LaMotte Dissolved Oxygen Water Quality User Manual

Dissolved

Oxygen

Water Quality Test Kit

Instruction Manual Code 5860-01

Introduction ............................................................................ 3

Dissolved Oxygen General Safety Precautions Use Proper Analytical Techniques Kit Contents Test Procedure

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Part 1: Collecting a Water Sample ........................................... 11

Part 2: Adding the Reagents ..................................................... 12

Part 3: Titration ........................................................................ 13

Percent Saturation .................................................................. 15

BOD

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EPA Compliance

Short Form Instructions

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......................................................... back

WARNING! This set contains chemicals

that may be harmful if misued. Read

cautions on individual containers

carefully. Not to be used by children

except under adult supervision.

INTRODUCTION

Aquatic animals need dissolved oxygen to live. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration. Oxygen dissolves readily into water from the atmosphere until the water is saturated. Once dissolved in the water, the oxygen diffuses very slowly and distribution depends on the movement of the aerated water. Oxygen is also produced by aquatic plants, algae, and phytoplankton as a by-product of photosynthesis.

This test kit uses the azide modiﬁ cation of the Winkler method for determining dissolved oxygen.

DISSOLVED OXYGEN, PERCENT SATURATION & BOD

Oxygen is critical to the survival of aquatic plants and animals, and a shortage of dissolved species are more sensitive to oxygen depletion than others, but some general guidelines to consider when analyzing test results are:

Because of its importance to the ﬁ sh’s survival, aquaculturists, or “ﬁ sh farmers,” and aquarists use the dissolved oxygen test as a primary indicator of their system’s ability to support healthy ﬁ sh.

WHERE DOES THE OXYGEN COME FROM?

The oxygen found in water comes from many sources, but the largest source is oxygen absorbed from the atmosphere. Wave action and splashing allows more oxygen to be absorbed into the water. A second major source of oxygen is aquatic plants, including algae; during photosynthesis plants remove carbon dioxide from the water and replace it with oxygen.

Absorption

Oxygen is continuously moving between the water and surrounding air. The direction and speed of this movement is dependent upon the amount of contact between the air and water. A tumbling mountain stream or windswept, wave-covered lake, where more of the water’s surface is exposed to the air, will absorb more oxygen from the atmosphere than a calm, smooth body of water. This is the idea behind aerators: by creating bubbles and waves the surface area is increased and more oxygen can enter the water.

oxygen is not only a sign of pollution, it is harmful to ﬁ sh. Some aquatic

5–6 ppm Sufﬁ cient for most species

<3 ppm Stressful to most aquatic species <2 ppm Fatal to most species

Photosynthesis

In the leaves of plants, one of the most important chemical processes on Earth is constantly occurring: photosynthesis. During daylight, plants constantly take carbon dioxide from the air, and in the presence of water convert it to oxygen and carbohydrates, which are used to produce additional plant material. Since photosynthesis requires light, plants the photosynthesis

do not photosynthesize at night, so no oxygen is produced. Chemically,

reaction can be written as:

Light + nCO2 + nH2O (C2HO)n + nO

Light + Carbon + Water Carbohydrate + Oxygen Dioxide

WHERE DOES THE OXYGEN GO?

Once in the water, oxygen is used by the aquatic life. Fish and other aquatic animals need oxygen to breathe or respire. Oxygen is also consumed by bacteria to decay, or decompose, dead plants and animals.

Respiration

All animals, whether on land or underwater, need oxygen to respire, grow and survive. Plants and animals respire throughout the night and day, consuming oxygen and producing carbon dioxide, which is then used by plants during photosynthesis.

Decomposition

All plant and animal waste eventually decomposes, whether it is from living animals or dead plants and animals. In the decomposition process, bacteria use oxygen to oxidize, or chemically alter, the material to break it down to its component parts. Some aquatic systems may undergo extreme amounts of oxidation, leaving no oxygen for the living organisms, which eventually leave or suffocate.

