A UNIVERSAL HYDROGEN-ION INDICATOR FOR CHEMISTRY DEMONSTRATIONS HENRY I. GOECKEL Cranford, New Jersey
A UNIVERSAL pH indicator suitable for lecture demon- Demonstrations. Arrange tubes in proper pH sestrations is described below. It is novel in that, with quence containing 10 ml. of sample solutions and add changing pH, the color sequence of this indicator 5 to 10 drops of universal indicator. Intermediate colors can be demonstrated by the follows the color sequence of the visible spectrum.' Preparation of Indicator. Dissolve the following in proper mixing of the stock solutions. The indicator 450 cc. of alcohol U. S. P. (or reagent grade iso-prepyl reaction of dilute acetic acid and mineral acids, acetic alcohol): Phenolphthalein, 100 mg.; methyl red, 200 anhydride, concentrated and fuming sulfuric acid, mg.; methyl yellow (dimethyl amino-azobenzene), 300 syrupy phosphoric acid, and phosphoric anhydride nrg.; hromthymol blue, 400 mg.; and thymol blue, will suggest many questions to the inquisitive student, 500 mg. Then add N/10 sodium hydroxide until the for some of the concentrated acids will register disred color disappears and the color becomes yellow tinctly alkaline, but upon dilution give the colors (pH 6). Finally, add sufficient alcohol to make 500 characteristic of acids. cc. The color changes are, respectively: pH 2, red; Indicator Chart. A chart showing the ranges of the pH 4, orange; pH 6, yellow; pH 8, green; and pH 10, varions indicators can be used to explain why certain blue. indicators are specified in titrimetry; i. e., because of Stock Solutions. Prepare normal solutions of the the transition point and distinctive visual change in following: (a) phosphoric acid, H,PO4, (b) monosodium color. The indicator is essentially a primary standard dihydric phosphate, NaHeP04, (c) distilled water (not for determining pH.% a solution), (d) disodinm monohydric phosphate For a comparison of pH within the limited range of NazHP04, (e) trisodium phosphate, NaaP04, and ( f ) one of the indicators in the universal mixture, prepare sodium hydroxide, NaOH. This gives an extended two solutions. To one add sufficient acid to produce pH sequence of common substances of the same ele- maximum acid color for the indicator; to the other ments, in colorless solution. In describing these sub- add sufficient NaOH to produce the maximum alkaline stances, call attention to the following facts of interest: color for the range. Ten tubes, each containing 10 (a) H3P04,a corrosive acid administered in very dilute ml. of the acidified, and 10 tubes, each containing 10 solutions for reducing fever. (b) NaHePO1, a de- ml. of the alkalized solutions, are set up for comparison. cidedly acidic, but noncorrosive salt; biologically The tubes are then paired off and the number of drops highly soluble and producing alkaline-acid unbalance. of indicator solution stated in the following table axe It is used medicinally to produce a high degree of added: Indicator Solution urinary acidity; this indicates the attempt by the living organism to eliminate excess acid in maintaining Acid tubes Drops normalacid-baseequilibrium. WithexcessNaH~PO~,~ l k ~ l i ~ ~D~~~~ t ~ b eg ~ 8 7 6 5 4 3 2 1 the balance is ultimately destroyed. (c) Distilled water, being neutral, fits into this pH sequence at this For closer readings the indicator may be diluted point. (d) N&HPOI is a mildly alkaline salt, bio- 10-fold. The unknown is prepared by adding 10 drops logically only slightly soluble; in large amounts it is of indicator solution to 10 ml. of unknown solution. a distinct mucous irritant, a cathartic capable of up- The pH can then be accurately determined by finding setting the metabolic balance in an entirely different which pair matches the unknown most closely, and log (x/lO x ) in manner. (e) Na3P04, a mild caustic used as a de- applying the formula: pH * pK which x is the number of drops in the alkaline tubes tergent. If) NaOH, highly caustic. of the series.
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1 For s more oomplete account of lecture demonstrstions with a universalmdicator see THISJOURNAL, 14,274 (1937)-Editor.
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U. S. Army Laboratory Manual, 5th edition.
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