CHAPTER VI. 



INDICATORS AND INDICATOR SOLUTIONS 

 Revised 1951 by J. D. Ostrow 

 Reference: Clark, W. M. , The Determination of Hydrogen Ions. (3rd Ed) 



INDICATOR SOLUTIONS: 



In the colorimetric determination of pH's, aqueous solutions con- 

 taining 0.04% of the indicator are generally used. However, to simplify 

 preparation and increase stability, stock solutions of 0.4% concentration 

 are first prepared, and the test solutions made up as needed by dilutions 

 of the stock. Since the indicators, as such, are insoluble in water, they 

 are first converted to their monosodium salts, prior to dissolution in the 

 specified volume of water. 



To prepare the 0.4% stock solutions: weigh exactly 1.00 g. of the 

 indicator into an agate mortar. Add the number of ml. of 0.05N NaOH 

 specified in the table below, and grind with an agate pestle until dis- 

 solved. Transfer quantitatively, with rinsing, to a 250 ml. volumetric 

 flask, and dilute to the mark with distilled water. 



To prepare the 0.04% test solutions: dilute 1 part of the stock 

 solution with 9 parts of distilled water, using accurately calibrated 

 pipettes and flasks. 



INDICATOR SETS: 



For determination of pH' s to the nearest 0.1 pH unit, sets of indi- 

 cator color standards are used. Each set covers the detectable range of 

 color change of a given indicator, as listed in the table below, and pro- 

 ceeds in steps of 0.2 pH units, intermediate values being estimated visually. 



Each set consists of nine colorimetr ical ly matched 15 X 150 mm. test 

 tubes, each calibrated at 10 ml. If the tubes cannot be matched on a 

 colorimeter, the following method will suffice: all tubes whose 10 ml. 

 graduation falls at the same height above the base of the tube will have 

 the same inside diameter, and therefore be color imetr ical ly identical, 

 except for variations in the thickness of the walls of the tubes. They 

 may, therefore, be used in the same indicator set, with little resultant 

 error . 



Each tube is filled to the mark with buffer solution at the proper 

 pll, tlie bottom of the meniscus being read in each case. Five drops, or 

 0.25 ml., of the 0.04% indicator test solution is then added. The tube. 



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