ISO 18191:2026

International
Standard
ISO 18191
Second edition
Water quality — Determination of
2026-07
pH in seawater — Method using
T
the indicator dye m-cresol purple
Qualité de l'eau — Détermination du pH dans l'eau de mer —
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Méthode utilisant l'indicateur coloré au pourpre de m-crésol
Reference number
© ISO 2026
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Reagents . 3
6 Apparatus . 3
7 Sampling . 4
8 Procedure . 4
9 Calculation and expression of results . 5
9.1 Correction of measured absorbances .5
9.2 Calculation of the pH of the seawater and indicator .5
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9.3 Correction for pH change resulting from addition of the indicator .6
T
Annex A (informative) Performance data . 8
Annex B (informative) Storage stability .11
Bibliography .12

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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This document was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2,
Physical, chemical and biochemical methods.
This second edition cancels and replaces the first edition (ISO 18191:2015), which has been technically
revised.
The main changes are as follows:
— update of terms and definitions;
— addition of Clause A.1 dedicated to method performance data based on an interlaboratory comparison
on TRIS buffer and natural seawater;
— addition of guidance on the measurement procedure and calculation and expression of results;
— editorial improvements throughout.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The greenhouse effect induced by anthropogenic carbon dioxide, CO , in the atmosphere is one of the serious
global environmental issues. A key factor controlling the atmospheric CO is its absorption into the ocean.
As a result of the absorption, the pH in the upper layer of the ocean is observed to have fallen gradually, and
its influence on the living organisms is a matter of concern all over the world.
On the other hand, carbon capture and storage (CCS) technology is considered as a useful means of reducing
the CO emissions from fossil fuel. When ocean environment such as sub-seabed aquifer is selected as a
storage site, the monitoring of carbonate system including pH in seawater becomes very important. The
analytical method for pH (the total hydrogen ion concentration pH scale) in seawater samples requires
T
specific conditions and techniques essential to the precise and accurate determination. This document
describes a method for the determination of pH in seawater with the repeatability less than 0,003.
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This method provides international communities accurate data sets on pH in seawater being compatible
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with each other. This is the base of national and international operational observation or monitoring
programs of the oceanic carbonate system as well as individual research works.

v
International Standard ISO 18191:2026(en)
Water quality — Determination of pH in seawater — Method
T
using the indicator dye m-cresol purple
WARNING — Persons using this document should be familiar with normal laboratory practice. This
document does not purport to address all of the safety problems, if any, associated with its use. It
is the responsibility of the user to establish appropriate safety and health practices and to ensure
compliance with any national regulatory conditions.
IMPORTANT — It is absolutely essential that tests conducted in accordance with this document be
carried out by suitably qualified staff.
1 Scope
This document specifies a spectrophotometric determination of the pH of seawater on the total hydrogen
T
+
ion concentration pH scale. The total hydrogen ion concentration, [H ] , is expressed as moles per kilogram
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of seawater. The method is suitable for assaying oceanic levels of pH from 7,4 to 8,2 for normal seawater of
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practical salinity ranging from 20 to 40.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
total hydrogen ion concentration
+
[H ]
T
total hydrogen ion concentration of seawater, including the contribution of free hydrogen ions and the
hydrogen ions bound to sulfate ions in the seawater
Note 1 to entry: The total hydrogen ion concentration of seawater is defined in Formula (1) as:
 
[]HH[] 1 SK/[HH][ SO ] (1)

TF TS F 4
where
+
[H ] is the free concentration of hydrogen ion in seawater;
F
– 2
   
S is the total sulfate concentration HSOS O ;
T 
   
−
K is the acid dissociation constant for HSO .
S
The pH is then defined as the negative of the base 10 logarithm of the hydrogen ion concentration as given
T
in Formula (2):

 
[]H
t
 
pH log (2)
T 10
 
mol/kg
 
3.2
practical salinity
S
amount of dissolved salts in seawater defined as a polynomial function of the ratio R
Note 1 to entry: The ratio R is the electrical conductivity of the seawater sample at the temperature of 15 °C on the
International Practical Temperature Scale of 1968 and the pressure of one standard atmosphere, to that of a potassium
-3
chloride (KCl) solution, in which the mass fraction of KCl is 32,435 6 × 10 , at the same temperature and pressure.
4 Principle
The values of pH are determined by adding an indicator to seawater. For sulfonephthalein indicators such as
T
m-cresol purple, the reaction of interest at seawater pH is the second dissociation as given in Formula (3):
T
 2
HI aq HaqI aq (3)
  
where
I is the indicator, which is present at a low level in a seawater sample.
The total hydrogen ion concentration of the sample can then be determined as given in Formula (4):
2
 
