Question

Estimate the useful range for each indicator, from a minimum to maximum pH. The range usually spans 1.5 to 2 pH units. For example, bromphenol blue is considered to be useful in the range from 3.0 to 4.6.

Can someone explain what this means?

*the pH ranges from 1.3-7.3 for each tube (tube 1:1.3, tube2:2.3; tube 3:3.3, etc) if that helps? If you need anything else to help me solve it, just ask please! Thanks again!

Answer 1

an indicator usually uses a weak acid and its salt in solution to set up a buffer. The pH is calculated, in part, by using a ratio of [A-]/[HA]. When the concentration of the acid (HA) and it's conjugate base (A-) are equal, or nearly equal, the value of the ratio is 1, or very close to it.
As the amount of acid or base inside the solution changes, the ratio of the indicator changes. If the concentrations vary too much, the buffer loses its effectiveness. We generally say that an indicator is useful to +/- 1 pH unit from the middle of its range.

If tube #1 has a pH of 1.3, then its range is going to be in the region of 0.3 - 2.3pH units. If the pH that you need is outside this range, then the indicator in that tube would be a bad choice to use, since the desired pH is outside the effective range of that indicator. Thankfully, there are all sorts of indicators that function over all sorts of pH ranges.

Answer 2

Okay, so I had 7 tubes and I added a different chemical to each (4 sets). So can I use any of the chemicals? (Does that make sense?) Or do I have to only use the chemical I added to the set?

Answer 3

what are you trying to do with these tubes? find the pH? identify them? I need more info

Answer 4

Sorry if this is confusing, I'm confused myself. But I'm trying to find the range for each indicator. For set 1 of 7 tubes, I added bromothymol blue to change the color. To the second set I added methyl yellow to the third bromcresol green and lastly the fourth I added red cabbage extract. So I am trying to find the range in which each chemical added works best.

Answer 5

do you know the pH of each tube in each set?

Answer 6

For set 1..
tube 1 1.3
tube 2 2.3
tube 3 3.3
tube 4 4.3
tube 5 5.3
tube 6 6.3
tube 7 7.3
(I have to look on my paper for the other ones. I will get those)

Answer 7

Oh wait. They are all the same. Set 1-4 is identical.

Answer 8

ok, this will work then. Since you added bromothymol blue to each tube in set 1, there should be a point at which the color of the solution in the tube changes, from yellow, to green, and maybe blue. The useful range of the indicator is +/- 1 pH unit from the point at which it changes colors.

So what in tube from set #1 does the color change?

Since you added a different indicator to each set, each set will have a different set of colors and a different range.

Answer 9

For set 1, the color changes from yellow to blue at tube 7

Answer 10

Since the color change happens in tube #7, the "transition interval" occurs somewhere between tubes 6 and 7. The pH "at the transition interval" is between 6.3 and 7.3
Halfway between 6.3 and 7.3 is 6.8, so the useful range of bromothymol blue will be +/- 1pH unit from 6.8

bromothymol blue: 5.8 - 7.8, or roughly 6-8. The other 3 tubes will all change colors at different pH values, giving you the useful range of each indicator.

Answer 11

Okay, but what if it changes more than once within a set? Like for set 2, it changes from red (tube 1&2) to orange (tube 3) to yellow (tube 4-7)

Answer 12

there really is only 1 transition, even though it looks like 2. If you think about it, what do you get when you mix equal amounts of "red" and "yellow"? You get orange! So what is happening here, is you are seeing the "exact" midpoint of the indicator's range inside the orange tube. Both colors are present in equal (or about equal) amounts.

Answer 13

oh okay. So the exact range is 3.3? And what if it changes 4 times...?

Answer 14

Like in set 3 it changes from yellow (tube 1-3) to green (tube 4) to blue (tube 5) to purple (tube 6&7)

Answer 15

green is between yellow and blue, so there's 1 transition. and purple is inbetween blue and ...?

Answer 16

if the transition occurs at 3.3, the range is 2.3 - 4.3

Answer 17

Okay and purple is inbetween blue and pink? I'm not sure..

Answer 18

@theequestrian i think and im assuming that the way @jfraser gave is appropriate

Answer 19

I thin so too, but I don't know what to do when the color changes four times. This is what I am trying to figure out.

Answer 20

**think

Answer 21

well it should have u tried it in the LAB

Answer 22

I did try it in the lab and I got the color change. I just do not know how to calculate the range of the pH like how @JFraser had helped me with the other sets.

Answer 23

then follow this:
Recall that HNO3 is a strong acid. It completely ionizes into H+ and NO3- ions.

We first get the molarity of the solution. Before that, we need to convert 5.00 g HNO3 to moles by dividing by its molar mass.

5.00 g / 63.01 g/mol = 0.0794 mol

then, we divide 0.0794 by the volume of solution, 2.00 L.

M = 0.0794 mol / 2.00 L
= 0.0397M

Before ionization, the concentration of HNO3 is 0.0397M. After ionization, the concentration of H+ and NO3- ions is also 0.0397M (since the solution completely dissociated, the conjugate base and H+ ions will have equal concentrations).

the concentration of H+ ions is also 0.0397M. We now get the pH by getting the negative log of the concentration.

pH = -log[H+]
= -log (0.0397)
= 1.40

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To Becky T: Not all the time, you have pH paper available. pH paper usually gives one a crude measurement of the pH of the substance. it's not that reliable enough, especially if you have a different concentration. Different concentrations of acids and bases usually have different pH values. To be sure of the pH, it's very useful to calculate the pH. That way, you won't go into the hassle of throwing away money or wasting time just to get a pH meter or pH paper. you still need to calibrate those stuff.

Answer 24

An indicator might have more than one pH range in which it is useful, why is this an issue?

Answer 25

@Rohangrr okay, that makes sense.

Answer 26

thanks im \[\huge SENSIBLE \quad HUMAN\]

Answer 27

@theequestrian ^^

Answer 28

thanks for your help!