ISO 15705:2002 pdf download

ISO 15705:2002 pdf download.Water quality – Determination of the chemical oxygen demand index (ST-COD)-Small-scale sealed-tube method.
6.7 Dispensed premixed reagent (ST-COD range up to 1 000 mg/I).
Dispense 0,50 ml ± 0,01 ml of potassium dichromate (6.3) into individual digestion tubes (7.1.2). Add carefully 0,20 ± 0,01 ml of mercury(ll) sulfate solution (6.5), followed by 2,50 ml ± 0,01 ml of silver sulfate (6.6).
Swirl cautiously to mix, then cap the tubes. Allow to stand overnight to cool. Swirl again before use.
This dispensed reagent is stable for 1 year if stored in the dark at ambient temperature.
A large batch of digestion tubes (7.1.2) may be prepared in advance, using the reagents as specified here.
Sealed tubes containing mercury(lI) sulfate, concentrated sulfuric acid, potassium dichromate and silver sulfate may be prepared in-house or purchased commercially, if available.
These sealed tubes should be stored in the dark at ambient temperatures. They should be stable for at least 1 year. It is essential that tubes that have passed their expiry date are not be used and are discarded.
6.8 Reagents for photometric detection
6.8.1 Stock calibration standard solution of potassium hydrogen phthalate (KHP) [C6H4(COOH)(COOK)], ST-COD = 10 000 mg/I.
Dissolve 4,251 g ± 0,002 g of potassium hydrogen phthalate, previously dried at 105 °C ± 5 °C for 2 h ± 10 mm, in approximately 350 ml of water (6.1). Dilute with water to 500 ml in a volumetric flask.
Store the solution in a refrigerator at 2 °C to 8 °C and prepare fresh each month.
An alternative to storage by refrigeration is to add 2 ml of dilute sulfuric acid (6.4.2), prior to diluting to 500 ml, to inhibit microbiological degradation.
6.8.2 Instrument calibration standard solutions, ST-COD of 200 mg/I, 400 mg/I, 600 mg/I, 800 mg/I and
1 000 mg/I.
Separately, dilute 20 ml, 40 ml, 60 ml, 80 ml and 100 ml of the stock 10 000 mg/I calibration solution (6.8.1) together with 4 ml of dilute sulfuric acid (6.4.2) to 1 000 ml with water.
Store these solutions at 2 °C to 8 °C and prepare fresh each month.
For a low concentration range [e.g. up to 150 mg/I (0)], standards of 30 mg/I, 60 mg/I, 90 mg/I, 120 mg/I and 150 mg/I may be prepared (see annex C). Store these solutions at 2 °C to 8 °C and prepare fresh each month.
6.9 Reagents for titrimetric detection (used for sealed-tube digested samples exhibiting atypical colour and/or turbidity).
6.9.1 Phenanthroline iron(II) sulfate indicator solution (ferroin)
Dissolve 3,5 g ± 0,1 g of iron(ll) sulfate heptahydrate (FeSO47H2O) in 500 ml of water (6.1).
Add 7,4 g ± 0,1 g of 1,10-phenanthroline monohydrate (C12H8N2H2O) and shake until dissolved.
The solution is stable for at least 1 month.
6.9.2 Ammonium iron(Il) sulfate (FAS) solution, approx. 0,075 mol/I.
Dissolve 30,0 g ± 0,5 g of ammonium iron(II) sulfate hexahydrate [(NH4)2Fe(S04)26H20] in about 200 ml of water. Cautiously add 20,0 ml ± 0,5 ml of concentrated sulfuric acid (6.4.1). Cool and dilute with water to 1 000 ml in a volumetric flask.
Dissolve 5,0 g ± 0,1 g of potassium chromate in 100 ml ± 1 ml of water (6.1). Add silver nitrate (6.9.3) dropwise to produce a slight red precipitate of silver chromate. Filter this solution.
This solution is stable for up to 1 year.
7 Apparatus
7.1 Apparatus for the digestion stage
7.1.1 Heating block, capable of maintaining a temperature of 150 °C ± 5 °C without causing localized over-heating to the contents of the tubes being tested.
The heating block should have a capacity for holding at least 10 tubes. The holes in the heating block should be of such a diameter that the glass tube wall is in close contact with the metal block. The depth of the holes should be such that adequate heating of the contents occurs.
NOTE Blocks are available that hold more than 50 tubes.
The contents of the tubes shall reach simmering point within 10 mm of adding the tubes to the preheated block.
7.1.2 Digestion tubes, made from acid-resistant glass capable of withstanding a pressure resistance of 600 kPa at 150 °C (e.g. length 185 mm, external diameter 14 mm, wall thickness 1 mm).
The glass tubes shall fit into the heating block such that the wall is in close contact with the metal block. Before use they shall be inspected to ensure that they are not damaged or cracked in any way, and they shall be discarded if any slight defect is detected. The glass tubes will be supplied with suitable caps.
If the sealed tubes are to be used as the cuvettes for measuring absorbance, then it is essential that the outside of the tubes are scrupulously clean prior to being put into the photometer.
NOTE Annex C gives some information on the use of commercial small-scale ST-COD kits utilizing photometric detection.