Juglone and Lawsone as Acid-Base Indicators
K r is h n a C. J o s h i , P. S i n g h , and G. S in g h
Department o f Chemistry, University of Rajasthan, Jaipur, India
(Z. N aturforsch. 32 b, 890-892 [1977]; received May 10, 1977)
Juglone, Lawsone
Juglone and lawsone, both acid-base indicators, obtained from natural sources and give
pink and red colours with aqueous alkalies, respectively. Their transition ranges are found
to be pH 7.4-8.2 and 2.6-3.4. To establish the transition intervals, a buffer series ranging
in the pH values from 2 .0 -2 .2 -2 .4 -2 .0 -2 .8 -3 .0 -3 .2 -3 .4 -3 .6 -3 .8 -4 .0 -4 .2 -4 .4 -4 .6 -4 .8 -5 .0 5 .2 -5.4-5.6-5.8-6 .0 -6 .2 -6 .4 -6 .6 -6 .8 -7 .0 -7 .2 -7 .4 -7 .6 -7 .8 -8 .0 -8 .2 -8 .4 -8 .6 -8 .8 -9 .0 has been
used. Juglone can be used for the titrations o f strong acids with strong bases and weak acids
with strong bases and lawsone can be used for the titration of strong acids with weak bases
only. Permanancy of the colours at the end point is one of their advantages.
In continuation to our preliminary findings on
the possible use of hydroxy naphthoquinones as
acid-base indicators1-2, we have now examined
juglone and lawsone for this purpose. Juglone
[5-hydroxy-1,4-naphthoquinone] and lawsone [2hydroxy-1,4-naphthoquinone] are easily isolated
from the plants of families Juglandaceae and
Lythraceae, respectively.
Juglone C1 0 H 6 O3 is an orange crystalline solid,
m.p. 164-165 °C. It gives a wine red colour with
FeCl3 and a pink colour with magnesium acetate.
Lawsone CioH603 is a pale yellow crystalline solid,
m.p. 192 °C (dec.). It gives a red colour with FeCl3
and an orange red colour with magnesium acetate.
medium may be ascribed to following resonating
structures.
The transition interval of juglone towards basic
side (7.8 to 8.2) is probably due to intramolecular
hydrogen bonding forming six membered ring which
suppresses the ionization of phenolic hydrogen. In
case of lawsone, there is intramolecular hydrogen
bonding involving five membered ring which does
not suppress the ionization to such an extent.
Experimental
Both juglone and lawsone give a slight pale yellow
colour in acidic medium and pink and red colours
respectively with aqueous alkalies. Therefore, we
thought it worthwhile to examine their utility as
acid-base indicators. Change in colour in alkaline
pH range determination
The determination of the transition intervals has
been done by the following simple procedure. A
buffer series (mixture of M c I lv a in e ) 3, according to
the transition interval to be expected, is prepared in
a number of test tubes of equal colour and diameter
in such a way that the initial pH value of the series
is 0.8 pH units less than the lowest limit, and the
final pH value of the series is 0.8 pH units higher
than the upper limit of the transition interval to
be expected. The remaining test tubes are filled
with buffer solutions of the pH values, lying be­
tween these two limits, in a succession of gradually
increasing and then decreasing intervals of pH
values. Juglone and lawsone have slight pale yellow
colour on acid side while their alkaline colours are
NaO H
PINK
CO LO UR
JUG LO N E
Requests for reprints should be sent to Prof. K. C.
Department of Chemistry, University of
Rajasthan, J a ip u r, India.
J o s h i,
RED
COLOUR
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K. C. Joshi et al. • Juglone and Lawsone as Acid-Base Indicators
pink and red and their transition intervals lie
between pH 7.4 to 8.2 and 2.6 to 3.4, respectively.
The volume of each buffer solution is taken in
equal amount, e.g., 20 ml, in each test tube. Now
0 . 2 ml, 0 . 1 % alcoholic solution of indicator is added
in each test tube proceeding from lower towards
higher pH values. The first pink and red shade
appears at pH 7.4 and 2.6 and the intensity of the
colours gradually increases till pH 8.2 and 3.4
respectively. Whereas in the reverse direction, the
decrease in pink and red shade and colour intensity
of the solutions is first perceptible at pH 8.2 and 3.4
and go on decreasing up to pH 7.4 and 2.6. These
two limits of transition intervals have also been
examined with the help of a pH meter.
Titration of strong acids with strong bases and viceversa
The pH values belonging to every single stage of
the neutralisation have been measured potentiometrically with glass electrodes. The alteration of
the pH value in the course of the titration is
illustrated by the so called neutralization curves
which can be obtained if the pH value of the
solution is depicted as a function of the volume of
the titrant consumed or of the percentage neutrali­
zation. Table I shows the variations of pH on
titrating 10 ml of 0.1 M hydrochloric acid with
sodium hydroxide.
