Calculator inputs
Responsive: 3 columns large, 2 small, 1 mobileExample data table
| Sample | Method | Inputs | Result (PO4) | Result (P) |
|---|---|---|---|---|
| Example A | Mass & Volume | 0.150 g KH2PO4 in 1.000 L | ≈ 104.7 mg/L PO4 | ≈ 34.1 mg/L P |
| Example B | Molarity | 0.0020 mol/L phosphate | ≈ 189.9 mg/L PO4 | ≈ 61.9 mg/L P |
| Example C | Spectrophotometry | A=0.420, m=0.006 A/(mg/L), b=0.030, DF=2 | ≈ 130.0 mg/L PO4 | ≈ 42.4 mg/L P |
Formula used
- Moles of compound: n = mass(g) / MM(compound)
- Moles of phosphate: n(PO4) = n × PO4_units
- Molarity: C(mol/L) = n(PO4) / V(L)
- PO4 mg/L: mg/L = C × MM(PO4) × 1000
- P mg/L: mg/L = C × AM(P) × 1000
- Calibration: A = m·C + b
- Concentration: C = (A − b) / m
- Apply dilution: C_actual = C × dilution_factor
- Convert PO4 ↔ P using molar mass ratio.
- Molarity from mg/L: mol/L = (mg/L ÷ 1000) ÷ MM(PO4)
How to use this calculator
- Choose the method that matches your measurements.
- Enter values with the correct units and rounding.
- If using absorbance, confirm slope and intercept.
- Apply dilution factor if you diluted your sample.
- Review PO4, P, and molarity results in the card.
- Use “Custom compound” for uncommon phosphate salts.
- Set “PO4 units” for formulas like Ca3(PO4)2.
- Switch display units to mg/L, µg/L, or ppm.
- Download CSV for spreadsheets and audits.
- Download PDF for quick sharing and records.
Practical concentration reporting
Phosphate is commonly reported as PO4³⁻ or as elemental phosphorus (P). This calculator returns both, using molar masses: MM(PO4) ≈ 94.97 g/mol and AM(P) ≈ 30.97 g/mol. The conversion is PO4→P = 30.97/94.97 ≈ 0.326, and P→PO4 ≈ 3.066. For dilute aqueous samples, ppm is equivalent to mg/L, and 1 mg/L equals 1000 µg/L.
From salts to phosphate in solution
When preparing standards, you often weigh a phosphate salt rather than PO4 directly. The Mass & Volume method converts weighed mass to moles via the compound molar mass, then multiplies by the number of PO4 groups per formula unit. For example, 0.150 g KH2PO4 (MM ≈ 136.09 g/mol) in 1.000 L provides about 0.00110 mol/L phosphate, which corresponds to ≈ 104.7 mg/L as PO4 and ≈ 34.1 mg/L as P. For Ca3(PO4)2, the phosphate moles are doubled because the formula contains two PO4 units.
Interpreting spectrophotometric readings
Colorimetric methods typically fit a straight calibration line, A = m·C + b, where C is concentration in mg/L. This tool solves C = (A − b)/m and then applies a dilution factor to report the original sample. A negative result can occur when blank correction is too strong or the intercept is misapplied; the calculator clamps it to zero and flags a warning so you can review slope, intercept, and dilution inputs.
Quality checks and documentation
For routine monitoring, consistency matters as much as the number. Record sample identifiers, reagent lots, filtration steps, and dilution history. Use the decimals setting to match your reporting policy, then export CSV for audit trails or PDF for quick sign-off. Keeping PO4, P, and molarity together reduces transcription errors and helps validate unit conversions during reporting.
Typical uses across chemistry workflows
Phosphate calculations support water treatment optimization, nutrient balance studies, corrosion and scale control, fertilizer solution preparation, and laboratory proficiency testing. Because results can be expressed in mg/L, µg/L, or ppm, the same dataset can serve operational dashboards and analytical reports without manual rework. Pair results with method limits and controls to interpret trends.
FAQs
What is the difference between PO4 and P reporting?
PO4 reports the full phosphate ion mass, while P reports only phosphorus. The calculator uses constant molar-mass ratios, so you can switch labels without changing the underlying chemistry or sample preparation steps.
Why does ppm match mg/L here?
For dilute water-based samples, 1 ppm is approximately 1 mg/L because the density is close to 1 kg/L. If your matrix is very salty, viscous, or non-aqueous, treat ppm as an approximation.
Which salt should I choose in Mass & Volume?
Select the compound you weighed to prepare a standard or solution. The tool converts using the compound molar mass and the number of PO4 groups per formula. Use Custom when your salt is not listed.
How do I enter dilution factor correctly?
Enter how much the measured solution represents the original sample. A 1:1 dilution means the concentration was halved, so use 2. A 1:9 dilution (one part sample plus nine parts diluent) uses 10.
My absorbance result is negative. What should I do?
Negative concentration usually indicates an intercept or blank correction issue. Confirm your calibration slope, intercept, and blank handling. Re-check that absorbance is within the calibration range and that dilution factor is correct.
Can I use this calculator for solid samples?
Yes, if you have already converted your solid into a final solution volume. Enter the extracted solution volume and the mass of the compound added, or use absorbance after extraction. Interpret ppm carefully for non-aqueous matrices.