ISO-6840-1982.pdf

International Standard 6840 0 d!i!i 4 INTERNATIONAL ORGANIZATION FOR STANDARDIZATlON.MEWYHAPOLIHAR OPfAHM3AUb4R i-i0 CTAH/lAPTH3Al.WlH.ORGANlSATlON INTERNATIONALE DE NORMALISATION Cationic surface active agents (hydrochlorides and hydrobromides) - Determination of critical micellization concentration - Method by measurement of counter ion activity Agents de surface cationiques (chlorhydrates et bromhydrates) - D over a certain concentration range, which is usually very narrow, these curves show a singularity. This phenomenon has been attributed to the formation of oriented aggregates of molecules, and the concentration at which this change occurs has been termed the “critical micellization con- centration” (c.m.c.1. For a given hydrophobic chain length, the critical micellization concentration is usually higher in the case of ionic surface ac- tive agents than in the case of non-ionic surface active agents. Apart from the structure of the surface active agent, the temperature and the presence, nature and quantity of metallic salts, organic compounds cause the critical micellization con- centration to vary, and hence the operational conditions likely to influence the result must be clearly stated, i.e. the purity of the product and, as far as possible, its impurities, which may greatly alter the value of measurements of potential (it is very useful to know the organic or mineral nature of these impurities and, if possible, their concentrations). 1 Scope and field of application This International Standard specifies a method for the deter- mination of the critical micellization concentration of cationic surface active agents (hydrochlorides and hydrobromides) in solution in distilled water or other aqueous systems, by measurement of the counter ion activity using a membrane electrode. The method is applicable to purified or unpurified cationic sur- face active agents (hydrochlorides and/or hydrobromides), which are soluble in water and which have a Krafft temperature below 60 OC. 3 Definitions 3.1 micelle : See ISO/R 862. 3.2 critical micellization concentration (c.m.c.) : See ISO/R 862. 3.3 Krafft temperature : See ISO/R 862. 4 Principle Potentiometric determination of the chloride or bromide ion ac- tivity of a series of solutions of cationic surface active agents the concentrations of which cover the expected critical micellization concentration. Plotting the graph of potential as a function of the logarithm of the concentration; the c.m.c. cor- responds to the singularity on this curve. 5 Reagents During the analysis, use only reagents of recognized analytical grade and only distilled water or water of equivalent purity. 6.1 Sodium chloride and sodium bromide, standard solu- tions of concentration approximately IO-4 to IO-2 mol/l. 6 Apparatus Ordinary laboratory apparatus and : 6.1 Polycrystalline membrane electrode (sensitive to chlorides (Ag$ + AgCI) or to bromides (AgzS + AgBr). NOTE - It is very important to clean the electrodes according to the manufacturer s instructions. 2 References IS0 607, Surface active agents and detergents - Methods of sample division. 6.2 Reference electrode : mercury-mercury(l) sulphate with a bridge containing saturated potassium sulphate solution, or a calomel electrode with a double bridge. In the latter case, the outer bridge shall be filled with saturated potassium nitrate solution. ISOIR 862, Surface active agents - Glossary. NOTE - If the bridge contains sulphate ions, the most concentrated and most dilute surface active agent solutions should be tested with a 1 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/28/2007 07:34:19 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6840-1982 (El portion of this bridge solution and no precipitation should be observed. If a precipitate does form, another type of bridge should be used. 6.3 Potentiometer, sensitivity 2 mV (potential of -500 to +500 mV) with a multi-range scale facility. 6.4 Thermostatically controlled water-bath, enabling the temperature of the solution to be controlled to with- in f 0,5 OC. 6.5 Jacketed glass vessel, with an opening suitable for in- troducing the electrodes. 6.6 Magnetic stirrer. 7 Sampling The laboratory sample of cationic surface active agent shall be prepared and stored according to the instructions given in IS0 607. 8 Procedure 8.1 Test portion and preparation of solutions Weigh, to the nearest 0,l mg, a quantity of the laboratory sam- ple such that 500 ml of a solution about ten times more concen- trated than the c.m.c. is obtained, dissolve in hot water and dilute to 500 ml in a one-mark volumetric flask. Let the concen- tration of this solution be c. Then, prepare by dilution 200 ml each of solutions of concentration c/2 - c/4 - c/8 - c/16 - cl32 - cl64 and c/128. Keep these solutions in the thermostatically controlled bath (6.4) at the measurement temperature for at least 1 h, but not more than 3 h, before the measurements. 