Dissolved Oxygen Measurement Techniques and Aplications for the Water Quality of the Passaic River

Dissolved Oxygen Measurement

Techniques and Applications for the

Water Quality Measurements of the

Passaic River

What is dissolved Oxygen?

It is the amount of oxygen gas dissolved in water

Water has a molecule of oxygen in its composition, but that is not the oxygen that we are talking about

Think of it as dissolving a spoonful of sugar in a glass of water

How does water gets its dissolved oxygen? The oxygen dissolved in the water gets in there by

contact with the atmosphere, and sometimes through underground water fountains depending on the body of water in question.

Water in constant movement gets more oxygen on it than still water.

Dissolved Oxygen Factors

Temperature: Cold water holds more oxygen than hot water

Salinity levels: Fresh water holds more DO than salt water

Why is this so important?

From keeping your pet fish alive, to keeping healthy ecosystems.

It is important what factors are messing with the dissolved oxygen levels because if they go dangerously low, it will both kill the fish in your tank, or destroy a whole ecosystem in nature.

Examples

Runoff of salt from roadsRunoff of fertilizer from gardes causing

eutrophication in lakesFactories treating waste water and

returning it to a body of water warmer than it should be

Methods of Measuring DO

Electrochemical devisesElectrochemical tests

Clarck electrochemical probesOptical based sensing methods

Chemical testsTitrimetric testsColorimetric tests

Electrochemical Probes

Clark Electrochemical probes: They are also known as membrane covered probes Are called this way in honor to their inventor Doctor

Leeland Clark They worked by using the membrane that is ony permeable

to gasses. When there is Oxygen present, it will diffuse trough the membrane at the cathode, the oxygen will get reduced and this will send an electrical signal that will be analyzed by the computer in the device.

Clark Electrochemical Probes

Sources of Error:Calibrating the probes improperly.Improperly installed electrodeWeak electrolyte inside the

membrane

Titrimetric Tests

They consist on having the dissolved oxygen react, and then titrate with a solution that will react with the fixed oxygen. The amount of oxygen in the sample will correspond to the amount of the titration solution added to the sample until the endpoint is reached.

Titrimetric Tests

Sources of error:Human error: corresponding to the level of skill of the person performing the test.Oxygen introduced to the sample at the moment of sample retrievalThe titration solution not being in optimal conditions

Colorimetric Tests These are also chemical tests. The procedure

will cause a change in color in the sample. The concentration of DO will correspond on how dark the color obtained is, which will be compared to a chart.

Colorimetric Tests

Sources of error:Human error: Low analytical skills,

bias while reading the sample colorIntroduction of oxygen while

obtaining the sample: The sample vial is never capped.

Tests Evaluated

Titrimetric: HACH DO Kit LaMotte DO Kit

Colorimetric CHEMetrics DO kit Profi O2 test

Clark Electrochemical Probes GSWA probe CSE probe

Set up

The first set up used in lab consisted in a 3500 mL beaker filled with tap water at room temperature. It was then placed over a stirring plate at velocity seven. A funnel connected to a lab air pump was used in order to introduced air to the sample and maintain a steady level of it.This was changed in order to make the experiment more replicable by replacing the lab pump with an aquarium air pump that deliver air at the same rate at all times.

With Lab Pump and aquarium pump

GSWA probe vs CSE probe

0 2 4 6 8 10 12 14 166

6.5

7

7.5

8

8.5

9

DO (ppm) vs time (mins)GSWA (ppm) CSE (ppm)

GSWA probe vs CSE probe The values were compared over a period of 25

minutes. As we can observe in the graph, the water has

around the maximum amount of dissolved oxygen that can be achieved at this temperature, which is around 8 ppm of oxygen. The difference over 15 minutes was measured in order to make sure that the levels of oxygen in the water were not fluctuating too much and that is what was found.

The average difference was of only 6.11% This may have been because of the errors while

calibrating.

Test Comparison at Room Temperature

La Motte GSWA meter CSE meter Chem ETS

8

8.958.51

88

8.868.58

87.9

8.858.49

88

8.888.53

8

Test comparison at 20 C

Trial 1 Trial 2 Trial 3 Average

Test Comparison at Room Temperature The difference between the GSWA meter and the LaMotte

testing kit was of 5.21%, which is still a small difference. This sort of difference was expected with titrimetric methods. One source for error could have been not closing the sample bottles tightly immediately after retrieving the water sample

CHEMetrics kit. Since this is a color comparison test, it was more vulnerable to human bias. Not only was this a problem, but also the solution that was indicating the color for a sample containing 8ppm of oxygen was lower than the rest, meaning that the dye would be more concentrated. This means that it was even less accurate, so the final color was compared to the color that indicated 6 ppm and the one that indicated 12 ppm of oxygen. Since it was darker than 6 ppm and lighter than 12 ppm, and the other tests had indicated 8 ppm, we simply assume that was the color for 8 ppm

Dissolved Oxygen at low Temperature

La Motte GSWA meter CSE meter Profi O2

10.4

12.3311.75 12

10

12.9 12.6612

10

13.0112 12

10.1

12.7412.13 12

Dissolved Oxygen at 7 C


Dissolved Oxygen at Low Temperatures As it was to be expected, the lower the temperature, the higher the

concentration of DO in the water sample. However, for some reason the LaMotte kit had a much bigger

difference from the electrochemical meters than in past experiments. The difference between the LaMotte and the GSWA meter was of 23%

The same sodium hydrosulfide solution was used to calibrate the samples so that may explain why, in the case an error was made during calibration, the two meters would be skewed similarly.

