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Potassium dichromate is added to the acidified sample as an oxidizing agent along with silver sulfate as a catalyst; mercury sulfate is also added to prevent the formation of elemental chlorine from chlorides. After oxidation of the organic substances in the sample (the dichromate ion is reduced to the chromium(III) ion in an acidic solution), the chromate required to achieve this is determined through reverse titration of the excess potassium dichromate with iron(II) solution (adjusted) against ferroin as an indicator [1].

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O

The water sample is diluted with oxygen-rich water “free of oxidizable organic substances” so that after the incubation period there is still at least 2 mg oxygen per liter left in the water. Afterward, the dissolved oxygen in the diluted sample should be determined immediately and then again after five days using an iodometric method or with oxygen electrodes. The difference between the two values yields the BOD₅ (taking into account the dilution factor).

The basis for these O₂ measurements is the detection of photoluminescence produced by an oxygen-sensitive layer. The change in photoluminescence depends on the partial pressure of the oxygen. Given the values for the partial pressure of the oxygen and the temperature, the amount of oxygen gas dissolved in the liquid can be calculated. The oxygen sensor determines the O₂ content of the liquid by means of optical detection through a photoluminescent process, in which an oxygen-sensitive layer is exposed to blue light. In doing so, the molecules in this layer become excited and reach a higher energy state. In the absence of oxygen, the molecules emit a red-colored light. If oxygen is present, it collides with the molecules in the oxygen-sensitive layer. The molecules in the oxygen-sensitive layer, which have collided with oxygen, cease to emit light (refer to figure 1). For this reason, a relationship exists between the oxygen concentration and the intensity of the emitted light as well as the intensity and the rapidity with which the intensity of the light diminishes. The intensity of the light is reduced at higher oxygen concentrations, although the rate at which it does so increases. The temperature of the product and the time interval between the light signal and the emission of light (phase shift) are both measured and used to calculate the oxygen content.

The device’s construction enables the state of the blue LED to be monitored using a photodiode. Another photodiode – with a red filter – measures the oxygen-dependent red light (refer to figure 2). This light is emitted by the luminophores due to photoluminescence (fluorescence) after they reach an excited state through exposure to the blue light. As a result of this exposure, the electrons of the luminophores are elevated to a higher energy level. As they return to their original energy level, they emit a red light.

Oxidizable substances in the water sample react with the sulfuric acid potassium dichromate solution in the presence of silver sulfate as a catalyst. Chloride is masked with mercury sulfate. The decrease in the yellow color of Cr⁶⁺ is measured.