284 TITRIMETRIC METHODS 



Alkaline Indicator Solution. Add 0.5 ml. M/15 secondary sodium 

 phosphate to 2 ml. 40 milligram per cent bromocresol green (pH 

 4.6) and 2.5 ml. boiled water. Prepare fresh each day and keep 

 in well-closed vessel. 



0.01 N Hydrochloric Acid containing 10 ml. 40 milligram per cent 

 bromocresol green/100 ml. 



Color Standard. Prepare by titrating a blank (45 /xl. water + 7 fi\. 

 0.015 A'' sulfuric acid + 18 /xl. indicator soln.) with the 0.01 N 

 hydrochloric acid until the color matches that of the indicator 

 in citrate buffer at pK 4.6. The slight difference in shade of the 

 indicator in citrate buffer makes the titsated blank the better 

 color standard. 



PROCEDURE 



1. Pipette a drop A of distilled water (about 20 /xl.) onto a clean 

 paraffin block (Fig. 94). 



Fig. 94. Use of the paraffin block 

 in transfer of the digest. 

 From Brilel et at. (1946) 



Paraffin block 



2. Draw up about one third of A into a hand pipette without a 

 constriction (Fig. 55, page 175) and carefully introduce the stem 

 of the pipette into the digestion tube until the tip touches the center 

 of the concave bottom of the tube. See that the pipette stem is 

 properly centered in the tube so that liquid is not drawn up along 

 the walls. Warm the tube just before adding the water to prevent 

 crystallization of potassium sulfate, which might block the pipette. 



3. Blow out the water into the digested sample (prepared as 

 described on pages 234-236) and immediately suck up the mixture 

 into the pipette. Deposit the liquid as a drop (B) on the paraffin 

 block. 



4. Draw up another third of A into the pipette and use the water 

 to wash the walls of the digestion tube. Draw the liquid back into 



