methyl red has the following color changes red 4.2 -8.3 yellow
a)write the ionic equilibrium equation
b) If a small quantity of methyl red is placed in a solution with a pH of 4.5 . calculate the approximate value of [In]/[HIn]. What colour would be observed on the basis of the colors of HIn and In
c) Predict the color change of the following indicators over the entire pH range from 0 to 14
Bromocresolpurple, cresol red , thymol blue
http://antoine.frostburg.edu/chem/senese/101/acidbase/indicators.shtml
a) To write the ionic equilibrium equation for methyl red, we need to understand its acid-base properties. Methyl red is an azo dye and acts as an acid-base indicator. It exists in two forms in aqueous solutions: In- (anionic form) and HIn (acidic form).
HIn ⇌ H+ + In-
In the above equation, HIn represents the acidic form of methyl red, and In- represents its conjugate base. This equilibrium equation indicates that methyl red can donate or accept a hydrogen ion (H+) depending on the pH of the solution.
b) To calculate the approximate value of [In]/[HIn] in a solution with a pH of 4.5, we need to use the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
pKa is the negative logarithm of the acid dissociation constant (Ka) for the acid HIn. In this case, the pKa for methyl red is given as 4.2-8.3.
Let's assume [In]/[HIn] = x. We are given that the pH of the solution is 4.5. So, substituting the values into the Henderson-Hasselbalch equation:
4.5 = 4.2-8.3 + log(x)
Simplifying,
0.3 = -4.1 + log(x)
Now, rearrange the equation:
log(x) = 0.3 + 4.1 = 4.4
Taking the antilog,
x = 10^4.4
Approximately, x = 2512
This means that the ratio of [In] to [HIn] is approximately 2512.
Based on the colors of HIn and In, HIn is red and In- is yellow. With a ratio of 2512, the color observed would be mostly yellow.
c) To predict the color change of the indicators over the entire pH range from 0 to 14, we need to know the transition pH ranges for each indicator:
- Bromocresol purple: It transitions from yellow (below pH 5.2) to purple (above pH 6.8)
- Cresol red: It transitions from red (below pH 7.2) to yellow (above pH 8.8)
- Thymol blue: It transitions from red (below pH 1.2) to yellow (between pH 8.0 and 9.6) to blue (above pH 9.6)
These transition pH ranges indicate the color changes that occur in the respective pH ranges for each indicator.