Sample Handling and Dilution Procedures for Dissolved Oxygen Determination

Dissolved oxygen (DO) is a critical parameter for assessing water quality and self‑purification capacity. Accurate DO measurement depends heavily on proper sample collection, handling, and dilution procedures. Although the specific steps may vary between the iodometric method (GB/T 7489‑1987) and the electrochemical probe method (HJ 506‑2009), both require strict adherence to well‑established operational protocols to avoid errors caused by oxygen exchange or contamination.

Sample Collection and Immediate Fixation

DO measurements are particularly sensitive to sample storage conditions. Ideally, the determination should be performed on‑site. When immediate analysis is not feasible, the sample must be fixed promptly at the sampling point. Special DO bottles should be used: the water sample is gently introduced along the bottle wall until it overflows slightly, then the stopper is inserted immediately to ensure that no air bubbles remain trapped inside.

For samples intended for the iodometric method, manganese sulfate solution and alkaline iodide‑azide solution are added without delay to convert the dissolved oxygen into a stable brown precipitate. The fixed samples are then stored in the dark. For the electrochemical probe method, fixation is unnecessary, but if measurement cannot be performed immediately, samples should be refrigerated at 0–4°C.

Sample Pretreatment

Before dilution, certain pretreatments may be necessary to eliminate interfering substances. If the sample pH falls outside the range of 6.5–7.5, it should be adjusted to near‑neutrality using dilute hydrochloric acid or sodium hydroxide solution, with the volume of the adjusting reagent not exceeding 0.5% of the sample volume. 

When oxidants such as free chlorine are present (e.g., >0.1 mg/L), sodium thiosulfate should be added to remove them to prevent interference. For samples containing reducing substances or excessive organic matter, additional pretreatment measures are required depending on the specific matrix.

Preparation of Dilution Water

Dilution water is a fundamental component of the dilution procedure. It is typically prepared from distilled water that has been aerated to achieve oxygen saturation or near‑saturation. According to standard protocols, the DO concentration of dilution water should be approximately 8 mg/L. 

The preparation method involves adding 1 mL each of four salt solutions per litre of distilled water, followed by thorough mixing and aeration for at least 24 hours. The aerated water is then placed in an incubator at 20°C for stabilisation, and its DO content is verified to be 8–9 mg/L before use. When seed microorganisms are required, a suitable volume of inoculum is added to the dilution water to obtain seeded dilution water.

Determination of Dilution Factors

Choosing the appropriate dilution factor is crucial for reliable results. The dilution should be designed so that during the test, the oxygen consumed is not less than 2 mg/L, and the residual DO after incubation also remains at least 2 mg/L. An optimal outcome is achieved when the remaining DO is approximately one‑third to two‑thirds of the initial concentration.

In practice, the expected oxygen demand of the sample is estimated from prior knowledge or preliminary tests, and three suitable dilution factors are selected to cover the likely range. For example, if the anticipated oxygen consumption is around 10 mg/L, dilution factors of 5×, 10×, and 15× may be chosen and run in parallel.