Understanding dissolved oxygen

1 Understanding dissolved oxygenTable of contentsWorkshop 5 Importance of Gas of pressure and of changing dissolved oxygen Methods calibrations into to determine dissolved sensing ) polarographic oxygen of influencing POS ) Luminescence dissolved dissolved oxygenWorkshop goals:My goal for this workshop is to provide participants with a full Understanding of whatgoes into the measurement of dissolved oxygen in natural waters. Because of the rapiddevelopment of technology and the production of advanced meters that are capable of generatingmuch information, the basics on which the technology is based are often forgotten , meaningthat operators may not be able to distinguish good and bad data. By having a thoroughunderstanding of what is involved with the measurement of DO, meaningful data will the end, participants should leave with an Understanding of how temperature andpressure influence the amount of dissolved oxygen in water; the basics of operation of differentsensors used to measure dissolved oxygen in water and their pros and cons.

2 And be comfortableusing a spreadsheet to automate and calculate the saturation concentrations of oxygen forcalibrations at different elevations and atmospheric dissolved oxygenBackground:- oxygen discovered in 1773-1774 by Carl Wilhelm Scheele, in Uppsala, and JosephPriestley in Wiltshire- named by Antoine Lavoisier in 1777 Highly non-metalic reactive element - typically forms oxidesPresent in all major classes of structural molecules in living organisms, such as proteins,carbohydrates, and present in major inorganic compounds such as animal shells, teeth, and is the third most abundant chemical element in the universe, after hydrogen andhelium3 Ozone O- in stratosphere - pollutant at low level = smog2At standard temperature and pressure, oxygen is a colorless, odorless gas O, in which thetwo oxygen atoms are chemically bonded to each - double produced by fractional distillation of liquefied air (79% N, O +others)Importance of oxygenNecessary to all life that has aerobic metabolismBiological oxygen demand - respiration of biota including bacteria6126222 CHO + 6O > 6CO + 6HO + heatoxygen is the final electron acceptor in cellular respirationChemical REDOX reactionsChemical oxygen demand (COD - in water column SOD in sediment) , nitrificationIn the presence of nitrifying bacteria, ammonia is oxidized first to nitrite, then tonitrate423+2NH + 2O > NO + 2H + HO+-2 The stoichiometric requirement for oxygen in the above reaction is mg of O per4mg of NH-N oxidized.

3 + , oxidation of ironUnderstanding dissolved oxygen2 -O434 FePO (insoluble) ---->Fe(PO4) (soluble) <----> 3Fe +2PO (free)3+2+2+3- <----2 +OBecause the well-being of aquatic organisms is dependent on the availability of oxygen , manyregulating agencies have adopted some form of an oxygen standard - a minimum. Regulatoryaction is taken if concentrations fall below this what the minimum is for your jurisdiction/regulating authorityThus we need a reliable method(s) with which to measure the oxygen content of water dissolved oxygenStandards- In physical sciences have standard conditions for temperature and pressure forexperimental measurements- allows comparison between different sets of data - should make effort to record - gettemperature of water anyway - pressure is a bit more difficult- whole variety of different sets- most common is that of International Union of Pure and applied chemistry (IUPAC) andthe National Institute of Standards and Technology (NIST)- far from being universal standards within an organization - the International Organization forStandardization (ISO), the United States Environmental Protection Agency (EPA) andNational Institute of Standards and Technology (NIST)

