Oxygen, determination sampling bottle

HMSO Series Methods for the Examination of Waters and Associated Materials Determination of the pH Value of Sludge, Soil, Mud and Sediment-, and the Lime Requirement of Soil, 2nd edn., 1992 The Determination of the pH in Low Ionic Strength Waters, 1988 Fluoride in Waters, Effluents, Sludges, Plants and Soils, 1982 The Determination of Carbon Dioxide in Natural, Treated and Beverage Waters with a Supplement of Sampling Bottled and Canned Waters, 1986 5 Day Biochemical Oxygen Demand (BODs), 2nd edn., 1988. 

As a rule, subsampling for the determination of dissolved oxygen should be carried out as soon as possible after the sampler has been recovered. The bottles are unstoppered and after flushing the nozzle of the hydrocast sampler with sample water, the end of the nozzle tube is inserted into the sample bottle almost to the bottom. The nozzle should be transparent and sufficiently narrow to ensure that the sample stream carries with it any air bubbles and the air segment in the tube but wide enough to fill the bottle rapidly. 

Another, though small, error is introduced by temperature differences between subsampling and the volume determination of the sample bottles. The resulting volume error depends very much on the geometry and material of the sample bottles. For high-precision oxygen measurements the determination of the volume-temperature dependence of the type of sample bottle and resulting correction is recommended. 

The reagents, 0.5 mL of dimethyl-p-phenylenediamine and 0.5 mL iron(///) chloride solution, are added in exactly the same way as the reagents for determining oxygen, i.e., simultaneously with piston pipettes or with dispensers. The tips of the pipettes/dispensers should touch the bottom of the oxygen bottles. No air bubbles must be trapped in the bottles. The colour develops within a few minutes. If other sizes of sample bottles are used, the amoimts... 

The same instrumentation as used for the determination of dissolved oxygen is required for the determination of thiosulphate in seawater (see Chapter 4). The same type of sample bottles is used, but with a content of about 100 mL instead of SOmL. The piston burette should be calibrated carefully and be readable to 0.01 mL. 

In our study, contact of the anoxic samples with oxygen was avoided. The samples were transferred from the samplers (Go-Flo, General Oceanics, 5 L) into bottles equipped with three-way taps under N2 pressure. The bottles had previously been flushed with N2, and they were completely filled. Filtration in the laboratory with acid-cleaned 0.45- xm cellulose nitrate filters (Sartorius) and a polycarbonate filtration unit (Sartorius) was also carried out under nitrogen gas. Most analytical determinations were performed in the laboratory on the sampling day. 

The direct technique is appropriate for ordinary clean river waters. It requires complete filling of two 300-mL BOD bottles with the sampled water, and measurement of the oxygen content of one within 15 to 20 min of collection. The other bottle is incubated in the dark, to avoid photosynthetic contribution to the oxygen content of the water, for 5 days at 20°C. The extent of biochemical oxidation of any organics in the sample is then determined by measuring the oxygen content of the second bottle. The difference between the measured oxygen content of the first and second bottles is a direct measure of the BOD of the river sampled (Eq. 4.29). 

The method is particularly suitable for measuring very low concentrations of hydrogen sulphide, e.g., in the Baltic Sea. The blank values are negligible, and Beer s law is applicable up to around 60 pmol/L, but determinations of up to 300/miol/L are possible without dilution of the sample. By using the ordinary bottles used for oxygen samples to collect hydrogen sulphide samples, rapid work is possible with minimum risk of losses of sulphide and contact with oxygen. 

Samples should be taken from the ordinary hydrocast samplers immediately after the oxygen sample and in the the same manner as these samples (see Chapter 4). The hydrocast samplers should preferably be all-plastic, since sulphide reacts with many metals. When no oxygen is present in the water, samples for hydrogen sulphide determination should be taken first (see also Chapter 2). We use 50 mL oxygen bottles. Hydrogen sulphide is probably present in the water when the manganese hydroxide precipitate in the oxygen sample is completely white instead of brownish. 

Samples for the determination of thiosulphate are taken in the same way as for oxygen. It is, however, only necessary to rinse the 100 mL bottle by allowing about 50 mL to overflow so that sample water is not wasted, as it might be needed for the determination of other constituents. The analysis must be performed within about 60 min after sampling, but the sulphide should be precipitated immediately (see Section 6.2.4.2). 

Fixed-volume sampling is more accurate when the water is subsampled into calibrated reaction bottles as, e.g., for oxygen, and excess sample water is then displaced by reagent addition. This procedure considerably reduces the contamination risk and generally improves the determination of ammonia. 

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