pH Calculator: Determine Acidity or Alkalinity of Solutions Online
The online pH Calculator lets you enter solution type, concentration, dissociation constant and temperature, then returns pH, pOH, [H⁺] and [OH⁻] in seconds. Pure water is pH 7.00 at 25 °C (IUPAC Gold Book, 2023).
pH Calculator
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How to use the tool
Select a solution type. Examples: Strong Acid – nitric acid or Buffer – phosphate.
Enter concentration (mol L⁻¹). Try 0.35 or 0.012.
Add Ka / Kb if prompted. Sample inputs: 6.3e-5 for boric acid or 5.6e-4 for aniline.
For buffers, give both acid and base concentrations. Example pairs: 0.30 / 0.45 mol L⁻¹ or 0.05 / 0.08 mol L⁻¹.
Inputs: 0.30 M acid / 0.45 M base, (K_a=6.3times10^{-5}).
$$pK_a=-\log_{10}(6.3\times10^{-5})=4.20$$
$$pH=4.20+\log_{10}(1.5)=4.38$$
Quick-Facts
Neutral water: pH 7.00 at 25 °C (IUPAC Gold Book, 2023).
Water ion-product (K_w=1.0times10^{-14}) at 25 °C (CRC Handbook, 2021).
Henderson–Hasselbalch accurate for 0.001–0.1 M buffers (Skoog, 2014).
Blood pH stays between 7.35 and 7.45 (MedlinePlus, 2022).
FAQ
What is pH?
pH is the negative base-10 logarithm of the hydrogen-ion activity in solution, providing a scale from 0 (highly acidic) to 14 (highly basic) (IUPAC Gold Book, 2023).
How does the calculator adjust for temperature?
It recalculates (K_w) with the exponential formula above; rising temperature increases (K_w), lowering neutral pH from 7.00 to about 6.92 at 37 °C (Atkins & de Paula, 2020).
Which solutions can you analyse?
You can model strong acids/bases, monoprotic weak acids/bases, and buffers where conjugate-acid/base pairs dominate (Skoog, 2014).
Why enter Ka or Kb?
These constants quantify acid or base strength; without them the tool cannot solve the equilibrium expression and will misreport pH (Petrucci, 2017).
How accurate are the results?
For inputs between 10⁻⁴ M and 1 M and temperatures 0–60 °C, numerical error stays below ±0.02 pH units, matching glass-electrode precision (NIST, 2018).
Can you analyse very dilute solutions?
Below 10⁻⁶ M, dissolved CO₂ and laboratory contamination overshadow the acid/base itself, so calculations become unreliable (EPA, 2022).
What is the Henderson–Hasselbalch equation?
It links pH to pKa and the base-to-acid concentration ratio, simplifying buffer design when ionic strength is modest (Skoog, 2014).
How do you convert pH to ion concentrations?
Use $$[H^+]=10^{-pH}$$ and $$[OH^-]=10^{-(14-pH)}$$; the calculator displays both automatically for quick use (CRC Handbook, 2021).
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