Henderson–Hasselbalch Buffer Calculator mechanism

Model scope

The core Henderson–Hasselbalch formula is correct, but the instrument specification needs an explicit inverse formula, variable-sufficiency rules, positivity validation, and concentration/activity assumptions before implementation.

• The stated forward formula is standard: pH = pKa + log10([A−]/[HA]), provided concentrations approximate activities in a sufficiently ideal, dilute solution.
• The inverse calculation required for a target pH is not explicitly stated; it should be [A−]/[HA] = 10^(pH − pKa).
• The phrase 'solve for any one variable' is underdetermined for solving an individual concentration: [A−] requires a known [HA], and [HA] requires a known [A−].
• No explicit constraints state that pH, pKa, [A−], and [HA] must be numeric, that both concentrations must be strictly positive, or that the ratio must be positive.

Units and conventions

• pKa and pH are dimensionless logarithmic quantities.
• [A−] and [HA] must use the same concentration units before forming their ratio; the ratio itself is dimensionless.
• The concentration form assumes activities can be approximated by concentrations, generally for dilute or sufficiently ideal solutions.
• The logarithm is base 10.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "pKa": 4.76,
    "A_minus_concentration_M": 0.1,
    "HA_concentration_M": 0.2
  },
  "expected_outputs": {
    "pH": 4.45897
  },
  "working": "[A−]/[HA] = 0.100 M / 0.200 M = 0.500. log10(0.500) = −0.30103. pH = 4.76 + (−0.30103) = 4.45897."
}

Failure boundary

• log10([A−]/[HA]) is undefined for a zero or negative ratio; both concentrations must be strictly positive.
• Division by zero occurs if [HA] = 0.
• The inverse calculation can overflow or produce impractically large or small ratios for extreme target pH − pKa values.
• Using mismatched concentration units without conversion produces an incorrect ratio.
• The Henderson–Hasselbalch approximation can be inaccurate for concentrated, highly nonideal, very dilute, or strongly reacting solutions.
• A single known concentration is insufficient to determine both individual concentrations from only a target pH; an additional amount or concentration constraint is required.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche4/tranche4-results.json