In aqueous (water-based) solution, acidity is defined as pH below seven. Several methods can reveal presence, and extent, of acidic character. Titrations, indicator paper and digital pH meters can all determine pH, and therefore acidity. Each of these methods has advantages and disadvantages. Typically, acidity tests involve a trade-off between cost of determination and precision. Corrosion can hint at acidic behavior. Redox reactions can be analyzed and, in conjunction with other theory and/or experimental data, reaction acidity can be determined.
Perform a titration. In a titration, the unknown solution, which could be acidic or basic, is neutralized with the “other” substance class. An acidic solution will eventually respond to addition of base, such as sodium hydroxide (NaOH). Color-changing indicators are added to titrated solutions in order to visually determine approximate pH range and therefore extent, if any, of acidic characteristics.
Use indicator paper. Indicator paper is a quick and reliable way to find solution pH. Place a drop of unknown solution on the paper and watch for the instant color change. Using the Hydrion Jumbo Size pH paper as an example, orange and red color indicates acidic characteristic. Remember to place solution sample onto a strip of paper. Do not dunk paper into the solution.
Take advantage of digital pH meters. These instruments measure pH to within as little as 0.02 pH units. Digital instruments let the user know not just whether or not a solution is acidic, but how acidic to an extent that few other methods can match. Adjustments for temperature (pH varies slightly with temperature change) are available on many marketed pH meters.
Investigate unusually fast corrosion. Acidic liquids tend to corrode metals such as copper. Note that acid is not the only corrosive factor. Bases, salts, electric current and suspended grit (sand, for example) can accelerate corrosion. If these other factors are ruled out, metal corrosion can be attributed to acidic solutions. Confirm acidic corrosion with other methods. Digital pH meter or titration analysis can solidify confidence in corrosion cause.
Analyze redox reactions. By observing changes in product formation with changing acid (H+ source) concentration, it is possible to calculate pH and therefore extent of acidity. Washington University in St. Louis demonstrates this method in detail using aluminum and manganese oxide-hypochlorous acid redox processes.
Remember that pH is a logarithmic scale. Solution with pH=4 is 100 times more acidic than solution with pH=6.
If doing experimental work, make sure to follow safety procedures and use common sense when dealing with strong acids (and strong bases).