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Determining the pH of a Solution
 

Most professional laboratories today use a digital pH meter the read the pH of solutions directly. These instruments provide a very quick and precise reading. Bench type pH meters (right) have the electrode on an adjustable arm and are capable of large-scale use, with results to five significant figures.

Portable pH meters (right) are battery-powered for use in the field. They are obviously less expensive, but still provide readings with two or three significant figures. These are common in high school chemistry labs

pH meters are calibrated by testing their reading of a buffer solution of a known pH.

Before pH meters, chemical indicators were used to determine pH. Indicators are weak organic acids and bases whose colors differ from the colors of their conjugate acids or bases.

Liquid indicators have two major weaknesses:

(1) they only provide a "ballpark" determination of pH

(2) they only work well with solutions that begin "colorless"

But they are still useful for basic testing. You may even have used an indicator if you have a swimming pool or hot tub. When using liquid indicators in a general chemistry lab, the color is best viewed from above against a white background.

The most convenient "indicators" are test paper strips. When using these in the lab, place a drop of the solution to be tested onto the paper - DO NOT dip the paper in the solution (unless directed to do so). This may cause contamination of the solution.

The simplest test strip is Litmus paper, which will just indicate if a solution is acidic or basic.

  • Red litmus paper turns blue in a base.
  • Blue litmus paper turns red in an acid.
There is a litmus liquid with similar color response.

Hydrion paper (simply called "pH paper") is similar to litmus paper, but provides a range of pH values instead of just acidic or basic.

  • The color changes possible with Hydrion paper are shown on the container.
  • These are available in several pH ranges, depending on the type of solution to be tested.
There are many different liquid indicators. The two most often found in general chemistry labs are Phenolphthalein and Universal Indicator solution.

Universal Indicator Solution is useful because the solution experiences a wide range of color changes - making it useful for many different tests.

This liquid indicator is similar to pH paper, because of its several possible colors, indicating several possible pH values.

Phenolphthalein is a common indicator used in neutralization titrations. The "end point" of the titration has been reached when the solution holds a very faint pink color for half a minute or more.
 
Phenolphthalein Color Chart
colorless in an acid
pH below 7

Acidic

faint pink in a neutral solution
pH = 7

Neutral

dark pink in a base
pH above 7

Basic

 

Other liquid indicators and their color change range

Acid / Base Indicators
IndicatorLower ColorpH RangeUpper Color
methyl violetyellow-green0.0 - 2.5violet
methyl orangered2.5 - 4.4yellow
congo redblue3.0 - 5.0red
bromocresol greenyellow4.5 - 5.5blue
methyl redred4.8 - 6.0yellow
bromocresol purpleyellow-green5.4 - 6.8violet
bromothymol blueyellow6.0 - 7.6blue
phenol redyellow6.4 - 8.2red-violet
cresol redyellow7.1 - 8.8violet
alizarin yellow Ryellow9.9 - 11.8dark orange

 

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Jim Askew  

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