The potential of hydrogen, or pH, is a scientific measurement that quantifies the acidity or alkalinity of an aqueous solution. This measurement operates on a logarithmic scale that typically ranges from 0 to 14, where a change of one whole number represents a tenfold difference in hydrogen ion concentration. A pH strip provides a quick, visual approximation of this value, functioning as a colorimetric tool impregnated with chemical dyes. These indicator dyes react with the solution, causing a distinct color change that corresponds to a specific point on the scale.
Necessary Preparations Before Testing
Reliable pH measurement starts with the integrity of the strip itself. Confirm the expiration date on the packaging, as chemical indicators can degrade over time and deliver inaccurate readings. You should also review the manufacturer’s instructions, which specify the exact reaction time and the specific color chart designed for that batch of strips.
The sample must also be ready for testing to ensure a consistent result. For non-liquid samples, such as soil, create a slurry or extract by mixing the material with distilled or deionized water, then allow particulate matter to settle. Testing solutions near room temperature (around 25°C) is recommended, as temperature can influence the concentration of hydrogen ions and shift the pH value.
Step-by-Step Instructions for pH Testing
The physical process requires careful handling to prevent contamination of the indicator pads. Before testing, dispense a single strip from its container, holding it only by the end furthest from the reactive pads. Touching the pads with your fingers can transfer oils or moisture, which may skew the color change and compromise the measurement.
To expose the strip to the sample, dip it into a small volume of the liquid or hold it briefly in a flowing stream, such as urine. The immersion should be just long enough to completely wet all the indicator pads, which typically takes only a second or two.
Immediately after removing the strip, gently shake off any excess fluid to prevent the chemical pads from bleeding colors onto one another. The manufacturer’s instructions designate a precise waiting period, often 15 seconds, during which the color development is finalized. Reading the color change too early or too late will introduce error, as the reaction is time-sensitive.
Interpreting the Color Chart and pH Scale
Once the specified reaction time has passed, quickly compare the color of the wet strip against the reference color chart provided with the packaging. This chart displays a spectrum of colors, each corresponding to a specific whole or fractional pH number. Hold the strip next to the chart in good light, determining which color block provides the closest visual match to the tested strip.
The pH scale is divided into three categories. Solutions with a pH value below 7 are classified as acidic, with colors typically falling into the yellow, orange, or red spectrum. A reading of exactly 7 is considered neutral, representing a balance between hydrogen and hydroxide ions, as is the case with pure water.
Values above 7 are classified as basic or alkaline, which often manifest as shades of green, blue, or purple on the color scale. Since the color change is visual, the resulting number is an approximation, often accurate to within one half of a pH unit. For applications requiring greater precision, narrow-range strips, which cover a smaller pH span with finer increments, are available.
Maximizing Accuracy and Troubleshooting
Proper storage is necessary to protect the sensitive indicator dyes and maintain accuracy between uses. Strips must be kept in their original, tightly sealed container away from direct light, heat, and moisture, as humidity can prematurely activate the chemicals. Always use a fresh strip for each new test to avoid cross-contamination between samples.
Certain sample characteristics can introduce complications and affect the final reading. Highly colored liquids, such as wine or dark tea, may obscure the color change, making it difficult to match the result accurately. Viscous fluids may not fully penetrate the indicator pads, resulting in incomplete or ambiguous color development. If the strip color does not match any block on the chart, the pH may be outside the measurable range, and a different type of test may be necessary.