Experiment 23: Dissolved Oxygen Content of Water Samples PDF

/ EXPERIMENT 23 ESTIMATION OF DISSOLVED OXYGEN CONTENT OF WATER SAMPLES Structure 23. I Introduction Objec~ives 23.2 Principle 23.3 Materials Required 23.4 Procedure 23.5 Calculations and Results 23....

/ EXPERIMENT 23 ESTIMATION OF DISSOLVED OXYGEN CONTENT OF WATER SAMPLES Structure 23. I Introduction Objec~ives 23.2 Principle 23.3 Materials Required 23.4 Procedure 23.5 Calculations and Results 23.6 SAQ - - 23.1 INTRODUCTION Oxygen is nccessary for aerobic ~spiration.Aquatic organisms for respiration obtain the oxygcn from water, where it remains in dissolved form. In addition the dissolvcd oxygen in water affects the oxidation-kduction state of many other chcmical variables, such as nitrate and ammonia, sulphate and sulphite, and fcnous and ferric ions. The amount of oxygcn present in aquatic environmcnt is higliiy variable and generally low. Many factors such as temperature, salinity, respiration, photosynlhcsis and decomposition of decaying plants and animals affect the amount of dissolvcd oxygcn. As such oxygen is not very soluble in water and the solubility decreases with increasing tcmperature. The photosynthetic acilivity of watcr plants increasc the amount of dis~olvedoxygen during day time, whereas during night il bccomcs depleted duc lo respiration of plants and animals. During the process of decomposition microorganisms use thc dissolved oxygen thus making it dcficient. This adversely affects Ihe other aquatic organisms. You can sce in Table 23.1 the oxygen content in some respiratory mcdia. Table 23.1: Oxygen content of some samples of water and air Samples Dissolved Oxygen content millilitresllitre Sea water at 5' C 6.4 Fresh watcr at 5" 9.0 Fresh water at 25" 5.8 Air 209.5 The.arnount of oxygen dissolved in.watcr can bc measurcd and is usually.expressed as mg/l (equivalent to parts per million or ppm). There are two methods of estimating dissolved oxygen: by using oxygen electrodcs and by Winlder 's titration method. Laboratory Course-I Winkler's method is the most commonly used method for estimation of dissolved oxygen in water. In this lab exercise you will be estimating the dissolved oxygen by Winkler's method from at least from two different water sources such as a pond and a well, or tap water and well water. or a river and pond. Objectives At the end of this lab exercise you should be able to: describe the principle behind the estimation of the dissolved oxygen in water, (b perform the experimental procedure without any difficulty, ' become familiar with the calculations for the estimation of oxygen, and e discuss that the oxygen cQntent of the different aquatic habitats differ significantly. Winkler's method is a volumetric procedure in which manganous ions (MnZ'). are oxidised into manganic ions (Mn3') which reacting with an alkali precipitates into MnO(OH), and Mn(OH),. The extent 'of oxidation is directly relatkd to the amount of dissolved oxygen. In the presence of iodide ions in dilute sulphuric acid, the manganese hydrox@ is converted into rnanganous sulphate [MnSO,] and simultaneously the iodide ions are oxidised to molecular iodine Q. It is the concentration of this iodine that is directly proportional to the concentration of oxygen in the original water sample. The amount of iodine liberated at the end of the reaction can be determined by titration with a thiosulphate solution using starch as an indicator to determine the end product. 23.3 MATERIALS REQUIRED 1. Burette and Burette stand 2 300 ml. glass stoppered reagent bottles 3. 250 ml. conical flasks 4. 10 ml. pipettes 5. Measuring cylinder 6. &SO, solution (36 grns of MNSO,dissolved in 100 ml. of distilled water. 