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What is an indicator in chemistry

Acid Base indicators change color depending on the pH of the solution. Below is a table that includes the pH range of the change and the colors involved for many different indicators.

Tricks to using NYS Chemistry Reference Table M

Tip 1- Notice "Common Acid Base Indicators", acid is on the left and base is on the right.

Low pH numbers are on the left and high pH numbers are on the right. They match up.

Tip 2- Look at methyl Orange, red is on the left, 3.2 is on the left. To the left (or below) a pH of 3.2 the color is red.

To the right (above) a pH of 4.4 the color is yellow.

Tip 3- Between the 2 numbers will never be asked on a test. This is where the indicator changes color.

Past Regents questions involving Acid Base Indicators

Aug 2010 - 27 Based on the results of testing colorless solutions with indicators, which solution is most acidic?

(1) a solution in which bromthymol blue is blue

(2) a solution in which bromcresol green is blue

(3) a solution in which phenolphthalein is pink

(4) a solution in which methyl orange is red

June 2009- 48 Which indicator would best distinguish between a solution with a pH of 3.5 and a solution with a pH of 5.5?

(1) bromthymol blue (3) litmus

(2) bromcresol green (4) thymol blue

Aug 2008- 25 Which indicator is blue in a solution that has a pH of 5.6?

(1) bromcresol green (3) methyl orange

(2) bromthymol blue (4) thymol blue

Jan 2008 - 47 Which indicator, when added to a solution, changes color from yellow to blue as the pH of the solution is changed from

5.5 to 8.0?

(1) bromcresol green (3) litmus

(2) bromthymol blue (4) methyl orange

Jan 2006 - 47 Which indicator is yellow in a solution with a pH of 9.8?

June 2005 - 50 In which solution will thymol blue indicator appear blue?

(1) 0.1 M CH3 COOH (3) 0.1 M HCl

(2) 0.1 M KOH (4) 0.1 M H2 SO4

Jan 2005 - 26 According to Reference Table M, what is the color of the indicator methyl orange in a solution that has a pH of 2?

Adding hydrogen ions:

Methyl orange


he pH range of indicators

The importance of pKind

Think about a general indicator, HInd - where "Ind" is all the rest of the indicator apart from the hydrogen ion which is given away:

Because this is just like any other weak acid, you can write an expression for Ka for it. We will call it Kind to stress that we are talking about the indicator.

Think of what happens half-way through the colour change. At this point the concentrations of the acid and its ion are equal. In that case, they will cancel out of the Kind expression.

You can use this to work out what the pH is at this half-way point. If you re-arrange the last equation so that the hydrogen ion concentration is on the left-hand side, and then convert to pH and pKind. you get:

That means that the end point for the indicator depends entirely on what its pKind value is. For the indicators we've looked at above, these are:

Category: Forex

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