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How many grams of reagent should you weigh for this buffer?

Buffer preparation calculator (molarity to grams and %w/v)

From the target molarity, the volume and the reagent's molecular weight, gives the grams of reagent to weigh out, with a purity correction and conversions to %w/v and mM.

When to use itUse it when making Tris, phosphate or other buffers from powder, to check the weight to measure out.

Prefilled with an example: making 1 L of 50 mM Tris buffer. Change any value and the result updates.

mol/L

Enter 0.05 for 50 mM.

L

The final volume of the solution.

g/mol

121.14 for Tris. For hydrates (·nH₂O) and salts (sodium salts and so on), use the molecular weight of that form.

%

Enter 100 for no correction.

Weight to measure out6.057 g
As %w/v0.6057 %w/v
As mM50 mM

Weigh out 6.057 g of reagent, dissolve it, adjust the pH, then make up to 1 L for a 50 mM solution.

Tips for making it

  • Adjusting pHWeigh, dissolve, adjust pH, then make up to volumeMost buffers are weighed and dissolved, brought to pH with acid or base, and finally topped up with water to the mark (making up to volume). The weight calculation is the starting point.
  • Hydrates and salt formsUse the molecular weight of the reagent you haveHydrates (·nH₂O) and salt forms (sodium salts and so on) have different molecular weights. Use the one for the reagent on your shelf, or the concentration will be off.
  • Ionic strength and osmolalityCheck the whole formulationCheck that ionic strength and osmolality meet the product's requirements with all components included, such as salts and sugars, not just the buffer.

How it is calculated

This gives the weight to measure out when making the buffer from powder. To make it by diluting a 10× stock or similar, use the dilution calculator (C1V1 = C2V2).

mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol) ÷ (purity/100) %w/v = mass (g) ÷ volume (mL) × 100

Source: Mass needed = molarity × volume × molecular weight ÷ purity. This follows from the definition of molarity and is the basis of solution preparation.

With your numbers

  1. mass = molarity × volume × molecular weight ÷ (purity ÷ 100) = 0.05 × 1 × 121.1 ÷ 1 = 6.057 g
  2. %w/v = mass ÷ volume (mL) × 100 = 6.057 ÷ 1,000 × 100 = 0.6057 %w/v

Limits of this calculation

  • The calculated weight is theoretical. In practice, purity, hydration, weighing error and solubility all affect it.
  • %w/v is weight (g) ÷ volume (mL) × 100. It is not the same as w/w (weight to weight) or v/v (volume to volume).
  • Check pH, ionic strength and buffer capacity separately, by measurement and formulation design. Treat the result as an estimate for learning.

Primary sources

Every calculator gives an estimate for learning, built on published information and primary sources. Real decisions follow the applicable guidance and expert judgment.