A European standard “EN 50267-2-1 General test methods for cables under fire conditions - Tests on gases evolved during the combustion of materials from cables - Part 2-1: Procedures - Determination of the amount of halogen acid gas” describes a procedure for determining the amount of halogen acid gas, other than hydrofluoric acid, evolved during the removal of compounds based on halogenated polymers and compounds containing halogenated additives from cable structures.
For accuracy reasons, it is not recommended to use this method when the amount of halogen acid released is less than 5 mg/g of the sample taken. This method is not suitable for identifying compounds or materials defined as zero halogen or halogen-free. For such compounds or materials and for all compounds or materials containing less than 5 mg/g of halogen acid equivalent, it is recommended to use the method specified in “EN 50267-2-2 General test methods for cables under fire conditions - Tests on gases evolved during the combustion of materials from cables - Part 2-2: Procedures - Determination of the acidity of gases for materials by measuring pH and conductivity”.
For the purposes of this standard, combustion means the exothermic reaction of a substance with an oxidant and the emission of waste material, usually accompanied by flames or smoke emission.
When performing the test, in principle, the apparatus under test is heated in a stream of dry air and the gases absorbed into the 0,1 M sodium hydroxide solution are retained in the wash bottles. Each wash bottle should contain at least 220 ml of 0,1 M sodium hydroxide solution. The amount of halogen acid is then determined by acidifying the solution with nitric acid, adding a measured volume of 0,1 M silver nitrate solution and back titrating the excess with 0,1 M ammonium thiocyanate, using ferric ammonium sulphate as an indicator or any equivalent analytical method of at least the same accuracy.
Repeated tests should be performed on the sample and a blank determination should be made without the sample. The result should be taken as the average of the two determinations. Individual values should not vary more than plus/minus 10 percent from the mean.
A sample should consist of 500 mg to 1000 mg of the material. The sample should be taken from representative samples of the material and cut into small pieces. The samples should be stored at 16 percent plus/minus 50 percent relative humidity for at least 5 hours. After conditioning to the nearest 0,1 mg, the sample to be weighed is placed in a combustion vessel. The sample should be evenly distributed over the bottom of the vessel. The vessel is then placed in the combustion tube placed in the tube furnace.
The aluminum flow rate shall be adjusted by the rotation speed of the needle valve and shall be kept constant throughout the test. The sample shall be heated at an approximately uniform heating rate for a period of 800 minutes plus/minus 10 minutes to raise the sample temperature to 40 degrees plus/minus 2 degrees and then held at this temperature for 20 minutes plus/minus 1 minute. The heating rate and sample temperature shall be controlled by a suitable method.
When determining the halogen acid content, after cooling to ambient temperature, 200 ml of the solution is measured into a flask using a pipette or burette, and then 4 ml of concentrated nitric acid, 20 ml of 0,1 M silver nitrate and 3 ml of nitrobenzene are added in sequence. The contents should be shaken well to agglomerate and coat the silver halide. After this, 6 ml of a 40 percent aqueous solution of ferric ammonium sulfate containing a few drops of 1 M nitric acid is added and all is mixed together. The solution is then titrated with a 0,1 M ammonium thiocyanate solution using magnetic stirring.
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