13 DO meter

Dr. Heena Rekhi Rekhi

  1. Description

 

Dissolved oxygen pertains to the degree of free, non-compound oxygen present in water or other liquids. It is a crucial parameter in evaluating water quality due to its influence on the organisms living within a body of water. Dissolved oxygen plays a crucial role in deciding the aquatic fauna, as it varies with the depth of the lake. A variation in the dissolved oxygen level i.e., either too high or too low, can be harmful to aquatic life.

When oxygen is not bonded to any other element it is usually known as free oxygen (O2) or non-compound oxygen. This free O2 molecules within water is known as the dissolved oxygen. The bonded oxygen in molecular water (H2O) is termed compound oxygen, does not count towards dissolved oxygen levels. Thus, the free O2 molecules dissolve as does the sugar or salt dissolve in water and are being held to water by week intermolecular forces. These forces are so weak that even slight variation in physicochemical conditions namely, pH, conductivity, temperature etc. of water leads to the variation of DO level in water.

 

Dissolved oxygen is essential to a all forms of aquatic life including fish, plants, invertebrates, and bacteria. These organisms utilize the dissolved oxygen in respiration, alike to organisms on land. For respiration fishes and crustaceans acquire oxygen with the help of their respiratory organs known as gills, and in case of phytoplankton’s they take dissolved oxygen (under dark conditions) for respiration.

The requirement of dissolved oxygen varies from species to species based on their metabolic processes. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen in the form of DO (1-6 mg/L), while shallow water fish need higher DO levels (4-15 mg/L). These organisms use DO to break down organic material at the bed of a body of water. Microbial decomposition is a significant contributor to nutrient recycling. The oxygen at lower water levels in a water body get used up faster if there is a surplus of decomposing organic materials from dying organisms.

 

Where Does DO Come From?

 

There are two ways by which dissolved oxygen paves way in water one is by the air and other as a plant by-product. A slow dissemination of oxygen occurs from the air to the surface of water from the adjacent atmosphere or it can mix quickly via aeration by natural or non-natural processes. The aeration of water can be caused by waterfalls, wind, ground water discharge or other forms of running water. The causes of aeration from synthetic mode or man-made fluctuate from an aquarium air pump to a hand-turned waterwheel. In additional as a waste product of photosynthesis from phytoplankton, algae, seaweed and other aquatic plants significantly contributes to dissolved oxygen. The shallow water plants and algae at the surface of a water body mostly contribute to the photosynthesis still seaweed, sub-surface algae and phytoplankton in underwater gives a prominent contribution to it.

 

Presence of light scattering elements in the water, the penetrating light varies through the depth. Depth also affects the wavelengths accessible to plants, with red being absorbed rapidly and blue light being visible past 100 m. In clear water, there is no longer abundant light for photosynthesis to happen past

 

200 m, and aquatic plants no longer grow. In turbid water, this photic (light-penetrating) zone is considerably shallower.Irrespective of wavelengths accessible, the cycle doesn’t get altered. In addition to the required light,CO2 is promptly absorbed by water (it’s about 200 times more soluble than oxygen) and the oxygen produced as a byproduct remains dissolved in water. The basic reaction of aquatic photosynthesis remains:CO2 + H2O → (CH2O) + O2

 

As aquatic photosynthesis is light-dependent, the dissolved oxygen produced will peak during daylight hours and wane at night.

 

Measurement of dissolved oxygen

 

The amount of dissolved oxygen in a liquid is measured with the help of a dissolved oxygen (DO) meters. Oxygen marks its way into water through a diverse processes, comprising aeration, as a byproduct of photosynthesis, and from surrounding air. A healthy aquatic system maintaining an aerobic life must cover a specific amount of oxygen; e.g., 5 mg/L of dissolved oxygen is an indication of a healthy water body. DO levels less than 5 mg/L can result in strained aquatic organisms, and levels which endure as low as 1 mg/L for even a few hours can result in prevalent fish kills.

 

Conversely, dissolved oxygen saturation can also be detrimental to aquatic life. The information discussed above, it becomes clear that dissolved oxygen measurement is particularlyvital when assessing the health and sustainability of aquatic ecosystems.

 

Dissolved oxygen levels are hardly fixed and vary with changes in season, temperature, and time of day. Even inside a single body of water, DO levels fluctuate within a vertical water column (in lakes or large rivers) or horizontally within the waterway (in smaller rivers and streams).Time of day and temperature in specific have a profound outcome on dissolved oxygen levels, partially because cold water tends to have additional dissolved oxygen. For example, a measurement taken at sunrise, when water temperatures are lowermost, may possibly be more than one full mg/L higher than a measurement taken at mid-day, when water temperatures are warmest. This is the only reason that the investigators regularly take hourly DO readings over the course of a 24-hour period and create a dissolved oxygen profile of a lake or river, rather than relying on a single measurement.