Table I. Neutralization o f 0.1 M HC1 with NaOH.
Neutralization [%]
0
50
90
99
99.9
100
Over titration
0.1
1
10
pH
1.1
1.3
2.0
3.0
4.0
7.0
10.0
11.0
12.0
In the titration of 0.1 M solutions, a neutralization
of 99.9% corresponds to pH 4.0 whereas an over
titration of 0.1% corresponds to pH 10; in titration
with 0.01 M solutions, the 99.9% neutralization is
corresponding to pH 5, and the 0.1% over titration
to pH 9. By plotting volume of the titrant consumed
with alteration of the pH value, the neutralization
curve of the titration of strong acids with strong
bases is obtained. The jump in equivalence point is
larger, the more concentrated are the solutions that
are titrated with one another. Since the curves are
completely symmetrical, the given Fig. 1 is also
valied for the reverse case, i.e., if strong bases are
titrated with strong acids.
Suitability of Indicator: For the indication of end
point, those indicators are suitable whose transition
891
intervals he between the pH values corresponding
to ± 0 . 1 % accuracy, i.e., on the steep part of the
titration curves. The steep part of the titration
curves lies between pH 3 and 11 in the case of 1 M
solutions, pH 4 and 10 in the case of 0.1 M solutions
and pH 5-9 in 0.01 M solutions. Juglone with pH
range 7.4 to 8.2 can be frequently used as indicator
in these titrations. But lawsone can not be used in
these titrations.
Fig. 1. Titration of strong acids with strong bases.
Titration of weak acids with strong bases: Table II
shows the variation of pH on titration of 10 ml of
0.1 M acetic acid with hydrochloric acid.
Table II. Neutralization of 0.1 M acetic acid with
NaOH.
Neutralization [%]
0
50
90
99
99.9
Over titration
0.1
1
10
pH
2.9
4.8
5.7
6.8
7.7
9.8
11.0
12.0
In the titration of weak acids with strong bases,
one may usually apply indicators changing their
colours in the alkaline pH range. According to
Table II, a neutralization of 99% may be indicated
with such indicators whose transition point is equal
to or greater than pH 6 .8 . If 0.1 M acetic acid is
titrated with 0.1 M NaOH in presence of juglone,
the colour change at transition interval will be
easily perceptible because its pH range is 7.4-8.2.
But in presence of lawsone the colour change at
transition interval will not be perceptible because
its pH range is lower than 6 .8 .
K. C. Joshi et al. • Juglone and Lawsone as Acid-Base Indicators
892
Fig. 2. Titration o f weak acids with strong bases.
Fig. 3. Titration of strong acids with weak bases.
Titration of weak bases with strong acids: The pH
values according to the percentage neutralization
are listed in Table III.
Table III. Neutralization of 0.1 M N H 4OH with HC1.
Neutralization [%]
0
50
90
99
99.9
Over titration
0.1
1
10
pH
11.3
9.3
8.3
7.3
6.3
4.0
3.0
2.0
The course of the titration curves is similar to
that of weak acids. Considerations applicable to
titration of weak bases are also analogous to those
for weak acids. For the indication of the end point
of titrations those indicators are generally used
which change their colour in the acid pH range.
According to the data listed in the Table III, in
order to attain 99.9% neutralization the transition
of the indicator must be equal or less than pH 7.3
and 6.3, if an accuracy of 0.1% is required. For the
indication of the end point of titration of ammonium
hydroxide, any of those indicators may be used
whose transition point falls between pH 7.3 and 3.0.
Juglone can not be used in these titrations but
lawsone can be used.
Conclusions
From the above observations, it has been con­
cluded th at:
I) Juglone and lawsone are one colour indicators.
II) The transition interval of juglone lies in base
pH range, i.e., pH 7.4 to 8 . 2 and lawsone lies
in acid pH range i.e. pH 2.6-3.4.
III) The transition intervals of juglone and lawsone
are narrow.
IV) The appearance of the first pink and red shade
is easily perceptible in each indicator.
V) The colour of the solution remains clear at
every pH value.
VI) Juglone is a suitable indicator in the titrations
of strong acids with strong bases and strong
bases with weak acids and vice-versa and
lawsone is suitable in the titration of strong
acids with weak bases.
VII) That appearance of the pink and red colours
at the end point remains permanent for a
longer time.
J o s h i , P. S i n g h , and G. S i n g h , Talanta 23,
325 [1976].
2 K. C. J o s h i , P. S i n g h , and G. S i n g h , Chem. Ind.
London 1976, 834.
3 T. C. M c I l v a i n e , J . Biol. Chem. 49, 183 [1921].
1
K. C.