4 Each have more than onedefinition of standard reference conditions in their various standards and standard reference conditions in current use TempAbsolute pressure Relative humidity Publishing or establishing entity C kPa % RH0 IUPAC (present definition)0 IUPAC (former definition), NIST, ISO1078015 0 ICAO's ISA, ISO 13443,20 EPA, NIST25 EPA F psi % RH60 OSHA, OPEC,59 78 Army Standard Metro59 60 ISO 2314, ISO 3977-2 Understanding dissolved oxygenIdeal gas lawsDeveloped for an ideal gas - states that for any gas, a given number of its particles occupythe same volume. Change in volume is inverse to changes in pressure and direct to temperaturePV = nRT1122 Boyle s Law - PV = PV or PV = constantWhere P = pressure and V = volume111222 From combined gas laws PV/T= PV/T or PV/T = constant T = temperaturewhere (in SI metric units):P = the absolute pressure of the gas, in Pan = amount of substance, in molV = the volume of the gas, in m3T = the absolute temperature of the gas, in KR = the universal gas law constant of m Pa/(mol K)3or where (in customary USA units):P = the absolute pressure of the gas, in psin = number of moles, in lbmolV = the volume of the gas, in ft/lbmol3T = the absolute temperature of the gas absolute, in RR = the universal gas law constant of ft psi/(lbmol R)3 The value of the ideal gas constant, R, is found to be as follows.

5 R = J mol K!1!1 = m Pa K mol3!1!1 = kPa L mol K-1-1 = L atm K mol!1!1 = L mmHg K mol!1!1 = ft psi R lb-mol (degrees Rankine)3!1!1 = ft lbf R!1 lbm (for air)!1- Technical literature is confusing because authors fail to indicate if they are using the universalgas law constant R, which applies to any ideal gas, or whether they are using the gas law constantRs, which only applies to a specific individual gas. The relationship between the two constants isRs = R / M, where M is the molecular weight of the dissolved oxygenUnits and conversion between themThe international SI unit for pressure is the pascal (Pa), equal to one newton per square meter(N m or kg m s). The conversions to other pressure units are:-2-1-2 Pressure Unitspascal(Pa)bar(bar)technicalatmosphe re(at)atmosphere(atm)torr(Torr)pound-for cepersquare inch(psi)1 Pa= 1 102!5!5!6!3!61 bar100,000= 10 at98, 1 atm101, 1 10!3!33= 1 Torr;= 1 1031 psi6, 1 lbf/in!

6 3!3!3 Example reading:1 Pa = 1 N/m = 10 bar = 10 at = 10 atm, !5!6!6kgf - kilogram forceUnderstanding dissolved oxygenSolubilityThe solubility of oxygen in water is temperature and pressure dependentAbout twice as much ( mg L) dissolves at 0 C than at 20 C ( mg L)!1!1 Less oxygen dissolves at high elevations (Mount Everest)compared to low elevations (sealevel) because the atmospheric pressure is less and thus the partial pressure is saturation = oxygen conc *100 oxygen solubility at saturation- important to know for animal health - , Total dissolved Gas (TDG) limits (EPA limit is110% currently- in this case % saturation is needed rather than a concentration (mg/L)Partial pressureThe pressure exerted by a particular component of a mixture of gases as if only that gas werepresentO2 Partial pressure of oxygen = DO/ * : DO measured concentration in mg/LO2 = Bunsen coefficient for oxygen (standard methods table 2810:I)The factor = 760/(1000K), where K is the ratio of molecular weight tomolecular volume of oxygen gasEffects of changing pressure (barometric or altitude)Typically any barometric pressure reported as part of a TV broadcast has been convertedto a value relative to sea level.)

7 This is to standardize data - and over large geographic areaspressures are typically similar, unless a storm is approaching. However, there are differenceswith altitude that can occur over a small distance. All avitation uses pressure altimeters todetermine where the ground determine true - uncorrected barometric pressure:1) obtain from calibrated mercury barometer - if you have another barometer close by -make sure it has been correctly set for your altitude (see reference on how tocalibrate barometer)2) call local airport or radio stationask if data are corrected to sea level, if yes - need to UNcorrect it23) use known O saturation tables / nomograms4) use tables / formulae in standard methods (we ll get to this in a minute) Understanding dissolved oxygenTo Uncorrect an airport or local weather station barometric pressure ex. inches Hga) - determine altitude (in feet) of your lab (Oklahoma City = 1295' or 395 m)Moscow, ID / Gritman Helipad = 2035 ft or above sea leavelPullman Airport = 2556' or above sea ) - determine the correction factor (CF):CF = [760 - (Altitude * )] 760= (760- (1295* )] 760 (Highlight = altitude for your location in feet)= [ ] 760= 760= ) therefore the true uncorrected barometric pressure = * = HgNote: - pressure drops about 26 mm (about 1 in) for every 1000 feet above sea level.

