Accurate BOD Measurements With Electrochemical and Optical Probes

Y S I Environmental Tech Note 0610 T605-01 YSI created this document to address some common problems that are encountered when conducting Biochemical Oxygen Demand (BOD) testing. Tere can be conusion about BOD and its relationship with dissolved oxygen (DO) readings. Even though the end result o BOD testing is a nal BOD reading, the calculations are all based on DO readings. Tereore, DO readings must be as accur ate as possible to ultimately get accurate BOD results. YSI has been a leader in the eld o DO measurements or 55+ years. Severa l o YSI’ s BOD probes incorporate the Clark Polarographic, membrane covered, sensor technology (some Model numbers incl ude the 5905, 5010, 200-BOD, ProBOD and 5750) while the YSI ProOBOD M uses a membran e-less optical DO technology Theo ry - Electrochemical Briey , the electrochemical technolo gy utilizes a gold cathode, silver anode, electrolyte solution, and eon® or Polyetheylene (PE) membrane in order to create a potential across the membrane. Te oxygen molecules are consumed at the gold cathode aer a voltage is applied rom the silver anode. In essence, the diusion rate o oxygen across the membrane is measured. Te sensors do not measure the concentration o oxygen but, instead, the sensor measures the partial pressure o dissolved oxygen in % saturation. W e know this because water can hold more oxygen in resh water than in se a water, however , the sensors raw output signal is identical in both media since the pressure is identical in both media. Te instrument then calculates the concentration rom the % saturation value (pressure value), the sample temperature, and a measured or en tered salinity corr ection value. Te calculation Ensure Accurate and Successful BOD Measurements with Electrochemical or Optical Sensors is based on the Oxygen Solubility chart ound in Standard Methods. Te instrument soware automatically calculates the mg/L values. For more in depth inormation about dissolved oxygen eel ree to visit www.YSI.com\weknowDO. Tere are several actors that can aect DO readings, includin g the probe care and use, calibration techniques, and other laboratory practi ces. At some point many labs, i not all, have some issue with their BOD results rom blanks coming out too high or poor glucose-gluta mic acid (GAA) result s. Te steps in correcting these issues can oen be numerous, time-consuming and outright rustrating. One easy step to take out o the equation is the unctionality or the instrument and probe. Probe Use and Care - Electrochemical In order or your DO reading, and ultimately your BOD result, to be as accurate as possible the ollowing issues surrounding the use and care o the probe must be addressed. 1. Te electrolyte solution should be resh. Once it has been hydrated, the shel lie o the solution is one year. It is best to allow the electrolyte solution a ull 24-hours aer mixing beore rst use to ensure proper mixing o the solution. Care should also be taken to ensure that the correct solution is being used or the correct membrane. Te solution you use or your eld DO probe may not be the same solution needed or your lab BOD probe and vice versa. Follow the instructio ns on the electrolyt e bottle or download the membrane selection guide at www.YSI. com/wekno wDO (located under the Guides tab). 2. Membranes must be installed correctly. Bubbles, wrinkles, and obvious tears and/or holes can create problems. However, occasional cosmetic blemishes o the membrane are normal and generally do not aect the successul operation and unctiona lity o the probe. Te cap membranes rarely have any issues when stored and used properly. Membrane caps are a much easier option than the stretch over style membranes common on older DO eld probes. Te cap membranes have been available or years and are capable o being changed within secon ds. Some manuactur ers may try to state that membrane changes are costly and cumbersome but it’s as simple as unscrewing the old cap and screwing on a new one. Proper membrane cap installation is shown below in Figure 2. (continued) ©2005, 2010 YSI Environmental +1 937 767 7241 Fax +1 937 767 9353 [email protected] www .ysi.com Figure 1. In electrochemical sensors, O 2 molecules diuse through the membrane to the sensor at a rate proportional to the pressure dierence across it.

Accurate BOD Measurements With Electrochemical and Optical Probes

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