I
  
pH pHK I log (4)

T 10

 
HI
 
The principle of this approach uses the fact that the different forms of the indicator have substantially
different absorption spectra. Thus, the information contained in the composite spectrum can be used to
2‑ ‑
estimate [I ] / [HI ].
At an individual wavelength, λ, the measured absorbance, A , in a cell with a pathlength L is given by the
λ
Beer-Lambert law as shown in Formula (5):
A
  22
   
HI HI  II B e (5)
 
 
   
L
where
B is the background absorbance of the sample;
λ
e is an error term due to instrumental noise.
‑ 2‑
Provided that the values of the extinction coefficients: ε (HI ) and ε (I ) have been measured as a function
λ λ
of wavelength, absorbance measurements made at two or more wavelengths can be used to estimate the
2‑ ‑
ratio [I ] / [HI ].
In the case that only two wavelengths are used, and provided that the background can be eliminated
effectively by a subtractive procedure, Formula (5) can be rearranged to give (assuming no instrumental
error) Formula (6):
2 
 
I AA//HI HI
 
12 12
 
 (6)
 2 2
 
HI IH//I  AA  IH/ I
   
1 21 2 22 2
 
where
1 and 2 are the wavelengths chosen.
2‑
For the best sensitivity, the wavelengths corresponding to the absorbance maxima of the base (I ) and acid
‑
(HI ) forms, respectively, are used. The various terms ε are the extinction coefficients of the specified species
at wavelengths 1 and 2, respectively.
5 Reagents
Use only reagents of recognized analytical grade.
5.1 m-cresol purple, containing no spectrophotometric impurities.
NOTE 1 Reference [13] showed the indicator can be characterized and purified using the HPLC system. The
‑ 2‑ ‑ 2‑
wavelength of isosbestic point for HI / I of the pure m-cresol purple, λ (HI / I ) depends on the following formula:
isos
‑ 2‑ ‑ ‑
λ ( HI / I ) = 490,6 – 0,10 t, where t is the temperature in degrees Celsius. That for H I / HI is also λ (H I / HI ) =
isos 2 isos 2
482,6 – 0,10 t.
NOTE 2 Reference [13] and Reference [16] describe the purification method of m-cresol purple.
NOTE 3 Purified m-cresol purple is used. When purified m-cresol purple is not available, the correction procedure
described in Reference [11] is applied.
5.2 Solution of pure m-cresol purple.
A concentrated (at least 2 mmol/l) pure indicator solution is prepared either in ultrapure water or in a
0,7 M NaCl ionic background. The pH value of the dye solution is adjusted to be in the range 7,9 ± 0,1 pH
units - chosen to match pH measurements from an oceanic profile by adding HCl or NaOH as needed; this
T
implies that for m-cresol purple A /A ≈ 1,6, where A and A are the corrected absorbances measured at the
1 2 1 2
wavelengths corresponding to the absorbance of 578 nm and 434 nm, respectively
NOTE The absorbance ratio of a concentrated indicator solution can be measured using a cell with a short
pathlength (0,5 mm).
5.3 Ultrapure water, of resistivity about 18 MΩcm.
To be used in the preparation of the pure m-cresol purple solution and in rinsing of equipment (e.g. cells).
6 Apparatus
Usual laboratory equipment and, in particular, the following shall be used.
6.1 Flexible drawing tube.
Approximately 40 cm long, sized to fit snugly over cell port. Silicone rubber is suitable. The drawing tube can
be pre-treated by soaking in seawater for at least one day. This minimizes the amount of bubble formation in
the tube when drawing a sample.
6.2 Spectrophotometric cell.
The spectrophotometric cells should be made of optical glass with a 10 cm pathlength, two ports and
stoppers. A sufficient number of cells are needed to collect all the samples that will be analysed from a
particular cast.
NOTE A flow through cuvette with a 10 cm pathlength is also available. Measurements with this cuvette require
sample bottles of at least 200 ml equipped with airtight caps.

