Methods to Test Water

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Winkler test for dissolved oxygenThe Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved Oxygen, abbreviated D.O., is widely used in water quality studies and

routine operation of water reclamation facilities. An excess of manganese(II) salt, iodide(I-) and hydroxide (HO -) ions are added to a water sample causing a white precipitate of Mn(OH) 2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. In the next step, a strong acid (either hydrochloric acid or sulfuric acid ) is added to acidify the solution. The brown precipitatethen convert the iodide ion (I-) to Iodine . The amount of dissolved oxygen is directly

proportional to the titration of Iodine with a thiosulfate solution [1].

Sample method

In the first step, Manganese(II) sulfate (at 48% of the total volume) is added to anenvironmental water sample. Next, Potassium iodide (15% in potassium hydroxide 70%)is added to create a pinkish-brown precipitate. In the alkaline solution, dissolved oxygenwill oxidize manganese(II) ions to the tetravalent state.

2 Mn(OH) 2(s) + O 2(aq) 2 MnO(OH) 2(s)

MnO(OH) 2 appears as a brown precipitate . There is some confusion about whether theoxidised manganese is tetravalent or trivalent . Some sources claim that Mn(OH) 3 is the

brown precipitate, but hydrated MnO 2 may also give the brown colour.

4 Mn(OH) 2(s) + O 2(aq) + 2 H 2O 4 Mn(OH) 3(s)

The second part of the Winkler test reduces acidifies the solution. The precipitate willdissolve back into solution. The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.

The Mn(SO 4)2 formed by the acid converts the iodide ions into iodine , itself beingreduced back to manganese(II) ions in an acidic medium.

Mn(SO 4)2 + 2 I -(aq) Mn 2+(aq) + I 2(aq) + 2 SO 42-(aq)

Thiosulfate solution is used, with a starch indicator, to titrate the iodine.

2 S 2O32-(aq) + I 2 S 4O62-(aq) + 2 I -(aq)

Analysis

From the above stoichiometric equations, we can find that:
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1 mole of O 2 2 moles of MnO(OH) 2 2 mole of I 2 4 mole of S 2O32-

Therefore, after determining the number of moles of iodine produced, we can work outthe number of moles of oxygen molecules present in the original water sample. Theoxygen content is usually presented as mg dm -3.

Limitations

The success of this method is critically dependent upon the manner in which the sampleis manipulated. At all stages, steps must be taken to ensure that oxygen is neither introduced to nor lost from the sample. Furthermore, the water sample must be free of any solutes that will oxidize or reduce iodine.

Instrumental methods for measurement of dissolved oxygen have widely supplanted theroutine use of the Winkler test, although the test is still used to check instrumentcalibration.

BOD 5

To determine five-day biological oxygen demand (BOD 5), several dilutions of a sampleare analyzed for dissolved oxygen before and after a five-day incubation period at 20degrees Celsius (68 degrees Fahrenheit) in the dark. In some cases, bacteria are used to

provide a source of oxygen to the sample; these bacteria are known as "seed". Thedifference in DO and the dilution factor are used to calculated BOD 5. The resultingnumber (usually reported in parts per million or milligrams per liter) is useful indetermining the relative organic strength of sewage or other polluted waters.

The BOD 5 test is an example of analysis that determines classes of materials in a sample.

The BOD 5 test

BOD measures the rate of oxygen uptake by micro-organisms in a sample of water at atemperature of 20C and over an elapsed period of five days in the dark.

There are two recognized methods for the measurement of BOD.

Dilution method

To ensure that all other conditions are equal, a very small amount of micro-organism seedis added to each sample being tested. This seed is typically generated by diluting
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activated sludge with de-ionized water . The BOD test is carried out by diluting thesample with oxygen saturated de-ionized water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen (DO) and then sealing the sample to preventfurther oxygen dissolving in. The sample is kept at 20 C in the dark to prevent

photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved

oxygen is measured again. The difference between the final DO and initial DO is theBOD. The apparent BOD for the control is subtracted from the control result to providethe corrected value.

The loss of dissolved oxygen in the sample, once corrections have been made for thedegree of dilution, is called the BOD 5. For measurement of carbonaceous BOD (cBOD),a nitrification inhibitor is added after the dilution water has been added to the sample.The inhibitor hinders the oxidation of nitrogen.

BOD can be calculated by:

Undiluted: Initial DO - Final DO = BOD Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor

BOD is similar in function to chemical oxygen demand (COD), in that both measure theamount of organic compounds in water. However, COD is less specific, since it measureseverything that can be chemically oxidised, rather than just levels of biologically activeorganic matter.

Manometric method

This method is limited to the measurement of the oxygen consumption due only to

carbonaceous oxidation. Ammonia oxidation is inhibited.

The sample is kept in a sealed container fitted with a pressure sensor . A substance thatabsorbs carbon dioxide (typically lithium hydroxide ) is added in the container above thesample level. The sample is stored in conditions identical to the dilution method. Oxygenis consumed and, as ammonia oxidation is inhibited, carbon dioxide is released. The totalamount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. Fromthe drop of pressure, the sensor electronics computes and displays the consumed quantityof oxygen.

