The laboratory measurement of chloride ion concentration has become increasingly important in many fields, such as the quality management, food, beverage and industrial sectors, as well as in clinical laboratories. It is important to measure chloride ions in body fluids since, together with sodium and potassium ions, they play a vital role in maintaining normal water distribution between cells, plasma and interstitial fluid through the effect they exert on the osmotic pressure.
The most frequently used methods for measuring chloride are based on ion selective electrodes (ISE) or photometric technologies. However such approaches have limitations, especially when used outside normal conditions, and can be problematic, particularly with very low sample volumes of less than 20 µL. In addition, most classical chloride-measuring instruments are also capable of measuring several other parameters which, in practice, many laboratories do not need, thus creating an unnecessary cost burden. In all such cases a coulometric titrator is preferable.
The traditional way of measuring chloride in liquids is by the Volhard chloride titration method. This is carried out by first of all adding a known, excess amount of silver nitrate solution to the solution to be assayed. A precipitate of silver chloride is formed: Ag+ + Cl- → AgCl ↓
Ferric iron (III) is then added as an indicator and the solution is titrated with potassium thiocyanate solution. As long as there are free excess silver ions (the concentration of which depends on the amount of chloride ions in the original solution), the silver reacts with the thiocyanate ions to form a silver thiocyanate precipitate: Ag+ + SCN - → AgSCN↓
The solution remains pale yellow. However, when all the silver ions have reacted with the thiocyanate, the first excess of thiocyanate reacts with Iron(III) to form an easily detected dark red complex: Fe3+ + SCN - → [FeSCN]2+ ←
The concentration of chloride ions in the original solution can then be calculated by subtracting the mole amount of excess silver ions having reacted with the thiocyanate from the total mole amount of silver nitrate originally added.
Modern chloride titrators measure the amount of chloride in a sample directly, and are based on the principle of coulometric titration using an end point determined by measuring the current between two silver electrodes. The main difference compared with the classical Volhard method is that the acid buffer in the sample does not originally contain silver ions. Instead, these are generated through oxidation at the silver-containing anode. This is achieved by passing a generator current through the system. The silver ion concentration can be kept constant, under the control of the indicator electrodes.
When chloride is added, a precipitate of silver chloride is formed, causing the indicator current to decrease. This, in turn, results in an increased generation of silver ions from the anode to bring the silver ion concentration back to its original level before the chloride-induced precipitation. This process of generating fresh silver ions continues until all the chloride is precipitated. The concentration of chloride in the original solution can then simply be determined by measuring the length of time during which the silver-generating current flowed.
Modern chloridometer instrument
To meet the the current and future need for chloride determinations, a modern, cutting-edge digital instrument, the FKGO chloridometer [Figure 2] carries out the process described above precisely and automatically under tight electronic control. The instrument has an easy, intuitive user interface with a modern touch screen and multi-language user prompts, which lead the operator through the entire measurement process. The nominal sample volume is 20 µL, thus making the instrument suitable even for the analysis of sweat samples in the diagnosis of cystic fibrosis. However, the sample volume is not fixed and can be adapted to suit other assay requirements, e.g. those in industrial labs. The instrument comes factory-calibrated so manual calibration by lab personnel is not needed. Another feature is the optional dot-matrix printer, which provides a hard copy printout of the results. This means that copying thermal print-outs for long-term storage, which is necessary in many other measuring devices, is avoided. For computer-aided data evaluation and digital data storage a serial interface (RS-232) is available.
The FKGO Chloridometer meets all the needs a laboratory may require of a modern, accurate chloride titrator. The touch-screen, dot-matrix printer and the RS-232 interface of the chloridometer makes the life of laboratory staff easier and more convenient, without any sacrifice in data quality.