Oximetry

Oximetry , often also spelled oximetry , is the name for measuring the oxygen content. The determination of the content of dissolved oxygen in liquids is of particular interest because of its great importance in medicine , pharmacy , water analysis and waste management . Today there are essentially three methods of oxygen measurement: the Winkler method, the polarographic method and the luminescence method. The pulse oximetry method is primarily used in intensive care patient monitoring .

Medical devices are available for measurement, which are called oximeters or pulse oximeters .

Winkler method

Precipitation of manganese (II) hydroxide with partial oxidation to brown manganese (III) or manganese (IV) oxide hydroxide by atmospheric oxygen

The Winkler method was developed by Lajos Winkler in 1888 and is based on the oxidation of doubly positively charged manganese ions by dissolved oxygen. As a rule, water samples in the field are mixed with the two reagents for "fixing" the oxygen as manganese hydroxide (oxidation level III and possibly also IV) and titrated in the laboratory. The DEV G-21 - Determination of dissolved oxygen - iodometric method ( DIN EN 25813: 1993-01) is based on the Winkler method .

The fixing reagents are on the one hand a saturated solution of manganese (II) chloride (MnCl ₂ ) and on the other hand a sodium hydroxide solution containing potassium iodide . When these reagents are added to the sample in the absence of air, manganese (II) hydroxide is formed , which is converted into manganese (III) hydroxide , sometimes even to manganese (IV) hydroxide or manganese (IV) oxide hydroxide (MnO (OH) ₂ ) is oxidized and precipitates as a cinnamon-brown to coffee-brown precipitate:

${\displaystyle \mathrm {Mn^{2+}+2\ OH^{-}\longrightarrow Mn(OH)_{2}} }$

${\displaystyle \mathrm {2\ Mn(OH)_{2}+0,5\ O_{2}+H_{2}O\longrightarrow 2\ Mn(OH)_{3}} }$

or

${\displaystyle \mathrm {2\ Mn(OH)_{2}+O_{2}\longrightarrow 2\ MnO(OH)_{2}} }$

After adding sulfuric acid or hydrochloric acid , the manganese (III) is reduced by the iodide . Manganese (IV) is first converted to manganese (III) ions in an acidic medium with manganese (II) ions still present via a comproportionation reaction .

${\displaystyle \mathrm {2\ Mn(OH)_{3}+2\ I^{-}+6\ H^{+}\longrightarrow 2\ Mn^{2+}+I_{2}+6\ H_{2}O} }$

The amount of substance of the iodine formed in this way corresponds to twice the amount of substance of the originally present oxygen (in mol) and is determined titrimetrically with sodium thiosulphate :

${\displaystyle \mathrm {2\ S_{2}O_{3}^{2-}+I_{2}\longrightarrow S_{4}O_{6}^{2-}+2\ I^{-}} }$

Here, twice the amount of sodium thiosulfate is converted compared to the amount of iodine.
In these reactions, 1 m mol of oxygen corresponds to 2 mmol of thiosulfate ions. 1 ml sodium thiosulphate solution with a concentration c = 0.01 mol / l corresponds to 0.08 mg oxygen.

The mass concentration of dissolved oxygen can be calculated using the following formula:

${\displaystyle \mathrm {\beta ={\frac {1}{4}}\cdot {\frac {M\cdot V_{2}\cdot c\cdot F}{V_{1}}}} }$ (in milligrams per liter)

variable	description
M.	molar mass of oxygen
V 1	Volume of the titrated sample (in ml)
V 2	Volume of sodium thiosulfate used (in ml)
c	Mole concentration of sodium thiosulphate (in mmol / l)
F.	V 0 / ( V 0 - V ′ )
V 0	Volume of the sample bottle (in ml)
V ′	Total volume of the added fixing reagents (in ml)

Polarographic method

In this method, oxygen is reduced electrochemically:

${\displaystyle \mathrm {O_{2}+2\ H_{2}O+4\ e^{-}\longrightarrow 4\ OH^{-}} }$

The electrical current is diverted via electrodes and its size is used as a measuring signal for a measuring device that indicates the oxygen concentration directly in mg O ₂ / l. This method, which is still widespread today, dates back to 1897 , and its first use on humans was in the middle of the 19th century . The best-known implementation of this method is known as the Clark electrode . The measuring cell is filled with an electrolyte containing anode and cathode , which in turn are separated from each other by a semipermeable membrane that is only permeable to oxygen . The reduction reaction takes place at the cathode, which is usually made of noble metal (e.g. gold ). Due to its simplicity, this method is also suitable for use in the field, but it does not achieve the accuracy of the Winkler method.

Luminescence method

While the theoretical work on this process dates back to 1947, it was not put into practice until 1987. This method uses the luminescence radiation of a suitable phosphor ( luminophor ), which is excited by radiation from normal light . The excitation energy of the Lumiphores is delivered to the oxygen molecules with different time constants, which can be observed in a characteristic damping in the intensity-time curve.

Depending on the material of the luminescent material (usually metalloporphyrin - Albumin - complexes ) and the wavelength of the incident light is the luminescence regarding maximum intensity and temporal. Decay depending on the surrounding material, the irradiated oxygen concentration. The advantage over Winkler and polarographic methods is their greater user-friendliness, since the luminescence method is, among other things, drift-free. In addition, the membrane and electrolyte do not have to be changed or serviced, as an oxygen-sensitive layer is used instead.

literature

• Leonhard A. Hütter: Water and water investigation, Verlag Diesterweg Salle & Sauerländer 1988, p. 335 ff, ISBN 3-425-05075-3

Web links

• Chemical investigation methods: Oxygen determination according to WINKLER , plingfactory.de

Individual evidence

1. ↑ Lajos Winkler : The determination of the dissolved oxygen in water . In: Reports of the German Chemical Society . 21, No. 2, pp. 2843-2855. doi : 10.1002 / cber.188802102122 .