The Profi O2 test gave an approximate value.

Dissolved Oxygen at High Temperatures


5.2 5.255.85 6

55.79

5.186

55.62

5.186

5.15.55 5.4

6

Test Comparison at 40 C


It was noticed that at higher temperatures than 40oC the CSE meter would not function properly and it would display error, so in order to keep the equipment from getting harmed, no higher than 40o C was reached for this test

Do not use to measure DO in hot springs. With the chemical experiments, it was decided to let

the sample sit at room temperature after it was collected in the closed bottle because all chemical reactions are faster at higher temperatures

Dissolved Oxygen at High Temperatures

Dissolved Oxygen at Different Temperatures


5.1 5.55 5.4 6

10.1

12.74 12.13 12

DO at Cold vs Warm Water

40 C 20 C

The difference can be seen clearly. In this experiment, by looking at the most trust worthy instrument which is the GSWA, there was a difference of 78.6 %. This is one of the reasons that the average temperature, as well as dissolved oxygen, is such an important factor when looking at the health of a body of water

Dissolved Oxygen at Different Temperatures

Testing in the Passaic River

Only four of the prior described procedures to measure dissolved oxygen were employed in the field testing of the Passaic River. In order to get an accurate result, we decided to take three readings for the Profi O2 and the LaMotte Kits

There were five sites: Newark (site#1), Lyndhurst (site #2), Great Falls (site#3), Chattam (site#4), Mendham (site#5)


Site 1 Site 2 Site 3 Site 4 Site 5

11 11.1 11.29.6 10.2

1312 12 12 1211.7

14.7 14.0412.3 12.87

14.44 14.39 14.2

12.2413.42

Chart Title

LaMotte Profi O2 GSWA CSE


The water for all of the sites had an average temperature of 3.18oC. Since it was so cold, it was expected to see high levels of oxygen, as the results indicate

The lowest amount of oxygen was found in site 4, which was Chattam. The temperature of the water in Chattam was not very different from the rest of the sites, so temperature could not be the reason there is less oxygen

The site of testing was close to a road. Since the field sampling was done in March, and there was still some recent snow, it could be that the salinity of the water was slightly higher than in the other sites


Biochemical Oxygen Demand

Biochemical oxygen demand, is the measurement of how much oxygen is consumed by microorganisms in the water sample, in a determined amount of time.

This is very important to know because if the biochemical oxygen demand is too high, then water dissolved oxygen levels would get dangerously low in a phenomenon called eutrophication. Eutrophication happens when there are too many nutrients in the water, which causes an algae bloom.

BOD in the Passaic River

site 1 site 2 site 3 site 4 site 5

4.74.2

2.5

3.16

BOD

BOD in the Passaic River While testing for the biochemical oxygen demand

for these sites, a couple of issues were found. First, although we had sterile bottles, the standard LaMotte test that was used was designed for samples of exactly 65 mL and not 265 mL. Site 2 was done a day before, and the data was completely ruined, so it was not included in the chart. In order to test for this, a proportion relationship was calculated, so if for the 65mL bottles 8 drops were needed then for the 265 mL 32.6 drops were needed. This we have to round to 33 drops for each of the reagents. The step of the titration was not changed


Another problem with this data is that it was taken seven days after the initial reading, instead of five days which is the necessary to test for the biochemical oxygen demand.


In conclusion, this biochemical oxygen demand data is not reliable and should be performed again correcting the mistakes made. First, only 65 mL bottles should be used for the water sample if the LaMotte kit is to be used. These bottles should be autoclaved to assure sterility. Second, the test should be measured after five days, no more and no less so that reliable results can be acquired.

About all the Methods

In conclusion, the most reliable, accurate and easier to use methods are the electrochemical probes. The chemical methods take a minimum of 20 minutes to perform, while the electronic probes will give the readings instantly. Of course, extra care should be taken while calibrating the probes. This accuracy and technology is the reason these probes are so expensive


The titrimetric methods would be next. They are pretty accurate as we were able to see with the LaMotte kit, but require more time and more care because toxic and corrosive reagents are being used. These can be purchased for less than a hundred dollars


Next are the low budget aquarium test kits like the Profi O2 test. Tests like these will do the job of approximately displaying the amount of dissolved oxygen, but it will not be very accurate. These are really meant for people who want to make sure that a lack of oxygen is not killing their fish in their ponds or aquariums

If there was more time, the BOD of each of the sites would be repeated.

dissolved oxygen at different salinity concentrations since this is also an interesting factor that affects DO.

BOD with different cultures of organisms.

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Dissolved Oxygen Measurement Techniques and Aplications for the Water Quality of the Passaic River