PERCENT SATURATION

The oxygen level of a water system is not only dependant on production and consumption. The potential dissolved oxygen capacity of water is limited by atmospheric pressure (altitude), salinity, and temperature. These factors determine the highest DO level possible. The percent saturation value expresses the quantity of dissolved oxygen in the sample as a percent of the theoretical potential.

When water holds all of the dissolved oxygen that it can hold at a given altitude, temperature, and salinity, it is said to be 100% saturated. If it holds a quarter as much as it could possibly hold under those conditions it is 25% saturated. It is possible to get percent saturation values over 100% when water becomes highly aerated by tumbling over rapids and dams. It can also become supersaturated on a sunny day when dense areas of plants or algae produce oxygen through photosynthesis.

Low atmospheric pressure found at higher altitudes slightly decreases the solubility of oxygen in water so the dissolved oxygen value must be corrected for altitude.

The various minerals dissolved in water lower the capacity of the water to hold oxygen. A correction factor can also be applied to dissolved oxygen measurements in saline waters. In fresh water, where the salinity is very low, this effect is insigniﬁ cant when compared to the effect of temperature. Therefore, a correction for salinity is not incorporated into the calculation.

Cold water can hold more oxygen than warm water. That is why ﬁ sh that require higher levels of oxygen, like trout, are found in cold water and dissolved oxygen concentrations are usually higher in the winter than they are in the summer at the same location. The percent saturation concentration can be corrected for water temperature.

Percent saturation levels from 80 to 120 percent are considered to be excellent. Levels between 60 and 79 percent are adequate. Above 125 percent and below 60 percent saturation, levels are poor. Fish and invertebrates that can move will leave areas with low dissolved oxygen and move to areas with higher levels. Slow moving, trapped or non-mobile aquatic animals may perish if levels become too low. Extremely high dissolved oxygen concentrations are harmful to ﬁ sh even for very short periods of time. Gas bubble disease, which is characterized by the rupturing of capillaries in the gills due to supersaturated water, is usually fatal.

MEASURING BOD (BIOCHEMICAL OXYGEN DEMAND)

Biochemical oxygen demand is determined by measuring the dissolved oxygen concentration in a freshly collected water sample and comparing it to the dissolved oxygen level in a sample that was collected at the same time but incubated under speciﬁ c conditions for a speciﬁ c length of time. The difference between the two oxygen levels represents the amount of oxygen required for the decomposition of organic material and the oxidation of chemicals in the water during the storage period, a measurement known as the BOD.

Unpolluted, natural waters will have a BOD of 5 ppm or less. Raw sewage may have levels of 150 to 300 ppm. Wastewater treatment plants must reduce BOD to levels speciﬁ ed in their discharge permits, usually between 8 and 150 ppm BOD.

TESTING DISSOLVED OXYGEN

The ﬁ rst step in a DO titration is the addition of Manganous Sulfate Solution (4167) and Alkaline Potassium Iodide Azide Solution (7166). These reagents react to form a white precipitate, or ﬂ oc, of manganous hydroxide, Mn(OH)2. Chemically, this reaction can be written as:

MnSO4 + 2KOH Mn(OH)2 + K2SO

Manganous + Potassium Manganous + Potassium Sulfate Hydroxide Hydroxide Sulfate

Immediately upon formation of the precipitate, the oxygen in the water oxidizes an equivalent amount of the manganous hydroxide to brown-colored manganic hydroxide. For every molecule of oxygen in the water, four molecules of manganous hydroxide are converted to manganic hydroxide. Chemically, this reaction can be written as:

4Mn(OH)2 + O2 + 2H2O 4Mn(OH)

Manganous + Oxygen + Water Manganic Hydroxide Hydroxide

After the brown precipitate is formed, a strong acid, such as Sulfamic Acid Powder (6286) or Sulfuric Acid, 1:1 (6141) is added to the sample. The acid converts the manganic hydroxide to manganic sulfate. At this point the sample is considered “ﬁ xed” and concern for additional oxygen being introduced into the sample is reduced. Chemically, this reaction can be written as:

2Mn(OH)3 + 3H2SO4 Mn2(SO4)3 + 6H2O Manganic + Sulfuric Manganic + Water

Hydroxide Acid Sulfate

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