Using pH paper and reading colour scales
In short
pH paper estimates how acidic or alkaline a suitable aqueous sample is through colour change. Select the range, use a clean subsample and compare at the specified time with that product's scale.
What pH describes
For typical aqueous examples around room temperature, below 7 is acidic, around 7 neutral and above 7 alkaline. The scale is logarithmic. It is not a universal hazard scale: similar pH values can accompany very different risks.
Broad or narrow range?
Universal paper, for example 0–14, provides an overview. A narrower range can distinguish smaller differences where appropriate. Read resolution from the supplied scale. Accurate quantitative work may require a suitable calibrated meter.
Avoid common mistakes
- Check instructions, expiry and storage.
- Where the method allows, transfer a small aliquot into a clean vessel.
- Wet the indicator as directed without touching it.
- Observe immersion and reading times.
- Compare in suitable light and record result and method.
Limitations
Strongly coloured or cloudy samples complicate comparison. Low buffering and unsuitable composition can affect results. Not every paper suits non-aqueous liquids. An uncertain colour must not be converted into unjustifiably precise decimals.
Read vessels and pipettes. Browse laboratory supplies.
Questions
- Can I dip into the stock bottle?
- An aliquot often prevents contamination. Follow the method, especially for heterogeneous samples.
- Can strips be reused?
- No; used indicator areas cannot provide reliable new measurements.
- Is laboratory paper suitable for medical self-testing?
- Only a product specifically intended for that purpose should be used; general laboratory paper is not equivalent.
Read next
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General guidance for study and teaching. For your actual work, your safety briefing, hygiene plan, lab instructions and the product instructions apply. Legal information refers to Germany.