8.2 Measurement temperature In order to reduce the effects of thermal and electrical hysteresis, take care that the temperatures of the electrodes, of the water used for washing, of the standard solutions and of the test solution are as close to each other as possible. The temperatures of the standard solutions and the test solution shall not differ by more than 0,5 OC. The measurement temperature should be 20 C whenever possible. 8.3 Calibration and controls of the potentiometer Calibrate the potentiometer (6.3) fitted with the membrane electrode (6.1) and the reference electrode (6.2) with standard reference halide solutions, the halide ion concentrations of which shall include the lowest and highest concentrations of halide ion which will be found in the surface active agent solu- tions on which measurements are performed. Assemble the potentiometer (6.3) as required, and switch on. Allow the potentiometer to operate, according to the manufac- turer s instructions, for a sufficient time to obtain good electric stabilization before beginning the measurements. Take care that the internal liquid of the reference electrode (6.2) is in equilibrium with atmospheric pressure, so that there is no in- hibition to its movement across the bridge. Note the temperature of the standard reference solutions, make the ap- propriate adjustments to the temperature-correction circuit, and check the zero of the potentiometer. These settings shall not be modified during the measurements. Introduce, in turn, each of the standard halide solutions (5.1) into the jacketed glass vessel (6.51, stir using the magnetic stir- rer (6.6) and immerse the electrodes at the same time. Continue stirring until the reading has become constant, i.e. for at least 60 s. Stop the stirrer before taking the final reading. Plot a calibration graph with the values of potential, in millivolts, as ordinates and the logarithms of the halide ion concentrations as abscissae. Verify that the halide ion concentration is a strictly linear func- tion of the measured potential. NOTE - The comparison with the calibration graph permits the calculation of the purity of the tested cationic surface active agent. 8.4 Determination of c.m.c. Proceed in the same manner as 8.3 using the solutions of con- centrations c/2 - . and c/128 (see 8:1), starting with the most dilute and finishing with the most concentrated solution. 8.5 Plotting the graph Plot a graph having the values of potential, in millivolts, as or- dinates and the logarithms of the surface active agent concen- trations, in grams per litre, as abscissae. The graph cor- responds approximately to two straight lines. NOTE - If the assumed c.m.c. was correct, four measured values of potential should be above and below the c.m.c. If this is not the case, prepare a new series of solutions based on the new estimate of the c.m.c. and measure the halide ion concentration, i.e. the potential. It is recommended to use smaller concentration steps in this second series. 9 Expression of results 9.1 Graph of potential From the graph (8.5) of potential as a function of the logarithm of concentration, determine the c.m.c. of the cationic surface active agents, expressed in grams per litre,ll as the abscissa corresponding to the intersection of the two straight lines. . I) In the case of chemically pure products, the c.m.c. may be expressed in moles per litre. 2 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/28/2007 07:34:19 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6840-1982 1E) 9.2 Reproducibility The difference between the results obtained on the same sam- ple in two different laboratories should not differ by more than 5 % from the mean value found. cl the range of concentrations used; d) the value of potential, as a function of the halide ion concentration; e) the types of electrodes used; 10 Test report The test report shall include the following particulars : a) the characteristics of the products, giving the fullest possible details relating to appearance, purity, content of active material and the relative molecular mass; b) the temperature of dissolution or dilution and the temperature at which the potential was measured; f) the graph showing the values of potential as a function of the logarithms of the concentrations; g) the numerical value of the c.m.c.; h) any operations not included in this International Stan- dard, or regarded as optional, as well as any incidents which may have affected the results. 3 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/28/2007 07:34:19 MDTNo reproduction or networking permitted without license from IHS -,-,-