8 Hence the multiplication by (26/1000)To convert inches Hg to mm Hg - multiply by = * = mm HgDetermining DO Saturationa) the temperature of the calibration sample is 21 C2b) from standard tables we can determine that the maximum O solubility at sea level andstandard pressure is mg/Lc) we know that the uncorrected barometric pressure is mm Hg2d) to determine the correction factor to adjust maximum O saturation to the actualpressure:Pressure correction factor = [True barometric pressure 760]= ( 760)= ) multiply the sea level saturation by the pressure correction factor= * mg/LUsing the standard method equations for determining the concentration at non-standardtemperatures and pressuresCp = C*P((1-Pwv/P)(1- P))/((1-Pwv)(1- ))Where: Cp = equilibrium oxygen concentration at nonstandard pressure, mg/LC* = equilibrium oxygen concentration at standard pressure of 1 atm, mg/LP = non standard pressure, atmPwv = partial pressure of water vapor, atm, computed from: lnPwv = ( )-(216961/T)2T = temperature in K ( K = C+ ) = ( 10t) + ( 10t)-5-82t = temperature, CUnderstanding dissolved oxygenExample.

9 At 20 C and atm, Cp = C*P( ) = mg/LTo calculate C* the equilibrium concentration at standard pressure and atmosphereslnC* = + ( 10/T) - ( 10/T) + ( 10/T) -572103( 10/T) - Chl[( ) 10) - ( 10/T) + ( 10/T)]114-2132where C* = equilibrium oxygen concentration a kPa, mg/LT = temperature ( K) = C + (for 0-40 C)Chl - chlorinityExample 1 - at 20 C and Chlorinity, lnC* = = mg/LMigrate to spreadsheet to set this up to calculate automatically for any pressure unit andtemperature or calibrations in perspective- pressure drops about 1 inch per 1000 ft (26 mm/1000ft)- Maximum DO saturation drops about mg/L for each 1000 ft- outside of storm systems, daily pressures fluctuate about 10 mm ( inches)- at 20 C the oxygen saturation decreases about mg/L for each degree rise intemperatureInstrument based barometers are making this sort of issue a thing of the past - however, you needto know that your on-board barometer is set correctly - many can be set to user defined points -and come factory set to correct to sea level!

10 You must realize how important pressure changes are to obtain accurate calibrations - this isespecially true if you take your oxygen meter for hikes up mountains and re-calibrate when youget to the if you are analyzing samples - at high and low pressure systems - need to be able toaccurately correct for pressure dissolved oxygenMethods to determine dissolved oxygenMethod must meet two important criteria:i) it must be accurate given low concentration (mg/L)ii) apparatus must be suitable to field conditions1) Bunsen method- boil oxygen out of water and measure in absorbent materials- too cumbersome for field and insufficient accuracy for lab2) nomograms - be sure you get a correct one, and not one that has been photocopied to death -as you are typically required to line up points and read off a scale3) Winkler method one of the best colorimetric methods (EPA Standard Methods approved)4) oxygen sensitive electrodei) Clark-style membrane and galvanic (EPA, Standard Methods approved)ii) Luminescence LDO (Chemical quenching of luminescence) Understanding dissolved oxygen2) NomogramsUnderstanding dissolved oxygen3) Chemical determination of oxygen in water (Winkler method)Principle behind the method:The Winkler titration method for the determination is based on the method developed byWinkler in 1888 (Winkler 1888).