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QC Calculations: %RSD, Assay, LOD & Karl Fischer
Everyday pharmaceutical quality-control calculations in one place: %RSD of replicate results, assay by HPLC or UV against a standard (API or % of label claim), assay by UV using specific absorbance, assay by titration, loss on drying and water content by Karl Fischer. Check %RSD against your acceptance limit, and print or download any result as a calculation record (CSV).
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How it works
HPLC/UV: Assay % = (Aᵤ ÷ Aₛ) × (Wₛ × Pₛ/100 ÷ Dₛ) × Dᵤ ÷ Wᵤ × 100
% of label claim = content per unit ÷ label claim × 100, where content per unit = amount found ÷ Wᵤ × average weight
Titration: Assay % = (V − blank) × F × E × 100 ÷ W (back titration: blank − V)
LOD % = (W2 − W3) ÷ (W2 − W1) × 100
KF factor F = water (mg) ÷ titre (mL); Water % = titre × F × 100 ÷ sample (mg)
Dried basis = result × 100 ÷ (100 − LOD or water %)
D is the total dilution in mL (for 50 mg in 50 mL, then 5 mL to 50 mL, enter 500). E is the equivalence from the monograph, in mg of substance per mL of titrant.
Worked examples
Tablet assay by HPLC
Standard 50 mg (99.5 %) diluted to 500; powder 250 mg (one average tablet weight, 250 mg) diluted to 500; areas 1,000,000 (standard) and 995,000 (sample); label claim 50 mg. Content = 49.50 mg per tablet, assay = 99.00 % of label claim.
Assay by titration
500 mg sample needed 27.6 mL of titrant (factor 1.002), blank 0.1 mL, and each mL is equivalent to 18.02 mg. Assay = 27.5 × 1.002 × 18.02 × 100 ÷ 500 = 99.31 %.
Karl Fischer factor with sodium tartrate
150 mg sodium tartrate dihydrate contains 150 × 0.1566 = 23.49 mg water; titre 4.70 mL, so F = 4.998 mg/mL. A 500 mg sample using 0.85 mL with F = 5.0 contains 0.85 % water.
Verified against the Indian Pharmacopoeia’s worked examples
We ran every worked example in the Indian Pharmacopoeia Commission’s guidance document on calculations (IPC/GD/10, Calculations in IP) through this calculator. Every result matches the IP answer.
| Calculation | Reference answer | LabKit result |
|---|---|---|
| %RSD of six HPLC areas | 0.47 % | 0.47 % |
| HPLC assay, ethionamide API (as is / dried basis) | 99.01 % / 99.13 % | 99.01 % / 99.13 % |
| UV assay against a standard, triamcinolone (as is / dried) | 99.38 % / 99.75 % | 99.38 % / 99.75 % |
| UV assay by specific absorbance, A(1%, 1 cm) = 389 | 99.58 % | 99.58 % |
| Titration assay with blank, albendazole (as is / dried) | 99.50 % / 99.62 % | 99.50 % / 99.62 % |
| Loss on drying, levosalbutamol sulphate | 0.91 % | 0.91 % |
| Water by Karl Fischer, cefaclor | 4.10 % | 4.10 % |
| KF factor with sodium tartrate dihydrate | F = W × 0.1566 ÷ V | F = W × 0.1566 ÷ V |
This check is our own; it is not an endorsement by the Indian Pharmacopoeia Commission.
Frequently asked questions
Which standard deviation does %RSD use?
The sample standard deviation (n − 1), as used for system suitability and precision in pharmacopoeial methods.
How do I enter the dilution?
Multiply out every dilution step into one number of mL: 25 mg in 100 mL, then 10 mL to 50 mL, is 100 × 50 ÷ 10 = 500.
When do I use dried basis?
When the monograph limit is stated “on the dried basis” or “on the anhydrous basis”, enter the LOD or water result; otherwise leave it empty.
Why is the water content of sodium tartrate 15.66 %?
Sodium tartrate dihydrate (C₄H₄Na₂O₆·2H₂O, 230.08 g/mol) contains two molecules of water (36.03 g), which is 15.66 % of its weight.
Related calculators
Results are for guidance. Use the formula, limits and rounding in your monograph or SOP, and verify calculations before entering them in GMP records.