6.3 Micropipette.
A micropipette is used to add the indicator to the cell. It should be of approximately 0,1 cm capacity with a
narrow tube attached to act as a nozzle.
6.4 High-quality spectrophotometer.
For work of the highest sensitivity and precision, a double-beam spectrophotometer is desirable. However,
good results can be obtained with a high-quality single-beam spectrophotometer.
6.5 Temperature-control system for spectrophotometer cell.
Commercially manufactured, thermostated spectrophotometer compartments that can accommodate 10 cm
cells are rarely available and one will probably have to be custom-made. Equipment needs to be maintained
in a closed room with controlled temperature, and all the materials, solutions and samples need to be
maintained in the same room for at least 2-h prior to start the test. The temperature should be regulated to
within 0,1 °C.
6.6 System to warm samples to measurement temperature.
It is possible to warm up the cells containing samples in sealed bags in a thermostat bath. However, it is
convenient to use a custom-made thermostated compartment that can hold approximately 12 cells at once
without getting them wet.
6.7 Thermostat bath.
A thermostat bath is used to control the temperature of both the cell compartment and the system to
±0,05 °C.
7 Sampling
Seawater should be collected from the sampler bottle immediately after opening, and before a significant
amount of water is removed, to ensure representative sampling. This is necessary to minimize exchange
of CO with the air space in the sampler which affects all carbon parameters except total alkalinity. It is
desirable that the carbon samples be collected before the sampler bottle is half empty and within 10 min of
it being first opened.
3 3
Rinse the sample bottle — If the bottle is not already clean, rinse it twice with 30 cm to 50 cm of sample to
remove any traces of a previous sample.
Fill the sample bottle — Fill the bottle smoothly from the bottom using a drawing tube which extends from
the sampler bottle drain to the bottom of the sample bottle. It is critical to remove any bubbles from the
draw tube before filling. Overflow the water by at least a full bottle volume. The air space within the sample
bottle is kept to a minimum, see References [9] and [10]. It is allowed to draw the sample directly from the
sampler bottle into the optical cell with two ports.
It is recommended that the pH analysis is performed immediately after sampling, although storage
experiments showed that the pH of seawater is stable up to 24 h even in the case of coastal water
(see Annex B). However, while awaiting analysis, store the samples in the refrigerator or icebox (not frozen).
The stability has not been assessed in case of important primary productivity (e.g. during a plankton bloom),
in this case, it is recommended to poison the seawater.
8 Procedure
In the case of optical cell with two ports, warm the sample cell to 25,0 °C ± 0,1 °C by placing a number of cells
in a thermostated compartment (see 6.6) for at least 1 h. Clean and dry the exterior of the cell before placing
it in the thermostated sample compartment of the spectrophotometer.

Measure and record the absorbances at three wavelengths: at the wavelengths corresponding to the
2‑ ‑
absorption maxima of the base (I ) and acid (HI ) forms of the m-cresol purple, 578 nm and 434 nm,
respectively, and a non-absorbing wavelength (750 nm).
NOTE 750 nm is chosen as a non-absorbing wavelength because it lies in a flat absorbance region, making it
relatively insensitive to wavelength accuracy. This allows for the correction of any baseline shifts that can occur
during the measurement process.
3 3
Remove one of the cell caps to inject m-cresol purple: add approximately 0,05 cm to 0,1 cm of m‑cresol
purple to the sample. Replace the cap and shake the cell to mix the seawater and m-cresol purple. Absorbance
values between 0,4 and 1,0 at each of the two absorbance peaks should be obtained by adding the appropriate
amount of m-cresol purple.
Return the cell to the spectrophotometer, again measure and record the absorbances at three wavelengths
of seawater after adding m-cresol purple.
Cells should be positioned to maintain consistent alignment(s) between two absorbance measurements.
In the case of a flow-through cuvette, the pH sample bottles are warmed to 25,0 °C ± 0,1 °C. The reference
absorbances at three wavelengths are measured and recorded as described above. m-cresol purple is then
added to the seawater, the sample is gently mixed, and the absorbances at three wavelengths are measured
and recorded immediately, ensuring that the absorbance values fall between 0,4 and 1,0 a
...