How ionic composition of Tissue fluids is determined

Determination of ionic composition of Tissue fluids is accomplished using a number of chemical analytical methods with the help of various instruments.

We shall look at some of these methods used to estimate the ionic composition of tissue fluids based on the elements analysed.  

Sodium and Potassium

As discussed in the previous article, Sodium and Potassium are routinely estimated by a Flame Photometer. In this instrument, a burner using an Air-gas mixture produces a normally non-luminous flame. Into this flame, is introduced a fine spray of a dilute solution of the ions to be estimated, at a constant rate. Solute atoms are excited at the high temperature, outer shell electrons are moved up to higher energy levels, and falling back emits quanta of light at wavelengths characteristic of the atomic species present. This emitted light is filtered into its separate wavelengths by optical means and the intensity of one wavelength corresponding to each of the species present is measured by means a photocell and recorded on a meter or chart recorder.

A typical concentration of Sodium or Potassium ion sprayed into the flame is about 0.5 mmol/l, which is quite dilute, considerable care must be taken to use very pure distilled water and scrupulously clean glassware.

The optical filtering is normally carried out by means of suitably dyed transparent film, although better instruments use half-wave filters, in which two glass plates are mounted with great accuracy at a spacing of a half wavelength at the desired optical wavelength.  For laboratory use a prism or grating spectrometer may be used for analysis; in this  instrument the wavelength to be measured maybe continuously varied by rotating a prism or optical grating by means of a finely calibrated dial.

In simple instruments, the photocell is used as source of current, which is measured by being passed through a sensitive microammeter. With care an accuracy of about 1% can be obtained from this type of instrument; by repeated comparison between standard solution and the unknown one. The more expensive instruments use a photomultiplier tube, whose output current is passed through a resistor and the resulting voltage drop measured by a digital voltmeter or by a potentiometer.

Calcium, Magnesium, and Trace ions

Many ions, including Calcium and Magnesium are insufficiently ionised in an air-gas flame to emit much light, even if acetylene is used as gas. These, however, may readily be estimated by use of an atomic absorption spectrometer. This instrument consists of an air-gas burner (usually air-acetylene) into which the solution to be estimated is sprayed. Light is passed through the flame from a special discharge lamp containing a trace of the element being investigated; the emitted light from this lamp contains the characteristic wavelength for that element. This light is received by a grating spectrometer and photomultiplier, which operates an output digital/voltmeter or potentiometer. The spectrometer is set to one of the wavelengths from the lamp, and the meter is adjusted to give a full scale reading when distilled water only is sprayed. When the solution to be estimated is sprayed, the flame absorbs energy from the lamp, if the element being investigated is present, and the meter reading drops by an amount which is a function of the amount of element present.

Very small concentrations of ions may be estimated in this way; this method is particularly useful for detecting toxic amounts of heavy metals, in food stuffs or accumulated in the body.

Chloride

Chloride estimations are usually performed by titration with a solution containing silver ions, the end point is reached when the precipitation of solid, silver chloride just ceases. In practice this is impossible to determine by eye, and therefore an electrometric titration is performed. This is based on the concept of reversibility of a silver chloride. If two silver electrodes have a potential of about 0.2 V applied between them as shown in the diagram below, a significant current will flow in the circuit only if free silver ions are present in the solution; this can only occur after all chloride ions have been precipitated.

The electrodes are wires of spectroscopic grade silver, of about 1 mm diameter. To give a sharp end-point, the conductivity of the solution is increased by the use of a supporting electrolyte. It is usual to dilute the original chloride-containing biological solution considerably by a known fraction and this is done using a solution of 0.1 mol/l chloride-free nitric acid with 10% acetic acid added, rather than with distilled water.

A few drops of chloride-free gelatin solution are added to give a uniform precipitate, and the solution is well stirred during the titration. The end point is reached when the meter suddenly commences to swing upward; an arbitrary point of 20 μA is suitable as a threshold. The small zero error this produces may be allowed for by titration of a chloride-free solution.

Instead of adding silver ion by means of a burette in the conventional way, Cot-love’s method may be used. Silver ion is added electronically, by inserting two more silver electrodes in the solution, and passing a constant current between them (9.46 mA will precipitate 0.1 μmol/sec. The time for which the current is passed before the end point is reached is then a measure of the amount of chloride present.