Henderson–Hasselbalch calculator

A Henderson–Hasselbalch calculator turns a weak-acid pKa and the ratio of conjugate base to undissociated acid into the buffer pH every chemistry lab and textbook uses. Enter pKa and either both concentrations [A−] and [HA] or the single ratio [A−]/[HA]; the tool returns pH equals pKa plus log base ten of that ratio, a short acidity label—acidic, neutral, or basic—and step-by-step substitution you can copy into a notebook. Homework checks, buffer prep, exam review, and automated graders all need that same closed formula without a spreadsheet or a full aqueous speciation package.

What the Henderson–Hasselbalch equation means for buffers

A buffer is a mixture of a weak acid HA and its conjugate base A− that resists large pH swings when small amounts of strong acid or base are added. For a monoprotic weak acid under the usual dilute aqueous approximations, the Henderson–Hasselbalch equation links buffer pH to the acid strength and the composition of the mixture: pH equals pKa plus the base-ten logarithm of the concentration ratio of conjugate base to acid, written pH equals pKa plus log ten of [A−] over [HA].

When the ratio equals one, the log term is zero and the buffer pH equals the pKa of the weak acid—the classic half-equivalence point of a weak-acid titration. When [A−] exceeds [HA], the ratio is greater than one, the log is positive, and the buffer pH sits above the pKa; when the acid form dominates, the ratio is less than one, the log is negative, and the buffer pH sits below the pKa.

Absolute concentrations cancel in the ratio, so you may work in moles per litre, millimoles per litre, or any other consistent unit as long as both species use the same unit. What never cancels is the requirement that both concentrations be strictly positive: a zero or negative value makes the logarithm undefined or meaningless for a real buffer, so this tool rejects those cases instead of inventing a pH.

Teachers and automated graders care about that rejection as much as the happy path, because a silent NaN would hide a data-entry error. The check path recomputes pKa plus log ten of the published ratio with the same fixed rounding used for the ph field, so a successful response always ships with steps you can assert in tests or lab notebooks.

Related classroom topics such as buffer capacity, polyprotic systems with multiple pKa values, or full charge-balance speciation are intentionally out of scope so this landing stays focused on the single search intent of converting a known pKa and base-to-acid ratio into buffer pH.

Practical tips, limits, and related searches

Keep pKa and the concentration ratio inside the published absolute range so the sum pKa plus log ten of ratio cannot overflow into a non-finite pH the solver must reject. Use consistent concentration units for [A−] and [HA]; if your source reports millimoles per litre for one species and moles per litre for the other, convert before calling rather than mixing unit systems inside one request.

Prefer this capability when you already know pKa and the buffer composition ratio; if your source only has [H+] without a buffer context, use a pH-from-concentration calculator instead. Round-trip intuition is easy: ten to the power of pH minus pKa recovers the original ratio within the same twelve-decimal rounding for typical aqueous buffers, and confirming acidity flips correctly when you move the computed pH across seven on the aqueous scale.