The main advantages of this method compared to the dilution method are:

simplicity: no dilution of sample required, no seeding, no blank sample direct reading of BOD value continuous display of BOD value at the current incubation time.

Furthermore, as the BOD measurement can be monitored continuously, a graph of itsevolution can be plotted. Interpolation of several graphs on a similar water may build an
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experience of its usual evolution, and allow an estimation of the five days BOD after asearly as the first two days of incubation .[2]

Test Limitations

The test method involves variables limiting reproducibility. Tests normally showobservations varying plus or minus ten to twenty percent around the mean .[3] :82

[Toxicity

Some wastes contain chemicals capable of suppressing microbiological growth or activity. Potential sources include industrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers in food processing or commercial cleaning facilities,chlorination disinfection used following conventional sewage treatment, and odor-controlformulations used in sanitary waste holding tanks in passenger vehicles or portabletoilets. Suppression of the microbial community oxidizing the waste will lower the testresult. [3] :85

Appropriate Microbial Population

The test relies upon a microbial ecosystem with enzymes capable of oxidizing theavailable organic material. Some waste waters, such as those from biological secondarysewage treatment, will already contain a large population of microorganisms acclimatedto the water being tested. An appreciable portion of the waste may be utilized during theholding period prior to commencement of the test procedure. On the other hand, organicwastes from industrial sources may require specialized enzymes. Microbial populationsfrom standard seed sources may take some time to produce those enzymes. A specializedseed culture may be appropriate to reflect conditions of an evolved ecosystem in thereceiving waters. [3]:85-87

History of the use of BOD

The Royal Commission on River Pollution , which was established in 1865 and theformation of the Royal Commission on Sewage Disposal in 1898 led to the selection in1908 of BOD 5 as the definitive test for organic pollution of rivers. Five days was chosenas an appropriate test period because this is supposedly the longest time that river water takes to travel from source to estuary in the U.K. In 1912, the commission also set a

standard of 20 ppm BOD 5 as the maximum concentration permitted in sewage worksdischarging to rivers, provided that there was at least an 8:1 dilution available at dryweather flow. This was contained in the famous 20:30 (BOD:Suspended Solids) + fullnitrification standard which was used as a yardstick in the U.K. up to the 1970s for sewage works effluent quality. The value of BOD is less than COD. [citation needed ]

The United States includes BOD effluent limitations in its secondary treatment regulations. Secondary sewage treatment is generally expected to remove 85 percent of
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the BOD measured in sewage and produce effluent BOD concentrations with a 30-dayaverage of less than 30 mg/L and a 7-day average of less than 45 mg/L. The regulationsalso describe "treatment equivalent to secondary treatment" as removing 65 percent of theBOD and producing effluent BOD concentrations with a 30-day average less than 45mg/L and a 7-day average less than 65 mg/L. [4]

Typical BOD values

Most pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L. Municipal sewage thatis efficiently treated by a three-stage process would have a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L in Europe and as low as200 mg/L in the U.S. , or where there is severe groundwater or surface water infiltration.(The generally lower values in the U.S. derive from the much greater water use per capitathan in other parts of the world.) [1]

Instructions

Things You'll Need: pH test kit or chlorine test kit

Step

Test the chlorine level in the water with a pH kit or other water testing kits.Testing usually involves dipping a paper tab into the water and comparingthe colors revealed against a chart. High chlorine levels or other pHimbalances should be treated.

1. Scope

1.1 These test methods cover the determination of ammonia nitrogen, exclusive of organic nitrogen, in water. Two test methods are included as follows:

SectionsTest Method ADirect Nesslerization 7 to 15

Test Method BIon Selective Electrode 16 to 24

1.2 Test Method A is used for the routine determination of ammonia in steamcondensates and demineralizer effluents.

1.3 Test Method B is applicable to the determination of ammonia nitrogen in the rangefrom 0.5 to 1000 mg NH 3 N/L directly in reagent and effluent waters. Higher concentrations can be determined following dilution. The reported lower range is based
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on multiple-operator precision. Lower limits have been obtained by two of the twelvelaboratories participating in the round robin.

1.4 Both test methods A and B are applicable to surface and industrial waters andwastewaters following distillation. The test method for distillation given in Appendix X1

has been used in the past to meet requirements for predistillation of samples beinganalyzed for ammonia.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associatedwith its use. It is the responsibility of the user of this standard to establish appropriatesafety and health practices and determine the applicability of regulatory limitations prior to use.

1.7 The distillation method now appears as Appendix X1 and is provided asnonmandatory information only. The automated colorimetric phenate method has beendiscontinued.

7.1 This test method is suitable for the rapid routine determination of ammonia nitrogenin steam condensates and demineralized water. See Appendix X1 for the distillation testmethod.

16.1 This test method is applicable to the measurement of ammonia in reagent andeffluent water.

Methods to Test Water

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