Winkler titration

The dissolved oxygen concentrations are determined immediately and after five days. The method for dissolved measurement involves either a modified Winkler titration or a membrane-covered oxygen electrode. The difference between initial and final dissolved oxygen multiphed by the dilution factor is the BOD value. 

The graphical method (Figure 2.17.1) of BOD calculation is shown in the above table. Sample DO in this example was 3.0 mg/L, determined by potenti-ometric or Winkler titration. From the above graph (Figure 2.17.1) ... 

Fig. 7 Vertical distribution versus density (agy kg m 3) of temperature (T), salinity (S), transmission (Xmiss), dissolved oxygen measured with YSI oxygen sensor (02SB), dissolved oxygen measured by Winkler titration (02), hydrogen sulfide (H2S), phosphate (PO4), silicate (Si), nitrite (N02), ammonia (NH4), dissolved manganese (Mn diss), bivalent iron (Fe(II)), and trivalent iron (Fe(III)) at a station near the Bosporus (Cast 16, RV Knorr 172-05 cruise, April 04,2003). Concentrations of chemical parameters are in xM...

DO can be measured by a fairly tricky wet chemical procedure known as the Winkler titration. The DO is first trapped, or "fixed", as an orange-colored oxide of manganese. This is then dissolved with sulfuric acid in the presence of iodide ion, which is converted to iodine by the oxidized manganese. The iodine is titrated using standard sodium thiosulfate. The original dissolved oxygen concentration is calculated from the volume of thiosulfate solution needed. 

Historic data on Secchi disk depth in the northern Adriatic Sea in 1911 through the present, with few interruptions of data collection, provide a measure of water transparency that could be interpreted to depict surface water productivity. These data coupled with surface and bottom water dissolved oxygen content determined by Winkler titrations and nutrient loads outline the sequence of eutrophication in the northern Adriatic Sea. Similar historical data from other coastal areas around the world demonstrate a decrease in water clarity due to phytoplankton production in response to increased nutrient loads that are paralleled by declines in water column oxygen levels. 

Briefly, any chlorine is removed from a water sample to be tested and the sample or a diluted version of it is adjusted to approximately pH 7. The pH buffer and nutrients are added along with a seed and/ or nitrification inhibitor if necessary. Two BOD bottles are filled to an extent such that insertion of the stoppers displaces all the air. The initial DO level (DOi) of the first bottle is determined using the Winkler titration method (a titrimetric determination) or using one of the modifications of this method (most commonly the azide modification). The second bottle is then incubated for 5 days, at which time the DO level (DOf) is determined ... 

Only one bottle will be needed for DOf and DOf if a DO electrode is used. If replicate determinations are carried out, it is customary to carry out a single determination on the initial DO bottle and have replicates for the final DO determinations, which are subject to greater variation i.e., for Winkler titration determination of DO, three bottles are prepared, one used immediately for DOf and the other two incubated and used for DOf determination. 

The classical method for DO determination is the Winkler titration method, which is still frequently used as a reference procedure for calibration of DO meters. It is inexpensive in capital cost outlay, requiring only common laboratory glassware (burettes, pipettes, measuring cylinders, conical flasks, volumetric flasks), burette stands, wash bottles, and chemicals. The principal reactions involved are... 

For both these methods - Winkler titration and the DO meter method - suspended and settled particulate organic material also contribute significantly to the BOD, though not as readily as dissolved organic matter. In cases where settled material forms a thick layer, dilution allows greater access of the bacteria to the settled material by making more surface area of the material available. 

A comparison of the processes involved in the Winkler titration and DO meter methods with alternative methods illustrates situations where they may not produce equivalent results. For example, where oil and grease are present in water, dilution allows greater accessibility by bacteria owing to the presence of finer oil droplets. In this case a higher BOD result is obtained using the Winkler and DO methods than is obtained by the lower-dilution manometric methods. 

An advantage of this system over the conventional BOD system (Winkler titration method) and the DO probe is that it can be used to monitor changes in DO levels with time. The procediues for ensuring that sufficient nutrients are available for the 5 days of the tests are not as rigorous as with the standard procedures, however, and this could lead to low BOD results. Additionally, air could leak into the manometer system, causing errors. 

Manganese(II)chloride 40g of MnCl2 5H20 are dissolved and made up to 100 mL with distilled water. This solution is the same reagent as is used for the determination of oxygen (Winkler titration Chapter 4). 

Data pairs of sensor signals (voltage or equivalent) and oxygen saturations determined by Winkler-titration are collected for various temperatures (commonly 5-30 °C in steps of 5 K). 

In the classic method, also called the dilution method, the DO is determined via Winkler titration [3]. Thus, manganese sulfate is added to the sample. After the redissolution of the precipitate, the solution is titrated with thiosulfate until the color of solution changes from dark blue fo clear, employing starch indicator. The volume of fifranf employed corresponds fo fhe DO value. 

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