Testing solutions for dissolved oxygen.

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Dissolved oxygen.

Testing methods for dissolved oxygen are of 3 types.

Chemical test kit. An example is the HI3810 Dissolved Oxygen test kit. See it at HI3810 kit. This kit has 5 chemical components and uses a modified Winkler method. The method is simple because the chemicals are pre-prepared and easy to dispense. The last step is a titration. The reagent is added with a graduated syringe until a desired colour change occurs. The value is read off and a simple calculation gives dissolved oxygen in ppm. Accuracy is 0.1 ppm between 0 and 10 ppm. The initial kit includes reagents for 110 tests and a replacement reagent kit can be purchased.

Photometer. A convenient solution is to combine chemical methods with an instrument which will detect the colour in the final solution. An example of this type is the HI93732N Disolved oxygen photometer. See details of this meter at HI93732N DO photometer. The HI93732N measures dissolved oxygen from 0.0 to 10.0 ppm. The reagent set is purchased separately. The kit uses 3 reagents and is based on a modified Winkler method. Using this meter is straightforward. Place a sample of the original water in the meter to obtain a zero reference point. Then follow the instructions to prepare a reacted sample. Fill the cuvette with from reacted sample, replace it in the photometer and read off the oxygen level. Accuracy is +/- 0.2 ppm. The meter gives the actual dissolved oxygen value. This meter is portable so can be used away from the lab.

Dissolved oxygen meter. This type of meter measures dissolved oxygen directly in water with an oxygen sensitive probe. An example of this is the HI9142. See it at HI9142 Dissolved Oxygen meter. These meters allow multiple rapid tests without the need to handle chemicals. The HI9142 is a waterproof meter designed for field use. The included probe with 4 m cable allows testing at different depths. This meters tests in the range 0.0 to 19.9 ppm. Only one calibration solution (0 ppm) is required and it is only required when calibrating a new probe. Slope adjustment calibration is required more frequently and is performed by adjusting a trimmer while the probe is in saturated air. The probe may need to be topped up periodically with filling solution (supplied) and sometimes the very delicate membrane at the bottom of the probe needs to be replaced so there is a small maintenance cost. These meters can be taken anywhere.

When the probe of a dissolved oxygen meter takes a measurement that measurement is influenced by the temperature of the water as well as the salinity of the water. Most meters internally calculate a correction for temperature and display the correct DO reading. However not all meters apply a correction for salinity. For example the HI9142 DO meter does not compensate for salinity. It assumes internally that the sample water is fresh (0 g/L salinity). If the sample water is slightly salty eg 10 g/L then at 10 deg C if the water is saturated with oxygen, the meter will display 11.3 ppm oxygen. The actual measurement should be 10.6 ppm oxygen. This subtraction to get the actual value has to be done manually. To help with this, the manual the comes with the HI9142 has a table that shows the differences between recorded and expected values for a range of temperatures and salinities. many of the more advanced DO meters do include salinity and even altitude compensation as well as temperature compensation so check the specifications for each meter.

Oxygen measurements are also used as part of the procedure to estimate organic matter loading. One of these indices is Biochemical Oxygen Demand or BOD. Depending on methods, oxygen levels are measured before and after incubation of a sample or at regular intervals to establish oxygen uptake rate. Instruments such as the HI4421 DO bench meter include a number of advanced features such as data logging and calculations. The HI4421 allows uses to input all parameters to enable calculation of BOD. The HI4421 supplies the oxygen readings and the final BOD value. Oxygen Uptake Rate (OUR) is a parameter that can be used to assess organic load in waters. The HI4421 provides all the calculations necessary to calculate OUR.

So which solution is best for you? There is relatively little difference in the accuracy of each of the 3 methods. The DO meters offers a wider range over the kit and photometer, 0 - 20 ppm or higher. The kit is less convenient to use under field conditions but the initial outlay for the kit is low. The photometer takes care of accurately reading colour differences in reacted samples. For frequent testing the cost per test using the photometer and meter will come down. If speed and ease of use in the field is important then the DO Meter is the best choice. Some DO meters also allow data logging and calculations.

 

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