7. Alkaline-iodide solution a) 100 gms of NaOHl100 ml. of distilled water b) 27 gms of NaIl100 ml. of distilled water C) Mix solutions a and b  8. Concentratcd H,SO, Estimation of Dissolved Oxygen Content of Water 9. Starch solution 1 gm of slarch per 100 ml. of distilled water. The water Samples musl be hcatcd to bearable warmth and the sluch dissolvcd in it. 10. 0.025N sodium thiosi:l i\llaLc (Na,S,O,) solution. (6.205 gms of Na2S20,. 5H20 pcr 1000 ml. of distilled watcr). 23.4 PROCEDURE , From each samplc obtain wntcr carefully and without air bubbles in 300 ml glass stoppcrcd rcagenl boltlcs. Labcl Lhc boltlcs as A and B. For accuralc ' dctermination of dissolvcd oxygcn it is vely nccessary llial specid carc in sampling and prcparation of walcr sclmplcs should bc taken. Any exposurc 01 the samplc to air will vitiatc your rcsulls. Thcrcforc, il is suggcstcd that you collecl walcr by kccping your bolllc undcr Lhc surfacc of walcr and dlow h e water to flow inlo Lhc boltlc vcry slowly without mixing wilh thc air. It is also neccssary that prior to Lhe filling of thc sample inlo thc botlle, you dctcrminc Lhe volume of Lhe bolllc. You niny usc a mcasu~ingcylinder for this purpose. Irnmcdialely aficr collecting thc samplc closc Ihc boltlc wilh a glass stoppcr. This hclps you Lo climinatc thc air spaccs. Now, you m:iy add thc various rcngcnts LO tlic s m ~ p l c:is dclailed bclow: 1. Remove lhc sloppcrs and add 2 n-rl. of MnSO, solution lollowed by 2 ml of alkaline-iodidc solulion in botrlcs A and B. Addition of thcsc rcagcnts should bc donc bclow tlie surracc or watcr by dipping thc pipcltc into thc wliter thus prcvcnting [he conta~ninalionwilh air. 2. Stopper the bolilcs and gcntly till Llicm scvcral limcs for tlic solulions Lo mix. You wiIl scc tl~cIonnalion o l yellowish brown prccipil;ilcs 01MII(OH)~ and MnO(OH),. Allow tl~cprccipi1;ltc Lo sctllc down and gcnlly sIlakc again. 3. Removc Lhc sloppcr and add cnrclully 2 nil or conc. 11,SO,, undcr thc surfacc of prcparcd s:lrnplcs. Stoppcr Lllc bollles again and mix wcll. The brown prccipilale con~plelclyclissovlcs 1c:lving a straw or brown colourcd solution. 4. Transfer 50 ml of tlic conlcnls of ~ h csarnlrlc bolllc A to a 250 nil conical flask. Add 1 nll of slarch indicator solulion. Tllc solulion turns bluc. Tilralc this solulion ng;iinst 0.025N sodium Lhiosulpllalc solulion. For tilralion you havc to fill the burctlc wilh lhc tlliosulphatc.solution. Opcn lhe slopcock of Ule bureltc and let thc solution run down once. Reffill Lhc burelle up10 zcro mark and pcrfurm Lhc Lilration. Tjlc cnd point is thc disappearance of thc bluc colour. Rccord tile burcltc rcading. You may repcal the tilration till you get thc concordant valucs. Thc concordant valucs may be oblaincd cvcn at thc cnd oI lhc sccond Litralion if you do Lhc~n chrefully. 5. Rcpcat thc above procc'durc with the samplc B. Fill in thc dala in your obscrvalion nole book in the form of thc table providcd below.  Volume of Burette reading Volume of Sample S. No. the sample initial final Na2S20, (me) consumed 23.5 CALCULATIONS AND RESULTS You can obtain the amount of dissolved oxygen per litre of water using the following calculations. K x 200 x vol. of Na2S203 x 0.698 Amount of oxygcn~litrc= Volume of the sample Volumc of bottle where K = volume of thc bottle-volume of the reagent added A sample calculation is shown below: Volume of the bottle = 300 ml Amount of reagcnt used = 4 ml (2 ml MnSO, h + 2 rnl Alkaline iodidc) Volumc of NaS,O, consumed = 4.5 ml K x 200 x 4.5 x 0.698 Amount of 0, = 50 23.7 SAQ Do you find any diffcrencc in the oxygen content of the two water samples? If the answer is yes, how do you account for the difference?