okay, the answer is A and here is the full answer explanation.
A solution that turns blue with bromthymol blue has a pH greater than 7.78 (pKa = 6.68, so the pure blue color starts at 6.68 + 1). A pH greater than 7.78 does not guarantee that the pH is greater than 8.79, the pKa for phenolphthalein. This means that the solution may or may not turn magenta with phenolphtihatein. Choice A is invalid. If a solution tums green with bromcresol green, the pH is roughly 4.37. if a solution turns bromthymol blue green, the pH is roughly 6.68. The pH cannot simultaneously be 4.37 and 6.68, so the stick cannot simultaneouily hurre two green marks. Choice B is a valid statement. A solution that turns methyl red yellow has a pH greater than 6.21 (pKa = 5.21,so the pure yellow color starts at 5.21' + 1). A pH greater than 6.2a guarantees that the pH is greater than 4.31, the pKa for bromcresol green' This means that the solution must turn blue with bromcresol green. Choice C is a valid statement. When the pH of the solution falls between 4.27 and5.37, it falls into the color blend range of two separate indicators. As a result, the pH can be approximated with twice the accuracy. Choice D is a valid statement.
I just do not understand their reasoning for choice D. Does the pH stick not estimate well for values beyond 5.37?
Indicators typically have a color band change in the range of pKa +/- 1. Now the passage says:
A pH stick is a device that can approximate the pH of an aqueous solution by reference to a color blend band.
The pH range of 4.21 and 5.37 would mean that the color blend band of both bromcresol green and methyl red would be changing from their respective protonated color to deprotonated colors. The pH stick would have two indicators it could reference to provide a more accurate pH reading. The 4.21 comes from the methyl red pKa minus 1 and the 5.37 comes from the bromcresol green pKa plus 1.
This isn't to say that the pH can't estimate well for values beyond 5.37, it's talking about how two indicators will provide a more accurate depiction of the pH than just one. Of the four indicators in table 1, let's look at what the ranges would be for the other two combinations (using the +/- of the pka):
1) Bromcresol Green & Methyl Red: 4.21-5.37 [This is the one from the question]
2) Methyl Red & Bromthymol Blue: 5.78-6.21
3) Bromthymol Blue & Phenolphthalein: 7.78-7.79
Methyl Red's pka is 5.21 and Bromthymol Blue's is 6.78... notice how their pka's are outside of the range. Again for the third, we know Bromthymol's pKa, and Phenolphthalein is 8.79, which is also outside the ranges.
Now since the pH stick is referencing the color blend band, it'll give a more accurate representation of when the indicators are in the midst of their color change. In scenario 2, the methyl red is predominantly yellow (even at the lowest part of that range), while bromthymol blue would only begin changing colors. In scenario 3, it's even worse, as the bromothymol blue would be completely finished changing colors, while phenolphthalein would only be beginning.
So answer choice D is valid for the indicators being used by the pH stick. That range of pH would produce the most accurate pH because the pH stick could reference the color band blend of two indicators instead of one. Not to say that the pH stick couldn't calculate the pH of a solution of a different pH (outside of that range)... it would just be less accurate.
Hope that helps.