 

DO changes due to temperature alteration are slightly tempered by changes to solar activity, which affects dissolved oxygen levels in a reverse fashion. As shown in the image above, improved levels of photosynthesis result in higher amounts of dissolved oxygen. During daylight hours, the photosynthesis upsurges with sufficient quantity of sunlight, hence results in greater DO levels and thus becomes independent of water temperatures. At night when photosynthesis activity declines and decomposition continues DO levels tend to fall independently of temperature factors. Considering the many variables listed above, DO levels are undoubtedly determined by complex interactions of environmental and biological conditions within a body of water.

 

The graph describes a typical DO profile comprising of several days’ worth of measurements. Kept this thing in mind that dissolved oxygen is more in cold waters, we can easily deduce that these readings probably began around dawn on a cool day. Water temperatures steadily increased from mid-day on day 3 (hour 70) and we can also determine that at ease, as all DO measurements were lower than the previous days. 

 

 Devices used for the measurement purpose:

 

A DO probe is coupled to a meter /analyzer of DO meter which is analogous in making to pH meters.In potassium chloride (KCl) electrolyte solution the two electrodes of the probe are suspended, and they are bounded with glass and/or a semipermeable membrane. These electrodes are linked to the meter, which delivers a small DC current to the electrodes via wiring. A measurable current change occurs when the sensor is submerged within a liquid, this is all due to the oxygen from the liquid which crosses the membrane and reacts with the cathode and finally the meter displayed this change as a millivolt output.

 

Dissolved oxygen meters typically measure more than DO similar to the pH meters. Multifunction meters may measure different liquid parameters such as pH, conductivity, oxygen reduction potential (ORP), temperature, etc. The methodologies for the maintenance such as ensuring proper levels of electrolyte solution, eliminating membrane leakage, and routine calibration of a DO

 

probe share similar lines to that of a pH probe. So, these remain the key factors in the cleaning and maintaining accurate meter output.

 

DO meters are generally portable, handheld devices as DO levels vary quickly when the water source is detached from it and this is one of the reasons for taking repeated field measurements. The image below shows a typical handheld dissolved oxygen meter.Milligrams per liter (mg/L) or percent saturation are the units in which DO is measured and expressed. The unit of Milligrams per liter (mg/L) is defined as the number of milligrams of oxygen within a liter of water and the amount of oxygen in a liter of water relative to the maximum amount of oxygen the water can hold at an identical temperature is known as percent saturation.

 

Applications

 

Aeration, diffusion, respiration, photosynthesis and decomposition are often affecting the DO concentrations. DO levels will also vary with temperature, pressure changes and salinity even after water equilibrates towards 100% air saturation. For pressure measurements by water level sensors a data logging system can be connected with an external barometer even though many DO meters contain an internal barometer. The interaction between oxygen and certain luminescent material is measured by optical dissolved oxygen sensors.

 

 

Furthermore, membrane electrode measures the diffusion current produced by concentration of dissolved oxygen to find its exact concentration. When sensor is inserted an air layer forms over the membrane. The oxygen partial pressure in air is in equilibrium with the concentration of dissolved oxygen in water. There are two types of membrane electrode methods are available galvanic and polarographic method. These two methods differ only in presence or absence of external voltage and have the same features, usage and performance.

 

Calibration of dissolved oxygen analyzer: It is performed in following situations

 

There are number of tools which are used for the process of calibration air calibration, span calibration and zero calibration in sodium sulfite. The most suitable method and easiest way to calibrate the analyzing system in ambient air is air calibration. Span calibration is needed if want to do it more accurately. Zero calibration in sodium sulfite needs much to perform and generally not required

 

Bibliography

  • Galen W. Ewing, Instrumental Methods of Chemical Analysis, 1985, McGraw-Hill Publishing. R.W. Murray, Analytical Chemistry is What Analytical Chemists Do, Editorial, Anal. Chem.,66 (1994) 682A
  • https://instrumentationtools.com/dissolved-oxygen-analyzer-working-principle/#.WmYnYK6Wafg
  • S.M. Khopkar, Basic Concepts of Analytical Chemistry, New Age International Publishers, New Delhi, 2004.
  • C. Dash, Analytical Chemistry, 2017, 2nd Edition, PHI Learning Pvt Ltd., Delhi.