Erlotinib HCl Salt vs Free Base DMSO Stock Molarity Tool
Calculate the exact reconstitution volume or required mass when preparing a DMSO stock from a hydrochloride or mesylate salt vial rather than the anhydrous free-base parent. This reference loads the checked erlotinib hydrochloride formula weight to demonstrate how using a parent database molecular weight under-doses a salt stock.
Uses the verified CIAAW 2021 formula weight for erlotinib hydrochloride (429.901 g/mol, PubChem CID 176871) alongside the free-base parent mass (393.443 g/mol) from the published formula-mass fixture. Illustrative 5 mg mass and 10 mM target inputs show the arithmetic volume difference.
Reconcile database parent structures with physical vial labels
Cheminformatics libraries and assay registration tables routinely normalise compounds to their desalinated parent structure so that different salt batches of the same drug share one canonical identifier. When a bench scientist looks up that compound record to prepare a 10 mM DMSO stock, copying the parent molecular weight directly into a reconstitution calculation introduces a systematic dilution bias whenever the physical bottle contains a salt or hydrate.
For a 393 Da kinase inhibitor such as erlotinib, a single hydrochloride equivalent adds 36.458 g/mol, representing an 8.48 percent mass shortfall. For a mesylate salt, methanesulfonic acid adds 96.10 g/mol, creating a 15.6 percent shortfall for dabrafenib mesylate and 16.3 percent for imatinib mesylate (96.10 of 589.72 g/mol). That error shifts every downstream IC50 curve rightward by the same factor.
Record lot-specific formula weights and solvent behaviour
Always inspect the bottle label and certificate of analysis for the exact salt stoichiometry, water content and chemical purity before adding solvent. Also note that hydrochloride and sulfate salts frequently have lower solubility in anhydrous DMSO than their corresponding free bases or mesylates; warm gently and inspect visually against a dark background to confirm complete dissolution before aliquoting.
Equations and variables
Salt-form reconstitution volume
Vsalt = (msalt × P) / (MWsalt × ctarget)
- Vsalt
- Final solution volume required for the weighed salt mass (L)
- msalt
- Weighed mass of the supplied salt or hydrate solid (g)
- MWsalt
- Complete formula weight including counterion and hydrate stoichiometry (g/mol)
- ctarget
- Target molar concentration of active parent species in solution (mol/L)
- P
- Mass-fraction purity on the stated certificate basis
Active-parent mass fraction and counterion shortfall
fparent = MWparent / MWsalt; shortfall (%) = 100 × (1 − fparent)
- MWparent
- Formula weight of the uncharged parent molecule (g/mol)
- MWsalt
- Formula weight of the salt form weighed on the balance (g/mol)
- fparent
- Fraction of weighed salt mass attributable to the active parent moiety
Scope and limitations
- A structure database often displays the parent free base while commercial vendors ship mono- or di-hydrochloride, mesylate, tosylate or hydrated lots whose exact stoichiometry must be read from the certificate of analysis.
- Chromatographic HPLC area-percent purity does not account for inorganic salts, residual solvent or bound water unless paired with quantitative elemental or Karl Fischer analysis.
- Reconstitution arithmetic computes nominal molarity and does not verify whether a hydrochloride salt remains soluble in neat DMSO at high millimolar concentrations.
Sources and provenance
- Calculator inputs and independent arithmetic verificationOriginal synthetic inputs; see individual fixture notices
- Independent CIAAW 2021 formula-mass verification fixtureOriginal exact decimal reference; PubChem CID 176871 erlotinib HCl
- IUPAC standard atomic weights 2021Cited atomic-weight constants
Technical questions
How much molarity error occurs if I use the erlotinib free-base molecular weight for an HCl vial?
Erlotinib free base has a formula weight of 393.443 g/mol, whereas erlotinib monohydrochloride is 429.901 g/mol. Calculating solvent volume with the free-base weight adds roughly nine percent too much DMSO, producing an actual concentration of about 9.15 mM instead of the intended 10 mM.
Why is the target concentration still expressed in molarity after dissociating in solution?
One mole of a mono-salt contains one mole of the active parent ion. Using the full salt formula weight in the mass-to-moles conversion ensures that the resulting solution contains the exact intended molar concentration of the active parent species.
What if the vial is both a salt and a hydrate, such as a monohydrate dihydrochloride?
Every stoichiometric counterion and water molecule present in the weighed powder contributes to the mass on the balance pan. Sum the complete formula weight of the exact lot form, parent plus all equivalents of acid and water, or enter the lot-specific formula weight printed on the vial label.
Connected workflows
- DMSO stock reconstitution volume
Compare free-base imatinib stock volume arithmetic with salt-corrected reconstitution.
- Chemical structure and canonical SMILES
Inspect parent canonical SMILES representations and how they differ from salt forms.
- 4PL IC50 curve fitting
See how a stock molarity error shifts fitted downstream dose-response midpoints.