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<br />ANALYTICAL -GENERAL <br />Hearine Instrucrions <br /> <br />Once the standards have been mixed, the colour is developed by hearing. For the <br />colorimetric standards, heat on a hot plate for 30 minutes (not less) near to <br />boiling (90° C). <br />The standards must be cooled to room temperature before they can be bulked up <br />and used. A water bath is commonly used. Unknowns must be at the same temperature <br />as the standards for best results since the colour intensity increases with temperature. <br />Reaeent Preparation <br />The following is the recipe for preparing "Buffered Picric Acid Reagept". <br />1) Dissolve 40 g DTPA and 16 g NaOH in approximately 600 mL pf distilled water <br />using a magnetic stirrer. <br />2) Add 6 g of moist picric acid (10% to 15% moisture content). if the moisture <br />content is 25% to 50%, drain excess water then weigh 7 g of the reagent <br />3) In a separate beaker, add 300 mL distlled water and either 14 g Na2B,O, or 27 <br />g Na2B,O~.lOH2O and warm until dissolved and add it to the 600 ml in step 1. <br />4) Add 8 g Na2C0, and stir until all is dissolved. <br />5) Cool to room temperature and filter through 0.45 µm paper (if necessary). <br />w„~~..,K.~ 13 <br />