Method for precisely determining the volume of a test liquid aliquot used to calibrate a liquid delivery device. Method is useful for determining volume of an aliquot of a complex and/or non-aqueous test liquid. Test-stock mixture contains test liquid and a stock solution containing a first dye, the first dye being distinguishable by absorbance from a second dye. Test-stock mixture is mixed with diluent containing second dye to form sample solution to be used to measure absorbaπces of both first dye and second dye. Volume of test-stock mixture aliquot is calculated with a formula based on the Beer-Lambert law which incorporates measured absorbance values and a dilution characteristic of the test-stock mixture. Test liquid and stock solution are combined gravimetrically in a controlled ratio, allowing flow characteristics of the test-stock mixture to correspond to flow characteristics of the test liquid without affecting distinguishable absorbance characteristics of first and second dyes.
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