Pulse oximetry
Oximetry, often written oximetry, is the term for measuring the oxygen content. The determination of the amount of dissolved oxygen in liquids is of particular interest because of their 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. In the intensive care patient monitoring is primarily the method of pulse oximetry used.
For measurement, there are medical devices, which are known as oximeters and pulse oximeter.
Winkler method
The Winkler method was developed in 1888 by Lajos Winkler and based on the oxidation of doubly positively charged manganese ions by the dissolved oxygen. In general, water samples ( also of the oxidation states III and IV may ) was added in the field with the two reagents to the " fixing" of the oxygen as manganese hydroxide and titrated in the laboratory. On the Winkler method based DEV G -21 - Determination of dissolved oxygen - Iodometric method (DIN EN 25813:1993-01 ).
The fixing reagents are for a saturated solution of manganese (II ) chloride ( MnCl2 ), and the other a potassium -containing sodium hydroxide solution. With the addition of these reagents under exclusion of air to the sample, manganese (II ) hydroxide, which by the oxygen to manganese (III ) hydroxide forms, sometimes even to manganese (IV ) hydroxide or manganese (IV ) oxide - hydroxide (MnO (OH ) 2) is oxidized and precipitates as cinnamon to coffee- brown precipitate:
Or
After the addition of sulfuric acid or hydrochloric acid, manganese (III) is reduced by the iodide. Manganese (IV) above in an acidic medium still present manganese (II ) ions, is converted by a comproportionation to manganese ( III) ions.
The molar amount of iodine thus formed corresponds to twice the molar amount of oxygen initially present ( in moles) and is determined by titrimetry with sodium thiosulfate:
Here, the double molar sodium thiosulfate is implemented compared to the number of moles of iodine. In these reactions, therefore, corresponds to 1 mmol of oxygen 2 mmol thiosulfate ions. 1 ml of sodium thiosulfate solution to a concentration of c = 0.01 mol / l corresponds to 0.08 mg of oxygen.
The mass concentration of dissolved oxygen can be calculated with the following formula:
Polarographic method
In this method, oxygen is electrochemically reduced:
The electric current is discharged through electrodes and its size is used as a measurement signal for a measuring instrument which indicates the oxygen concentration in mg O2 / l. This is still widespread method goes back to 1897 when the first human applications reach the mid-19th century. The most popular implementation of this method is referred to as a Clark electrode. Here, the measurement cell is filled with an electrolyte in which the anode and cathode are, in turn, are separated by a permeable for oxygen semipermeable membrane. The reduction reaction usually takes place at the noble metal (eg gold) existing cathode. Due to its simplicity, this method is also suitable for use in the field, but it does not reach the accuracy of the Winkler method.
Luminescence method
While the theoretical work of this process dates back to the year 1947, was the practical implementation until 1987. This method utilizes the luminescence of a suitable phosphor ( luminophores ) of which is excited by exposure to normal light. The excitation energy is emitted lumiphores with different time constants to the oxygen molecules, which can be observed at a characteristic absorption in the intensity-time curve.
Depending on the material of the luminescent material (mostly the metalloporphyrin complex -albumin ) and the wavelength of the incident light with respect to the luminescence intensity and the maximum temporal decay of the oxygen concentration surrounding the irradiated material is dependent. Advantage over Winkler and polarographic method is the higher user-friendliness, because the luminescence method is, among other things drift. In addition, must not be changed or maintained at their membrane and electrolyte, since this place an oxygen-sensitive layer is employed.