GB/T 11913-1989 English PDF (GBT11913-1989)

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GB/T 11913-1989: Water quality--Determination of dissolved oxygen--Electrochemical probe method
GB 11913-1989
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
Water Quality - Determination of
Dissolved Oxygen - Electrochemical Probe Method
APPROVED ON: DECEMBER 25, 1989
IMPLEMENTED ON: JULY 01, 1990
Approved by: State Bureau of Environmental Protection
Table of Contents
1 Theme Content and Applicable Scope ... 3
2 Principle ... 4
3 Reagents ... 4
4 Instruments ... 4
5 Procedures ... 5
6 Presentation of Results ... 7
7 Test Report ... 8
Appendix A (Addition)... 9
Water Quality - Determination of
Dissolved Oxygen - Electrochemical Probe Method
This Standard equivalently adopts the international standard ISO 5814-1984 Water
Quality – Determination of Dissolved Oxygen – Electrochemical Probe Method.
1 Theme Content and Applicable Scope
1.1 Theme content
This Standard specifies a method for measuring dissolved oxygen in water by an
electrode that separates the water sample from the electrochemical cell through a gas-
permeable membrane.
According to the different types of the used probes, the concentration of oxygen (mg/L),
or the percentage of oxygen saturation (% dissolved oxygen), or both may be
measured. This method may measure dissolved oxygen with a saturation percentage
from 0% to 100% in water. However, most instruments can measure supersaturation
values higher than 100%. This method may be used not only for determination in the
laboratory, but also for on-site determination and continuous monitoring of dissolved
oxygen. This method is suitable for the determination of water with high chromaticity
and turbidity. It is also suitable for the determination of water containing iron and
substances that can interact with iodine. All the above substances shall interfere with
the determination by iodometry. Some gases and vapors like chlorine, sulfur dioxide,
hydrogen sulfide, amine, ammonia, carbon dioxide, bromine and iodine can diffuse
and pass through the membrane. If these substances are present, they shall affect the
measured current and cause interference. The presence of other substances in the
sample shall cause the membrane blockage, membrane damage or electrode
corrosion, and further lead to interfere with the measured current. These substances
include solvents, oils, sulfides, carbonates and algae.
1.2 Scope of application
This method is applicable to natural water, sewage and salt water. If it is used to
measure salt water such as sea water or harbor water, the salt content shall be
checked.
4.1.2 Meter, the scale directly displays the concentration of dissolved oxygen, and (or)
the saturation percentage of oxygen or the current microampere.
4.2 Thermometer with a scale division of 0.5°C.
4.3 Barometer with a scale division of 10Pa.
5 Procedures
When using the measuring instrument, the instruction manual of the manufacture shall
be followed.
5.1 Measurement technology and precautions
5.1.1 Do not touch the active surface of the membrane with hands.
5.1.2 After replacing the electrolyte and membrane, or when the membrane is dry,
make the membrane wet. Calibration (see 5.2) can only be performed after the reading
is stable. The required time depends on the time required for the consumption of
dissolved oxygen in the electrolyte.
5.1.3 When the probe is immersed in the sample, it shall be ensured that no air bubbles
are trapped on the membrane.
5.1.4 When the sample is in contact with the membrane of the probe, a certain flow
rate shall be maintained to prevent the dissolved oxygen in the sample at that part from
being exhausted at the moment of contact with the membrane, and generating the
false readings. It shall be ensured that the flow rate of the sample does not cause the
reading to fluctuate. In this regard, refer to the instructions of the instrument
manufacturer.
5.1.5 For dispersed samples, the measuring container shall be able to be sealed and
isolate air; and be equipped with a stirrer (such as an electromagnetic stirrer). Fill the
container with the sample to overflow; seal it and then measure. Adjust the stirring
speed to keep the reading stable after reaching equilibrium, and must not to entrain air.
For flowing samples, such as river channels, it is necessary to check whether sufficient
flow velocity can be ensured. If it is insufficient, move the probe back and forth in the
water sample; or take out a dispersed sample and measure it according to the method
described in the previous paragraph.
5.2 Calibration
The calibration procedures are described in 5.2.1 to 5.2.3; but the instruction manual
of the instrument manufacture must be referred.
5.2.1 Adjustment
Adjust the electrical zero point of the instrument. Some instruments have
compensation zero point, so there is no need to adjust.
5.2.2 Check zero point
When checking the zero point (need to adjust the zero point, when necessary), the
probe can be immersed in each liter of distilled water adding 1g of sodium sulfite (3.1)
and about 1mg of cobalt salt (II) (3.2).
A stable reading shall be obtained within 10min.
NOTE: The new instrument only takes 2~3min.
5.2.3 Calibration close to saturation
At a certain temperature, aerate into the water to make the oxygen content in the water
saturated or close to saturation. Keep at this temperature for 15min and then determine
the concentration of dissolved oxygen; for example, by using iodometry.
5.2.4 Adjust the instrument
The probe is immersed in the bottle; and the bottle is completely filled with the sample
prepared and calibrated according to the above procedures. Allow the probe to
stabilize in the stirred solution for 10min (see 5.2.2 NOTE). If necessary, adjust the
instrument reading to the known oxygen concentration of the sample.
When the instrument can no longer be calibrated, or the response of the instrument
becomes unstable or low (see the manufacturer's instruction manual), the electrolyte
or (and) membrane shall be replaced.
NOTE: ① If the aeration time and air flow rate required for air-saturated samples have been
given in the past experience, Table A1 and Table A3 may be referred to replace
iodometry determination.
② Many instruments may be calibrated in air.
5.3 Determination
Determine the water to be tested in accordance with the manufacturer's instruction
manual.
After the probe is immersed in the sample, allow the probe to stay for enough time to
make the probe and the water temperature to be measured consistent and to stabilize
the reading. Due to different instrument models and different requirements for results,
the water temperature and atmospheric pressure shall be checked if necessary.
6.3 Corrected dissolved oxygen concentration of brine samples
The solubility of oxygen in water decreases with the increase of salt content. In
practical applications, when the salt content (expressed as total salt) is below 35g/L, it
may be reasonably considered that the above relationship is linear. Table 1 shows the
salt content per 1g/L minus the correction value during correction; that is ΔCs.
Therefore, when the salt content in water is ng/L, the solubility of oxygen in water is
equal to the corresponding solubility in pure water minus nΔCs.
6.4 Dissolved concentration expressed as saturation percentage
This is the actual dissolved oxygen concentration expressed in mg/L, which needs to
be temperature corrected if necessary; and divided by the theoretical value given in
Tables A1 and A3 and obtain a percentage:
7 ...