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What is an acid-base indicator?An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicatoris a different color than the iogenic form of the indicator. An Indicator does not change colorfrom pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color changeoccurs over a range of hydrogen ion concentrations. This range is termed the color changeinterval. It is expressed as a pH range.

How is an indicator used?eak acids are titrated in the presence of indicators which change under slightly alkalineconditions. eak bases should be titrated in the presence of indicators which change underslightly acidic conditions.

What are some common acid-base indicators?!everal acid-base indicators are listed below, some more than once if they can be used overmultiple pH ranges. "uantity of indicator in a#ueous $a#.% or alcohol $alc.% solution isspecified. Tried-and-true indicators include& thymol blue, tropeolin '', methyl yellow, methylorange, bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutralred, phenolphthalein, thymolphthalein, ali(arin yellow, tropeolin ', nitramine, andtrinitroben(oic acid. )ata in this table are for sodium salts of thymol blue, bromphenol blue,tetrabromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, and cresolred.

Table of Common Acid-Base Indicators

Indicator pH Range Quantit per !" ml Acid Base

Thymol *lue +.-. +- drops .+/ soln. in a#. red yellow

0entamethoxy red +.-.1 + drop .+/ soln. in 2/ alc. red-violet colorless

Tropeolin '' +.1-1. + drop +/ a#. soln. red yellow

,3-)initrophenol .3-3. +- drops .+/ soln. in 4/alc.

colorless yellow

5ethyl yellow .6-3. + drop .+/ soln. in 6/ alc. red yellow

5ethyl orange 1.+-3.3 + drop .+/ a#. soln. red orange

*romphenol blue 1.-3.7 + drop .+/ a#. soln. yellow blue-violet

Tetrabromphenol blue 1.-3.7 + drop .+/ a#. soln. yellow blue

Ali(arin sodium sulfonate 1.2-4. + drop .+/ a#. soln. yellow violet

8-9aphthyl red 1.2-4. + drop .+/ soln. in 2/ alc. red yellow

p-:thoxychrysoidine 1.4-4.4 + drop .+/ a#. soln. red yellow

*romcresol green 3.-4.7 + drop .+/ a#. soln. yellow blue

5ethyl red 3.3-7. + drop .+/ a#. soln. red yellow

*romcresol purple 4.-7. + drop .+/ a#. soln. yellow purple

;hlorphenol red 4.3-7. + drop .+/ a#. soln. yellow red

*romphenol blue 7.-2.7 + drop .+/ a#. soln. yellow blue

p-9itrophenol 4.-2. +-4 drops .+/ a#. soln. colorless yellow

A(olitmin 4.-. 4 drops .4/ a#. soln. red blue

0henol red 7.3-. + drop .+/ a#. soln. yellow red

9eutral red 7.-. + drop .+/ soln. in 2/ alc. red yellow

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bromothymol blue!bromthymol blue- a dye used as an acid-base indicator

http://www.thefreedictionary.com/bromothymol+bluehttp://www.thefreedictionary.com/bromothymol+bluehttp://www.thefreedictionary.com/bromthymol+bluehttp://www.thefreedictionary.com/bromothymol+bluehttp://www.thefreedictionary.com/bromthymol+blue