EDTA Disodium Stock pH Adjustment Calculator

Calculator inputs

The page layout is single column, while the form fields below follow a responsive 3-column, 2-column, and 1-column arrangement.

Desired stock concentration (M)

Final volume

Volume unit

Molecular weight (g/mol)

Default matches common EDTA disodium dihydrate use.

Reagent purity (%)

Target pH

NaOH stock concentration (M)

Planned or actual NaOH added (mL)

Leave blank to estimate pH from the recommended NaOH volume.

NaOH overage (%)

Model pKa3

Model pKa4

Reset

Plotly graph

The chart shows the estimated species distribution across pH using the two-step dissociation model.

Formula used

1) EDTA moles
moles EDTA = stock molarity × final volume in liters

2) Corrected reagent mass
mass (g) = [moles EDTA × molecular weight] ÷ [purity fraction]

3) Species ratios
r1 = 10^(pH − pKa3)
r2 = 10^(pH − pKa4)
denominator = 1 + r1 + r1×r2

4) Species fractions
H2Y2- fraction = 1 ÷ denominator
HY3- fraction = r1 ÷ denominator
Y4- fraction = (r1×r2) ÷ denominator

5) Estimated OH equivalents per mole EDTA
OH equivalents = HY3- fraction + 2 × Y4- fraction

6) Recommended sodium hydroxide
NaOH moles = EDTA moles × OH equivalents × (1 + overage %)
NaOH volume (L) = NaOH moles ÷ NaOH molarity

7) Predicted pH from your NaOH entry
Actual OH equivalents = actual NaOH moles ÷ EDTA moles
The page then numerically solves for the matching pH.

This model is intentionally practical rather than absolute. Final measured pH can differ due to temperature, reagent assay, ionic strength, hydration state, and the stage at which volume is adjusted.

How to use this calculator

Enter the desired EDTA stock molarity.
Enter the final preparation volume and choose mL or L.
Keep the molecular weight default or replace it with your exact reagent value.
Adjust purity if your reagent assay differs from the default.
Set the target pH and your sodium hydroxide solution molarity.
Optionally enter the NaOH volume you actually plan to add.
Press the calculate button.
Read the result section above the form for mass, NaOH need, and predicted pH.
Use the CSV and PDF buttons to export the output.
Verify the final preparation with a calibrated pH meter in the lab.

Example data table

Stock Molarity	Final Volume	Purity	Target pH	NaOH Molarity	Mass to Weigh	Estimated NaOH
0.25 M	250 mL	99%	8.00	10.0 M	23.5000 g	6.195 mL
0.50 M	500 mL	99%	8.00	10.0 M	94.0000 g	24.780 mL
0.50 M	1.00 L	99%	8.50	10.0 M	188.0000 g	50.627 mL

These example values use the built-in approximation model and are meant for planning, not as a replacement for measured pH adjustment.

FAQs

1) Why does EDTA disodium often need sodium hydroxide during stock preparation?

EDTA disodium may dissolve slowly until the mixture becomes more alkaline. Sodium hydroxide shifts the acid-base balance toward more soluble forms, helping the powder dissolve and bringing the stock closer to the desired pH.

2) Is the calculated pH exact?

No. The pH is an estimate based on a simplified dissociation model. Real stocks can differ because of temperature, ionic strength, reagent hydration, assay, water quality, and how much volume change occurs during adjustment.

3) Can I change the molecular weight?

Yes. That field is editable so you can match the exact EDTA disodium form written on your reagent label. If you use a different hydrate state or supplier assay, update the value before calculating.

4) Why is reagent purity included?

Lower purity means you must weigh slightly more material to deliver the same moles of active EDTA. Including purity improves the mass estimate when your certificate of analysis differs from an ideal reagent.

5) What does the overage percent do?

Overage increases the recommended sodium hydroxide amount by a chosen percentage. It is useful when you intentionally plan a slight excess, but you should still confirm final pH experimentally and adjust carefully.

6) Should I enter the actual sodium hydroxide volume?

Enter it when you want the page to estimate the pH produced by your real plan. Leave it blank when you only want the recommended sodium hydroxide volume for the selected target pH.

7) Does the graph help with stock preparation?

Yes. It shows how the estimated EDTA species distribution changes across pH. That helps you see why dissolution and final stock behavior improve as the solution shifts toward more deprotonated forms.

8) What is the best practice after using this calculator?

Weigh the reagent, dissolve in less than final volume, add sodium hydroxide gradually, measure pH with a calibrated meter, then bring to final volume. Record the actual additions for repeatable future batches.