Introduction
The determination of the halogene content in chemical substances by elemental analysis should be applied to prepared organic substances containing Cl and/or Br besides the constituents C,H (N,S) and O.
The determination of chlorine and bromine is done by mineralization followed argentometric titration.
The Mineralization is done by thermal decomposition in pure oxygen using either an electric oven or combustion according to Schöninger.
Besides that argentometric titration can be used to directly observe the chloride/bromide content of dissociating or hydrolizing compounds. In this case the sample is extracted or dissolved in an aqueous solution and the solute is investigated as described below without any mineralization step.
Method
Mineralization according to Schöninger
Mineralization in the electric oven
The sample is weighed in to an open platinum boat. The necessary amount of material is in the range from 2 to 7 mg. If substances containing little carbon are under investigation the sample weight may be as high as 50 mg.
The platinum vessel is placed in a fused silica tube. Pure oxygen is flushed through the system. Then a small electric oven slides along this tube heating the contents to about 900 to 950 °C. This makes the substances decompose. Oxidation reactions are completed in the following part of the fused silica tube which is stacked with quartz wool and contains a little platinum net as a catalyst. This part is heated by a stationary oven and the reaction temperature is at about 700 °C.
An absorption tube is attached to the fused silica reaction tube. The gaseous reaction products are collected there in an alkaline absorption solution containing hydrogen peroxide at room temperature.
When the mineralization is finished the absorption solution is flushed out using distilled water and about 10 to 20 ml of analyte solution are gained in a titration beaker. This solution contains the halogenides Cl- and Br-. Mind that fluorine cannot be detected as the HF gas immediately decomposes in contact to the large excess of silica to form SiF4. Iodide has a redox potential that enables elementary iodine to form at the oven outlet and can thus not be determined by argentometric titration.
Besides the reactions mentioned above the analyte solution also contains sulfate, carbonate (which readily evolves as carbon dioxide from the fairly acidic solution), nitrite, and nitrate.
Determination by Argentometry
The analyte solution is acidified by two to three droplets of 1:1 nitric acid to guarantee there is an acidic milieu. Then a silver electrode is immersed.
The titration is performed using an auto titrator (DL 21, Mettler-Toledo 1989) equipped with a 1 ml/2.000 steps micro burette filled with 0,1 N AgNO3 solution (Titrisol, Merck).
The data are transferred to a PC to perform improved evaluation. The evaluation process is semi automatic and gains a best fit between the data recorded and a theoretical curve.
The blank is run using benzoic acid which is mineralized in the freshly purified platinum boat.
Single point calibration is performed using chloro-benzoic acid and bromo-benzoic acid standard materials for elemental analysis.
Determination using Capillary Ione Electrophoresis
Working Range
Analysis recovers all the analyte elements in the sample. Various technical products ranging from PVC (57 w-% Cl) to biogene materials (<0,05 w-% Cl) have been investigated besides ordinary organic compounds.
The limit of determination has been found at about 0,05 w-% (500 ppm) with 6 to 10 mg of sample per analysis. The uncertainty was estimated to be at 0,02 w-%.
In the typical concentration region for organic material the uncertainty stays well below the 0,3 w-% limit and can be proven to be within 0,1 to 0,2 w-%.
Interferences
Handling of volatile substances or solvent mixtures are problematic. Normally samples are manipulated using a capillary for weighing to prevent evaporation. In investigation of mixtures the selective loss of the most volatile components can be a problem. In the analysis of highly volatile liquids the use of a Hamilton syringe has been tried successfully. Thus the sample could be introduced to the platinum boat immediately at the mineralization oven.
It is well known that some metals als Na, K, Mg, and Ca are not fully reacting to the oxides but still bind some halogenide under the degstion conditions. This can lead to significant daviation of the analysis result from the true halogene content. Addition of vanadine pentoxide (V2O5) can minimize this effect and helps to find the true values within 0,3 w-% as normally required as the metals preferably form vanadates and the hydrogen halogenides are easily released.
Mineralization of compounds and mixtures including elements which form volatile halogenides at 900 °C as Al, Cu, Zn, Ti etc. leads to interferences too. At high etla content eventually no significant result can be gained at all. Mind that under such circumstances hydrolysis of a substance or mixture under mild conditions can lead to a digest that can readily be investigated by argentometry or Capillary Ion Electrophoresis
Important Advice
We need as much information about your sample as available to be able to provide you with best results according to the state of the art.
Declaration of the expected elemental composition (sum formula) is required for standard elemental analysis on preparative samples. Additionally we need to be informed about remarkable instability and/or sensitivity of the material to bring your samples into the analysis in the expected state (e.g. how to store, dry etc.).
With technical products we require knowledge about the assumed amount of analyte in the sample as well as known additives. If we know about the background of analysis (e.g. verify the limit of 0,2 w-% halogene in waste material) we can guarantee results fitting your purposes best.