Lab Proposal: Determining the Effectiveness of Thymol Blue and Ethyl Bis Ethanoate compared to that of Phenolphthalein in Determining the Molarity of NaOH
Background
As described in our Atkins textbook, an acid-base indicator “changes color with pH because it is a weak acid that has one color in its acid form and another color in its conjugate base” (538). The equation of the proton-transfer that the indicator undergoes is as follows:
〖HIn〗_((aq) )+〖H_2 O〗_((l) )⇋〖H_3 O^+〗_((aq) )+〖In^-〗_((aq) )
The end point of the indicator is when the “concentrations of its acid and the base forms are equal”, so by the Henderson-Hasselbach equation, we know that pH=pKIn. Because the end point is a property of the indicator, and the equivalence point, or…show more content… Because NaOH is not a primary standard due to its hygroscopicity and reactivity with CO2 in the air, we could not use it directly to titrate phosphoric acid. Instead, we needed to standardize ~0.1M NaOH with KHP, which is a reagent that is pure enough to weigh out to directly find the number of moles of. We used phenolphthalein, which has a transition range of pH 8.2 (colorless) to 9.8 (pinkish red-violet). This was important because we were titrating a weak acid with a strong base, which has an equivalence point near pH=9, which is included in the pH range of phenolphthalein. At the equivalence point, the moles of analyte (NaOH in our solution) were equal to moles of titrant…show more content… The reaction equation is as follows:
〖KHP〗_((aq))+〖NaOH〗_((aq))→〖NaKP〗_((aq))+〖H_2 O〗_((l))
In class, we used phenolphthalein, but now, we want to see the effects of using other color indicators with similar rangers, and compare our calculated NaOH molarity with that determined by phenolphthalein. This is important to consider because if the result is not significantly different, we may be able to use different color indicators without impacting the accuracy in the standardization of NaOH. Considering the costs of different color indicators and the supply of the indicators, it may be beneficial to use a similarly ranged indicator if our results are not significantly impacted.
We will be comparing 3 indicators, with 3 titrations per indicator for a total of 9 titrations. We will compare the results of the titrations with thymol blue and ethyl bis ethanoate to the results with phenolphthalein. By performing 3 titrations per indicator, we can hopefully be more accurate with the added trials per indicator, and reduce the effect of random
