The relationship between the activity of hydrogen ions [exactly hydronium
ions, H(H2O)n+] and concentration of a solution is fundamentally important
to determine the extent of a chemical reaction, as it affects the equilibria and
kinetics of a wide variety of chemical and biochemical reactions. The
hydrogen-ion activity refers to the effective concentration of unassociated
hydrogen ions, the form that directly affects physicochemical reaction rates and
equilibria. The symbol, pH, numerically relates the hydrogen ions concentration
or activity. The pH is approximately equal to the negative logarithm of H+
concentration expressed in molarity. pH 7 is neutral; above it alkalinity
increases and below it acidity increases. pH indicators are usually weak acidic
or basic organic tautomers which exist in more than one structural form of
which at least one form is characteristically colored in relation to different
electronic configuration of the bound. Indicators should not change color
exactly at one pH value, but within a wide pH range. The transition point of an
indicator is defined as the point at which the acid and alkaline forms of the
indicator exist in equal concentrations. Visual transition intervals
are as below:
- alpha-Naphthol phthalein (CAS
#: 596-01-0): from pH 7.3
(pinkish-yellow) to pH 8.7 (greenish-blue)
- Bromocresol green (CAS #: 76-60-8):
from pH 3.8
(yellow) to pH 5.4 (blue)
- Bromcresol
Purple (CAS #: 115-40-2): from pH 5.2 (yellow) to pH 6.8 (purple)
- Bromochlorophenol Blue (CAS
#: 2553-71-1): from pH 3.2 (yellow) to
pH 4.8 (blue)
- Bromocresol purple, sodium salt (CAS
#: 62625-30-3): from
pH 5.2 (yellow) to pH 6.8 (purple)
- Bromophenol blue (CAS
#: 115-39-9): from pH 3.0 (yellow) to pH 4.6 (blue)
- Bromophenol blue, sodium salt (CAS
#: 62625-28-9): from pH 3.0
(yellow) to pH 4.6 (blue)
- Bromopyrogallol Red
(CAS #: 16574-43-9): suitable for complexometric or chelometric titrations
- Bromothymol blue (CAS
#: 76-59-5): from pH 6.0 (yellow) to pH 7.6 (blue)
- Bromothymol blue, sodium salt (CAS
#: 34722-90-2): from pH 6.0 (yellow) to
pH 7.6 (blue)
- Bromoxylenol Blue (CAS
#: 40070-59-5): from pH 6.0
(yellow) to pH 7.6 (blue)
- Chlorophenol Red, water
soluble (CAS #: 123333-64-2): from pH 4.6 (yellow) to pH 7.0 (purple)
- Cresol
Red (CAS #: 1733-12-6): from pH 7.0 (orange) to pH 8.8 (purple)
- Fluorescein (CAS
#: 2321-07-5)
- Fluorescein sodium
(CAS #: 518-47-8):
- Glycinethymol Blue (CAS
#: 3810-63-7):
- Iodophthalein sodium (CAS
#: 2217-44-9)
- m-Cresol purple
(CAS #: 2303-01-7): from pH 7.4 (yellow) to pH 9.0 (purple)
- Methylthymol blue, sodium salt (CAS
#: 1945-77-3): suitable for complexometric or
chelometric titrations
- o-Cresolphthalein (CAS
#: 596-27-0): from pH
8.2 (colorless) to pH 9.8 (red)
- o-Cresolphthalein Complexone (CAS
#: 2411-89-4): suitable for complexometric or
chelometric titrations
- Phenolphthalein Complexone (CAS #: 25296-54-2):
- Phenol Red
(CAS #: 143-74-8): from pH 6.8 (yellow) to pH 8.2 (red)
- Phenol Red Sodium Salt (CAS
#: 34487-61-1): from pH 6.8 (yellow) to pH
8.2 (red)
- Phenolphthalein (CAS
#: 77-09-8): from pH 8.5
(colorless) to 9.0 (red)
- p-Xylenol Blue (CAS
#: 125-31-5):
- Acid
range: from pH 1.2 (red) to pH 2.8 (yellow)
- Alkali
range: from pH 8.0 (yellow) to pH 9.6 (purplish-blue)
- Pyrocatechol
violet (CAS #: 115-41-3): suitable for determination of many metals (complexometric or
chelometric titrations) particularly rapid for tin
- Pyrogallol
Red (CAS #: 32638-88-3):
- Thymol Blue, sodium salt (CAS
#: 62625-21-2):
- Acid range: from pH 1.2 (red) to pH
2.8 (yellow)
- Alkaline range: from pH 8.0 (yellow) to pH 9.2 (blue)
- Thymolphthalein
Complexone (CAS #: 1913-93-5): suitable for complexometric or chelometric titrations
- Thymolphthalein (CAS
#: 125-20-2): from pH 8.8
(colourless) to pH 10.5 (blue)
Phthalein is the moiety in a dye
which shows color changes depend on particular hydrogen ion concentrations.
Certain phthaleins (e.g.
phenolphthalein) have a cathartic action.oncentrations.
Certain phthaleins (e.g.
phenolphthalein) have a cathartic action.
Bromocresol Green,
3',3'',5',5''-Tetrabromo-m-cresolsulfonephthalein,
is a pH used as an indicator form pH 3.8 (yellow) and 5.4 (blue).
It is a brown powder sparingly soluble in water, soluble in benzene,
and
very soluble in ethanol and diethyl ether. The sodium salt is a dark green powder.
Both of free acid and sodium salt are used as pH indicator.
It was used in serum albumin determinations. Bromocresol Green is
used in oil field industry for alkalinity test of mud filtrate.
Bromocresol Purple
is 5',5''-Dibromo-o-cresolsulfonephthalein,
a yellow powder soluble in water. The pH range is from
pH 5.2 (yellow) to pH 6.8 (purple). Bromophenol red (CAS RN: 2800-80-8) is 5',5''-Dibromophenolsulfonphthalein,
a brown powder with pH range from 5.2 (orange-yellow) to pH 6.8
(red)
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