2.5.12. WATER: SEMI-MICRO DETERMINATION …

1 Complexometric titrationsEUROPEAN PHARMACOPOEIA oflithium carbonate Rand dry in an oven before the substance to be examined in the paper and placeit in the sample carrier. Introduce into the flaskwater Rorthe prescribed solution designed to absorb the combustionproducts, displace the air with oxygen by means of a tubehaving its end just above the liquid, moisten the neck of theflask withwater Rand close with its stopper. Ignite the paperstrip by suitable means with the usual precautions. Keep theflask firmly closed during the combustion. Shake the flaskvigorously to completely dissolve the combustion and after about 5 min, unless otherwise prescribed,carefully unstopper the flask.

2 Wash the ground parts and thewalls of the flask, as well as the sample carrier, withwater the combustion products and the washings andproceed as prescribed in the COMPLEXOMETRIC TITRATIONSALUMINIUMI ntroduce mL of the prescribed solution into a 500 mLconical flask, add mL M sodium edetateand10 mL of a mixture of equal volumes of a 155 g/L solutionofammonium acetate Randdilute acetic acid R. Boil for2 min, then cool. Add 50 mL ofethanol Rand 3 mL of a freshlyprepared g/L solution ofdithizone Rinethanol excess of sodium edetate M zinc sulfateuntil thecolour changes from greenish-blue to M sodium edetateis equivalent to mg of the prescribed solution into a 500 mL conicalflask.

3 Dilute to 250 mL withwater Rand then, unlessotherwise prescribed, add dropwise, with shaking,concentratedammonia Runtil the mixture becomes cloudy. Add mLofnitric acid Cuntilthecloudinessdisappears completely. Add about 50 mg ofxylenol orangetriturate Rand titrate M sodium edetateuntil thecolour changes from pinkish-violet to M sodium edetateis equivalent to mg of the prescribed solution into a 500 mL conical flask,and dilute to 300 mL withwater sodiumhydroxide solution Rand about 15 mg ofcalconecarboxylicacid triturate M sodium edetateuntil thecolour changes from violet to full M sodium edetateis equivalent to mg of the prescribed solution into a 500 mL conical flaskand dilute to 300 mL withwater buffer solution pH Rand about 50 mg ofmordantblack 11 triturate R.

4 Heat to about 40 C then titrate at thistemperature M sodium edetateuntil the colourchanges from violet to full M sodium edetateis equivalent to mg of the prescribed solution into a 500 mL conical flaskand dilute to 200 mL withwater triturate Randhexamethylenetetramine Runtil thesolution becomes violet-pink. Titrate M sodium edetateuntil the violet-pink colour changes to M sodium edetateis equivalent to mg of the prescribed solution into a 500 mL conical flaskand dilute to 200 mL withwater orange triturate Randhexamethylenetetramine Runtil the solution becomes violet-pink. Add 2 g ofhexamethylenetetramine Rin excess. Titrate Msodium edetateuntil the violet-pink colour changes to M sodium edetateis equivalent to mg of :20512corrected WATER: semi -MICRODETERMINATIONThe SEMI-MICRO DETERMINATION of water is based upon thequantitative reaction of water with sulfur dioxide and iodine ina suitable anhydrous medium in the presence of a base withsufficient buffering apparatus consists of a titration vessel with: 2 identical platinum electrodes; tight inlets for introduction of solvent and titrant; an inlet for introduction of air via a desiccant.

5 A sample inlet fitted with a stopper or, for liquids, a systems for introduction of dry nitrogen or for aspirationof solvents may also be titration is carried out according to the instrumentsupplier s instructions. Care is taken throughout thedetermination to avoid exposure of reagents and solvents toatmospheric moisture. The end-point is determined using 2identical indicator electrodes connected to an electrical sourcethat maintains between the electrodes either a constant currentor a constant voltage. Where direct titration is used (method A),addition of titrant causes either a decrease in voltage whereconstant current is maintained or an increase in current whereconstant voltage is maintained, until the end-point is with automatic end-point detection are To the titration vessel, addmethanol R,dried if necessary, or the solvent recommended by the supplierof the titrant.

6 Where applicable for the apparatus used,eliminate residual water from the measurement cell or carryout a pre-titration. Introduce a suitable amount of water in anappropriate form (water Ror a certified reference material)and carry out the titration, stirring for the necessary time. intervals otherwise prescribed, use Method A. Introduce into the titration vesselmethanol R,orthe solvent indicated in the monograph or recommended by thesupplier of the titrant. Where applicable for the apparatus used,eliminate residual water from the measurement cell or carry outa pre-titration. Introduce the substance to be examined rapidlyand carry out the titration, stirring for the necessary R,orthe solvent indicated in the monograph or recommended by thesupplier of the titrant.

7 Where applicable for the apparatus used,eliminate residual water from the measurement cell or carry outa pre-titration. Introduce the substance to be examined rapidlyand in a suitable state of division. Add an accurately measuredvolumeofthetitrant,sufficienttog iveanexcessofabout1mLor the prescribed volume. Allow to stand protected from lightfor 1 min or the prescribed time, with stirring. Titrate theexcess of reagent usingmethanol Ror the prescribed solvent,containing an accurately known quantity of the information section on general monographs (cover pages)