Test strips or sticks used to measure pH are inexpensive and easy to use. So why would you want to use a pH meter, which is more expensive and takes more time to use?
1. Resolution
Resolution refers to the smallest measureable change in the quantity being measured. For pH strips or digital pH meters, this refers to the intervals over which the reading is given. For most pH meters, the resolution is at least 0.1 pH units, and resolutions of 0.01 and 0.001 units are common.
For test strips, the resolutions are much lower. Typical pH strips that cover a wide pH range have colors that are spaced 0.5 or 1.0 pH unit apart. Specialty pH strips that cover a narrow range can have color graduations as low as 0.2-0.4 pH units apart. Interpolation between colors is possible, but even the lowest resolution pH meters generally provide better resolution than the highest resolution pH strips.
2. Precision and Accuracy
Precision refers to the reproducibility of a measurement. That is, a more precise measurement technique will yield measured values that are closer to each other as the measurement is repeated. Because the resolution forms the upper limit of precision, low-resolution measurement techniques such as test strips have less potential to be precise compared with higher resolution techniques such as pH meters. Although higher resolution does not necessarily mean higher precision, pH meters generally have higher precision than pH strips.
Accuracy refers to the amount of uncertainty in a measurement, such that a technique with greater accuracy will yield measurements with less uncertainty. An accurate measurement technique will produce measurements that are close to the true value. As with precision, test strips have less potential to be accurate compared with pH meters because of their relatively low resolution.
In summary, pH meters are usually much more precise and accurate than test strips.
2. Objectivity and Consistency
When a test strip is used, it changes color and is compared with a color chart. A problem with this is that different people see colors differently and may tend to interpret the color comparison in different ways. Because of this, two different people may report two different pH values when shown the same test strip. In addition to this, colors appear differently under different lighting conditions. Unless lighting conditions are consistent, this may add to the uncertainty in test strip measurements.
Additionally, interpretation of the test strip requires a certain amount of judgement, especially when interpolation between graduations is involved. For example, if it is not clear which color on the chart is the best match for the test strip, it is a matter of judgement to choose one color or the other, or to report an intermediate value. Because of this, a single operator may not be entirely consistent in making these judgement calls from measurement to measurement. Even the same operator may report different pH values at different times, even if the color does not change.
Test strip results are also subject to variation due to the way the strips are handled. For example, the length of time that the strip is exposed to the solution, the manner in which the strip is handled after it is removed from solution, and the length of time between removing the strip and comparing the colors may all affect the final result. If all of these conditions are not kept consistent, then some additional variation in the results will likely occur.
The subjectivity of test strips can be partly removed by scanning the strip and having the color analyzed by computer, but care must be taken so that user handling and scanning/lighting conditions are consistent. Digital pH meters, on the other hand, produce unambiguous digital output, and measurement protocols using pH meters are relatively easy to keep consistent.
3. Shelf life
Test strips contain reagents that will tend to deteriorate over time. If properly stored under ideal conditions, a set of strips will stay good for approximately one year. However, exposure to moisture, sunlight, and high or low temperatures will tend to degrade the reagents in the strips, which will affect their performance. Test strips that have become degraded may become unacceptably inaccurate.
That said, pH meters also require care. The pH electrode must be properly cared for and stored, and the calibration solutions themselves have a limited shelf life.
4. Continuous Measurement
In some applications, such as acid-base titrations, it is desirable to monitor pH over time in order to detect or record changes. Aside from their other limitations, test strips are not as suitable for this as pH meters because each strip can only be used for one measurement. Digital pH meters, on the other hand, report pH values continuously, and automated recording of measurements is even possible.
Conclusion
In applications where relatively low levels of resolution and accuracy are acceptable, pH test strips may be a good choice because of their low cost and ease of use, as long as relatively fresh strips are used that have been stored under optimal conditions.
Where moderate to high levels of resolution and accuracy are needed, pH meters are most likely the best choice, even though they require a greater up-front investment and more skill to use. When making the choice of measurement technique, one should consider the possible hidden costs of inaccurate measurements, such as addition of excessive material to correct pH, sub-optimal growing conditions, or degradation of the quality of the final product.