pH testing strips are a quick and easy way to measure the acidity or alkalinity of a solution. The strips contain indicators that change color depending on the pH. However, sometimes the color change on the strip can be difficult to interpret or falls between two colors. If your test strip shows a yellow color, there are a few things this could indicate about the pH of your sample.
Reasons for Yellow pH Result
There are a few potential reasons why a pH test strip might show a yellow color:
- The pH is near the upper end of the acidic range around 6
- The pH is in the middle neutral range around 7
- The sample is dilute or weakly buffered
- The test strip is expired or contaminated
The exact yellow shade and how it compares to the reference colors on the dispenser bottle can provide clues as to the actual pH. A pale yellow suggests a pH in the 6 to 7 range, while a bright or orange-yellow points more towards a neutral 7 pH.
Evaluating Color and Troubleshooting
When faced with an ambiguous yellow test strip result, there are a few steps you can take:
- Carefully compare the yellow color to the reference scale. Look for the closest match.
- Confirm the test strips have not expired and the dispenser bottle is tightly sealed.
- Test a sample of known pH such as a buffer solution to validate the strips are working properly.
- Repeat the test with fresh strips and be sure to follow the instructions.
- Test progressively more diluted samples to isolate the color.
Pay attention to any changes in the yellow color after time. Temporary pH indicators like bromothymol blue that appear yellow may shift toward green or blue as the color develops if the pH is acidic or alkaline, respectively.
Interpreting Borderline or Approximate Results
What if the yellow color falls between reference colors or is difficult to discern? Here are some guidelines for interpreting borderline, approximate pH results:
- Between red and orange: pH around 6-6.5
- Between orange and yellow: pH around 6.5-7
- Bright yellow: pH close to 7
- Between yellow and green: pH around 7-8
- Between green and blue: pH around 8-9
For example, a test strip that appears orange-yellow likely indicates a pH between 6.5 to 7. If the exact pH is critical, consider testing the sample with an electronic pH probe or meter for the most accurate reading.
Causes of Inaccurate Results
Incorrect pH test strip results stemming from a misleading yellow color may be the outcome of:
- Expired reagents that are no longer accurately sensitive
- Test strips that were exposed to moisture and contamination
- Not following the proper technique outlined in the instructions
- Insufficient sample volume or highly diluted solutions
- Very weak buffers with low ionic strength
- Samples that contain interfering substances
Certain chemicals can distort the color change and throw off the accuracy of pH strips. Oils, fats, proteins, and heavy metals are common interfering compounds. Some sample preparation or extraction steps may be needed prior to testing to remove these interferences.
When to Retest with Fresh Strips
Yellow test strip results that seem ambiguous or borderline should be verified by retesting with fresh strips. Use a new vial and sample aliquot if possible. Repeating the test helps confirm the yellow color is an accurate pH result and not an anomaly or error.
Ensure you do not touch the flat test pad portion where the color will develop. Oils from skin can distort the sensitive reagents. Follow all timing and technique directions closely when retesting to avoid variables.
Consistent yellow colors on multiple sampled and tests likely represents the true pH. But if the shade varies drastically each time, the sample and strips may require troubleshooting to determine the real pH.
Using Dilution to Isolate Color
For yellow colors that fall between reference shades, diluting the sample may help isolate the exact color for a more accurate reading. Try making serial dilutions like 1:10, 1:100, and 1:1000 dilutions of the original sample. Test these progressively smaller concentrations of sample.
Often the color becomes less ambiguous at higher dilutions and allows matching the resulting yellow shade more precisely to the dispenser scale. Be sure to account for the dilution factor then when reporting the final pH.
Limitations of pH Strips
While pH strips are inexpensive and easy to use, they do have limitations in terms of accuracy and precision compared to electronic pH probes and meters. The finite number of reference colors means test strip readings are approximate, usually to the nearest 0.5 pH unit. Issues like ambiguous yellow colors illustrate the challenges of colorimetric pH measurements.
pH strips are best suited for quick screening tests rather than precision analytical quantitative pH data. The role of pH strips is to provide general acidity or alkalinity trends and broad pH ranges rather than exact pH values. Just keep in mind the limitations and interpret borderline yellow results accordingly.
Electronic Verification for Accuracy
For the highest accuracy pH measurements, electronic pH meters with glass probes provide greater precision and reproducibility down to 0.01 pH units. These instruments measure pH electronically rather than relying on visual color interpretation.
If the exact pH is needed, verify approximate strip readings against an electronic pH meter using properly calibrated standards. This dual-method testing combines the convenience of test strips with the performance of digital pH meters.
Conclusion
Yellow test strip results can be ambiguous, but with careful color evaluation and controls, an informed approximate pH can be obtained. Compare the specific yellow shade and intensity to the dispenser reference colors. Retest with fresh strips to validate. Use serial dilutions to intensify color development. And verify borderline readings electronically if needed for applications requiring accurate pH control.
| Color | Approximate pH Range |
|---|---|
| Red | pH 1-6 |
| Orange | pH 6-6.5 |
| Yellow | pH 6.5-7 |
| Green | pH 7-8 |
| Blue | pH 8-9 |
This table summarizes the typical pH range indicated by various test strip colors. Yellow suggests a pH near neutral around 6.5 to 7. But matching the specific yellow shade to the reference scale provides a more precise approximate pH. Electronic pH verification can give an accurate final pH if needed.
