / base used as an analyte or titrant provided it is weak. In a pH curve, an inflexion point is obtained around the equivalence point when the pH changes sharply on addition of a drop of the titrant. The mid-point of this region where the pH changes rapidly is marked as the equivalence point. A perpendicular can be drawn from the equivalence point to the y axes to deduce the pH at equivalence point and a perpendicular can be drawn from the equivalence point to the x axes to deduce the volume of titrant required at the equivalence point.

A pH curve can be mathematically treated to obtain the equivalence point. The equivalence point is basically the inflexion point of the curve. Thus, the polynomial equation that the pH curve follows can be used for a double derivative and the value of y axes (pH) and x axes (volume of titrant added; NaOH in this case) can be deduced.

 

Depending on the type of the acid and base used, there are four different types of pH curves

To delineate a comparative study of the indicators used, a pH curve will be used for the titration of a strong base NaOH with a strong acid HCl. The same titration will be performed using various indicators. From the pH curve, the following will be computed for each case, the pH at equivalence point, the volume of NaOH required at equivalence point, the pH range and also the color change. This method is known as pH metric titration.

This investigation is focused on four natural indicators- turmeric, tomatoes, cherries and beetroot. All of them contain some organic chromophores. These are organic molecules having extended conjugation (alternate single and double bonds) which enables them to absorb radiation in the visible spectrum and thus display the complementary color. Presence of Hydrogen ions or hydroxide ions causes them to alter their structural features and thus the color displayed also changes. The pigments present in various fruits are-

The natural indicators are equally efficient as the synthetic indicator – phenolphthalein and there is no significant difference in the efficiency of the different varieties of the natural indicator- turmeric, cherries, tomatoes and beetroot.

The synthetic indicator phenolphthalein is more efficient than the natural indicator. The natural indicators differ in their efficiency.

• A safety masks was worn so that the acid vapors are not inhaled as they may cause nausea.
• A laboratory coat was used to prevent exposing any of the chemicals to skin.
• Hair was kept tied so that it does not spill over the solutions made and used.
• The knife was used very carefully while cutting the fruits.
• The mixer blender was operated with care so that the paste does not spill out.
• Sanitizers were used as a part of the COVID protocol and physical distancing protocols were also maintained.

As the investigation involves the use of eatable substances, it was ensured that there is minimum wastage and to do so, the pulp left as a residue after filtering was used instead of disposing them. All COVID protocols were maintained while performing the investigation.

The procedure is such that it does not involve the emission of any green-house gas or major toxic wastage.

• The turmeric was cut into small pieces using a knife on a cutting board.
• The cut pieces of turmeric were inserted into the mixer blender and blended to obtain fine thick paste.
• The paste was transferred from the mixer grinder to a 100 cm³ glass beaker and distilled water was added to it.
• A glass rod was used to stir and make a suspension.
• The supernatant of the suspension thus obtained was filtered using a funnel and the filter paper. The filtrate was collected and used as the indicator solution.

 

The other indicator solutions were prepared in the same way. For Chinarose, petals of the flower was used.