Manual of basic techniques for a health laboratory - libdoc.who.int

28 Manual of basic techniques for a health laboratoryFig. 2.34 A demineralizerQuality controlApparatus with a control dialThe dial registers the resistivity of the water resulting from the presence of ions.The more complete the demineralization, the higher the electrical resistivity of thewater.1. Check that the control system is fitted with a battery in good working order.2. To check that the battery is charged, press the button marked “zero test”; theneedle on the dial should swing to zero (Fig. 2.35(a)).3. Let water flow into the cartridge.4. When demineralized water begins to flow out at the other end, press the buttonmarked “water test”. The needle should register a resistivity of over 2 megaohms/cm (2MW/cm) (Fig. 2.35(b)).5. If the needle stops at a point below 2MW/cm or stays at zero, the cartridge ofion-exchange resin granules has been used for too long and must be replaced orreactivated.The apparatus may indicate the resistivity (MW/cm) or the reciprocal value, theconductivity (cm/MW or Siemens, S).Apparatus without a control dialUsing an indicator paper, determine:— the pH of the water supply flowing into the apparatus, and— the pH of the demineralized water that flows out at the other end.If the pH remains the same (usually below 6.5), the resin is no longer active.Demineralized water should have a pH between 6.6 and 7.0.An additional check can be made using a 1.7% solution of silver nitrate (reagentno. 49). Pass a weak solution of sodium chloride (cooking salt) through theresin, then carry out the test described in section 2.4.2 for the quality controlof distilled water. If a slight whitish cloudiness appears, the resin must bereplaced.