Buffer solutions: the underrated foundation of every measurement

A buffer solution holds its pH even when small amounts of acid or base are added - which is exactly why it serves as the reference for calibration. This persistence has limits, though: carry-over, air contact and time wear down every buffer. Know the rules and you calibrate reliably; ignore them and the error is passed on to every following measurement.

Technical or NIST buffers: which numbers apply?

Commerce is dominated by technical buffers with the round values 4.01, 7.00 and 10.01. Alongside them exist the primary buffers per NIST and DIN 19266 with odd values such as 4.008, 6.865 and 9.180 - highest accuracy, but pricier and more delicate. What matters is that the meter's buffer recognition matches the set in use: if the meter is set to NIST and you dip into technical pH 7 buffer, it interprets the 7.00 as 6.865 - a built-in error of a good 0.1 pH. The colour coding of the bottles (commonly red for 4, yellow or green for 7, blue for 10), by the way, is manufacturer convention, not a standard - the label is what counts.

The golden handling rules

1. Always decant, never dip: the electrode belongs in a small beaker of buffer - never in the stock bottle. A single carry-over spoils the entire stock.
2. Discard used buffer: what has been in the beaker goes down the drain, never back into the bottle.
3. Rinse between buffers: rinse the electrode with distilled water and only dab it dry - rubbing charges the membrane statically.
4. Keep the order: first pH 7 (zero point), then pH 4 or 10 (slope).
5. Close the bottle immediately: pH 10 in particular is grateful for every minute less of air contact.
6. Mind the temperature: bring buffer and electrode to the same temperature - the actual values per temperature are in the tables at the electrode check.

Honest shelf lives: pH 10 is the diva

Unopened, buffers keep for the years stated on the label. After opening, their paths diverge: the acidic pH 4 buffer is chemically robust but, as an organic phthalate solution, is the one most prone to mould - manufacturers' biocide additives slow that down. The pH 7 phosphate buffer keeps unremarkably for some months. The pH 10 buffer, however, absorbs carbon dioxide from the air and thus drifts measurably downwards within weeks of opening - faster with frequent opening. Which is precisely why a failed calibration so often fails at the alkaline point. Practical remedies: small bottles or single-use sachets, and the opening date written on the bottle.

Spotting a buffer gone bad

Cloudiness, flakes or streaks are clear exclusion criteria - as is any visible film. Trickier is the invisibly aged pH 10 buffer: it looks like day one and is off nonetheless. The most reliable warning comes from the calibration itself: if the meter repeatedly reports poor slope although the electrode is freshly cared for, replace the buffers first - the troubleshooting guide leads the same way.

Mix your own? For once, no.

Unlike the KCl storage solution, mixing buffers yourself does not pay: the recipes demand high-purity chemicals, an analytical balance and CO₂-free boiled water - and in the end the traceable certification that ready-made buffers bring for little money is missing. The exception proves the rule: those who work with volumetric standards in the lab anyway know best themselves.

The most common mix-up: buffer is not storage solution

Parking an electrode permanently in pH 7 buffer sounds logical - but is wrong: the phosphate buffer is comparatively low in ions and, on top, diffuses into the junction. For storage, the electrode belongs in 3-molar KCl solution; only the pH 4 buffer will do as a short-term stand-in at a pinch.