, 2 ML .
5.3 . THERMOMETER , RANGE UP TO 50* C , GRADUATED IN 0,1 OR 0,2* C .
5.4 . ANALYTICAL BALANCE .
5.5 . WATER BATH , THERMOSTATICALLY CONTROLLED AT 20 MORE OR LESS 0,5* C .
5.6 . VOLUMETRIC FLASKS , 50 AND 100 ML WITH GROUND GLASS STOPPERS .
6 . PROCEDURE
6.1 . RINSE A TUBE ( 5.1 ) WITH THE LABORATORY SAMPLE , FILL TO THE MARK WITH THE SAMPLE AND THEN PLACE IN A WATER BATH ( 5.5 ) AT A CONSTANT TEMPERATURE OF 20 MORE OR LESS 0,5* C . AFTER 20 MINUTES REMOVE THE TUBE FROM THE BATH , ADD 2 ML OF POTASSIUM PERMANGANATE SOLUTION ( 4.1 ) FROM THE PIPETTE ( 5.2 ) . NOTE THE TIME . STOPPER THE TUBE IMMEDIATELY . SHAKE WELL AND PLACE THE TUBE IN THE BATH AGAIN .
6.2 . OBSERVE THE CHANGE OF COLOUR OF THE SOLUTION AND COMPARE THE COLOUR BY TAKING THE TUBE OUT OF THE BATH FROM TIME TO TIME AND PLACING IT AGAINST A WHITE BACKGROUND NEXT TO A SECOND TUBE FILLED TO THE MARK WITH THE COLOUR STANDARD ( 4.4 ) . TAKE CARE NOT TO EXPOSE THE TEST SOLUTION DURING THE TEST TO THE ACTION OF DIRECT SUNLIGHT . NOTE THE TIME AT WHICH THE COLOUR OF THE TEST SOLUTION BECOMES EQUAL TO THAT OF THE COLOUR STANDARD .
7 . EXPRESSION OF RESULTS
7.1 . INTERPRETATION
THE TIME OF CLEARING SHALL BE THE TIME REQUIRED FOR THE COLOUR OF THE SAMPLE TUBE TO MATCH THAT OF THE STANDARD TUBE .
7.2 . REPEATABILITY
THE DIFFERENCE IN THE TIMES OF TWO TESTS , CARRIED OUT SIMULTANEOUSLY OR IN RAPID SUCCESSION , BY THE SAME ANALYST , ON THE SAME SAMPLE , UNDER THE SAME CONDITIONS , SHALL NOT EXCEED TWO MINUTES .
8 . NOTES
8.1 . TRACES OF MANGANESE DIOXIDE HAVE A CATALYZING EFFECT ON THE REACTION ; ENSURE THAT PIPETTES AND NESSLER TUBES ARE USED WHICH HAVE BEEN SCRUPULOUSLY CLEANED AND RESERVED EXCLUSIVELY FOR THE PURPOSE . CLEAN THEM WITH HYDROCHLORIC ACID AND RINSE THOROUGHLY WITH WATER ; NO BROWN COLORATION SHALL BE VISIBLE ON THE GLASS APPARATUS .
8.2 . THE QUALITY OF THE WATER USED TO PREPARE THE DILUTE PERMANGANATE SOLUTION ( 4.1 ) SHALL BE CAREFULLY MONITORED ; IT MUST NOT CONSUME ANY PERMANGANATE . IF THE REQUIRED QUALITY CANNOT BE OBTAINED , DISTILLED WATER SHOULD BE BROUGHT TO THE BOIL AND A SMALL QUANTITY OF PERMANGANATE ADDED IN ORDER TO OBTAIN A VERY SLIGHTLY PINK COLORATION . THIS SHOULD THEN BE COOLED AND USED FOR THE DILUTION .
8.3 . FOR SOME SAMPLES THE DECOLORATION MAY OCCUR WITHOUT PASSING THROUGH THE EXACT SHADE OF THE REFERENCE SOLUTION .
8.4 . THE PERMANGANATE TEST MAY BE DISTORTED WHEN THE ALCOHOL SAMPLE FOR ANALYSIS HAS NOT BEEN STORED IN A PERFECTLY CLEAN GLASS FLASK , SEALED WITH EITHER A GROUND GLASS STOPPER WHICH HAS BEEN RINSED WITH ALCOHOL OR ANOTHER