CompoundList
Sign in

Purity and assay calculator

The everyday quality control maths in one place: percent purity, assay by titration with the equivalence factor, conversion between as is, dried and anhydrous basis, and how much of a batch to use for a given amount of pure substance.

Calculation
mL
mL

Leave at 0 if no blank was run.

mol/L

Use the standardised value.

1 for KHP with NaOH, 0.5 for Na2CO3 with HCl.

g/mol

Assay

99.659%

498.3 mg of analyte found in 500 mg of sample.

Equivalence factor

Each 1 mL of 0.1 M titrant is equivalent to 20.422 mg of analyte. This is the figure pharmacopoeia monographs quote.

assay % = (24.4 mL × 20.422 mg/mL) ÷ 500 mg × 100

Use the values from the certificate of analysis of the exact batch. Assay, water and loss on drying vary from batch to batch.

How the calculation works

Percent purity is the share of the sample mass that is the substance you want.

purity % = mass of pure substance ÷ mass of sample × 100
Percent purity

In a titration assay the titrant reacts with the analyte in a known mole ratio, so the volume used tells you how much analyte the sample contained.

assay % = (V − Vblank) × c × r × M ÷ m × 100
Assay by titration (volume in litres, molar mass in g/mol, sample mass in g)
V
volume of titrant at the end point, V blank is the volume used without sample
c
standardised molarity of the titrant
r
moles of analyte that react with one mole of titrant
M
molar mass of the analyte
m
mass of sample taken

Pharmacopoeia monographs give the product c × r × M as an equivalence factor, for example each mL of 0.1 M sodium hydroxide is equivalent to 20.42 mg of potassium hydrogen phthalate.

Certificates report assays on different bases. The as is result includes the water and volatiles in the sample. To compare batches fairly, specifications often use a dried or anhydrous basis.

dried = as is × 100 ÷ (100 − LOD) anhydrous = as is × 100 ÷ (100 − water)
Converting between bases

For production and compounding, potency correction tells you how much material to use: quantity = amount of pure substance needed ÷ potency, where potency is the as is assay as a fraction. If the assay is on the dried basis, multiply it by (100 − LOD) ÷ 100 first.

Worked example

The same steps the calculator follows, done by hand

A 500 mg sample of potassium hydrogen phthalate (KHP, 204.22 g/mol) needs 24.5 mL of 0.1 M sodium hydroxide, and the blank uses 0.1 mL. KHP reacts 1 to 1 with NaOH. The batch has a loss on drying of 0.8 %.

  1. 1
    Net titre: 24.5 − 0.1 = 24.4 mL = 0.0244 L.
  2. 2
    Equivalence factor: 0.1 × 1 × 204.22 = 20.42 mg per mL. KHP found: 24.4 × 20.42 = 498.3 mg.
  3. 3
    Assay as is: 498.3 ÷ 500 × 100 = 99.66 %.
  4. 4
    On the dried basis: 99.66 × 100 ÷ (100 − 0.8) = 100.46 %. A value just over 100 % after drying is normal within method uncertainty.

Frequently asked questions

What is the difference between assay and purity?
Purity describes how much of the material is the wanted substance. Assay is the measured content from a specific test, such as titration or HPLC. A batch can have a high assay by titration and still contain impurities that the titration does not detect, which is why certificates list both assay and impurity results.
What does assay on the dried basis mean?
The result is recalculated as if the sample had been dried first, using its loss on drying. It removes the effect of moisture so batches can be compared. The anhydrous basis does the same using the water content from a Karl Fischer test.
Why can an assay be more than 100 %?
Every method has uncertainty, and correcting to the dried or anhydrous basis raises the number. Specifications for reagents and APIs therefore often read 98.0 to 102.0 %. A result far above 100 % points to an error, such as an unstandardised titrant.
Should I correct quantities for potency?
In pharmaceutical and food manufacturing, yes, whenever the specification or recipe is set on 100 % pure substance. Use the as is potency of the batch you will dispense, from its certificate of analysis.

More free tools