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DEVELOPMENT OF pH INDICATOR STRIP FOR pH MEASURMENT OF WATER
SAMPLE
Weerawut Kacha,* Songporn Juengmunkong, Waew Kongsila
Department of Chemistry, Faculty of Pharmacy, Ubon Ratchathani University, Ubon
Ratchathani 34190, Thailand
*e-mail: [email protected]
Abstract: In this work the pH indicator strips for the pH measurement of water sample in the
pH range of 2 – 10 was developed. Three systems of the mixed indicators, such as
tetrabromophenol blue and methyl red, cresol red and tetrabromophenol red, and cresol red
and phenolphthalein were prepared to produced bad A, band B and brand C of the pH
indicator strips, respectively. The pH reading of water sample from the the developed strips
gave the same value as the litmus paper and commercial pH sticks, so the developed pH
indicator strips could be used in pH monitoring of water sample without the use of pH meter.
Introduction: pH is the measure of acidity or alkalinity of material. A measurement of pH is
one of the most important parameter and frequently used test in water chemistry. pH as
defined by Sorenson is –log[H+]. Use of the term pH assumes that the activity of the
hydrogen ion, [H+]. A logarithmic scale in the pH range of 1 – 141. Indicator is a weak acid or
weak base of a large organic molecule, the color change of indicator over a range of
hydrogen ion concentrations. This range is termed the color change interval, which expressed
as pH range.
Table1: pH indicators used in the pH indicator strips.
Composition of
pH (transition
range)
Indicators
Color change
indicator solution
(%)*
3.0 – 4.6
Tetrabromophenol blue
yellow - purple
0.5 (in water)
0.5 (in water)
0.5 (in water)
0.5 (in water)
4.4 – 6.2
Methyl red
red - yellow
7.2 – 8.8
Cresol red
yellow - red
8.3 – 10.0
Phenolphthalein
colorless - fuchsia
*Composition of indicator solution (%w/v) which used in this work.
Methodology: Indicators in the acid range in this work are tetrabromophenol blue (TB, 3.0 –
4.6), methyl red (MR, 4.4 – 6.2), neutral to basic range are cresol red (CR, 7.2 – 8.8) and
phenolphthalein (Phe, 8.3 – 10.0) from Carlo erba. Litmus paper and pH sticks from Merck
Serono (UK). pH indicator strips were prepared by using filter paper immersed fully in mixed
indicator solution (0.5%w/v), TB+MR (band A), CR+TB (band B) and CR+ Phe (band C). A
1x8 cm filter paper strips were immersed in the mixed indicator solution for 30 min. and
dried at 70 oC for 1 hour in hot air oven. The dried indicator strips used for pH measurement
of water sample and color stability test with buffer solution (pH 2 – 10). The pH value was
compared with the reading value of litmus paper and commercial indicator stick.
© 38th Congress on Science and Technology of Thailand (Full paper)
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Figure 1. The prepared pH indicator strips (1x8 cm, size) with three bands of mixed
indicator.
Results, Discussion and Conclusion: The changing of color in the three band indicator strips
were tested by using buffer solution and used as standard color of these strips to reading the
pH value of testing sample. pH value or pH range of testing water sample were tested by
using prepared indicator strip, shown as the same result as litmus paper and commercial
indicator sticks. In Fig.3 showed the color band change of prepared indicator strip, litmus
paper and commercial indicator sticks.
pH 2 pH 3 pH 4 pH 5 pH 6 pH 7 pH 8 pH 9 pH 10
A
B
C
Figure 2. Color band of prepared pH indicator strips showed color change at pH unit
intervals, 2-10.
Table2: pH value/pH range of testing water sample*.
Test kit
Studied pH strips Litmus paper Indicator sticks
Sample
1
6
6
6-7
2
6
6
6-7
3
6
6
6-7
4
8
9
8-9
5
8
8
8-9
6
6
6
6-7
7
6
6
6-7
8
6
6
6-7
9
6
6
6-7
10
6
6
6-7
* testing water sample = commercial drinking water 10 brands.
© 38th Congress on Science and Technology of Thailand (Full paper)
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Figure 3. Color change of testing water sample (S1- S10) by using prepared pH indicator
strips (TK), litmus paper (ก) and commercial indicator sticks ( ) (Merck Serono, UK).
(A)
(B)
(C)
Figure 4. Color change of prepared pH indicator strips (Rep1- 10) by using buffer solution
(A) buffer pH 4.0, (B) buffer pH 7 and (C) buffer pH 10.0.
© 38th Congress on Science and Technology of Thailand (Full paper)
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To evaluate the stability of indicator color which adsorbed on filter paper, pH indicator strips
were tested with buffer solution pH 4, 7 and 10 (10 repeat testing per solution). The color
change of the prepared pH indicator strips showed the same color band. The testing result
was shown in Fig.4.
In conclusion we found a good correlation between the changing color of prepared pH
indicator strips and litmus paper and Merck Serono strips (with pH strips 2–9). We conclude
that the pH strips accurately represent the actual pH of water sample and can therefore be
used in the pH measurement of water.
References:
1. Nourmohammadian F., Mahnaz D., Abdol-Ali A. Dyes and Pigments 2007; 74:741743.
Acknowledgements: The authors acknowledge financial assistance received from the
department of chemistry, faculty of pharmacy, Ubon Ratchathani University, Ubon
Ratchathani 34190, Thailand.
Keywords: pH strip, pH stick, indicator strip
© 38th Congress on Science and Technology of Thailand (Full paper)