Dilution ratio

In chemistry and biology, the dilution ratio and dilution factor are two related (but slightly different) expressions of the change in concentration of a liquid substance when mixing it with another liquid substance. They are often used for simple dilutions, one in which a unit volume of a liquid material of interest is combined with an appropriate volume of a solvent liquid to achieve the desired concentration. The diluted material must be thoroughly mixed to achieve the true dilution.

For example, in a solution with a 1:5 dilution ratio, entails combining 1 unit volume of solute (the material to be diluted) with 5 unit volumes of the solvent to give 6 total units of total volume.

In photographic development, dilutions are normally given in a '1+x' format. For example '1+49' would typically mean 1 part concentrate and 49 parts water, meaning a 500ml solution would require 10ml concentrate and 490ml water.

Dilution factor
The "dilution factor" is an expression which describes the ratio of the aliquot volume to the final volume. Dilution factor is a notation often used in commercial assays. For example, in solution with a 1/5 dilution factor (which may be abbreviated as x5 dilution), entails combining 1 unit volume of solute (the material to be diluted) with (approximately) 4 unit volumes of the solvent to give 5 units of total volume. The following formulas can be used to calculate the volumes of solute ($V_{solute}$) and solvent ($V_{solvent}$) to be used: $$\begin{align} V_{\rm solute} &= \frac{V_{\rm total}}{F} \\[4pt] V_{\rm solvent} &= V_{\rm total} - V_{\rm solute} \end{align}$$ where $V_{total}$ is the desired total volume, and $F$ is the desired dilution factor number (the number in the position of $F$ if expressed as "$1/F$ dilution factor" or "$xF$ dilution"). However, some solutions and mixtures take up slightly less volume than their components.

In other areas of science such as pharmacy, and in non-scientific usage, a dilution is normally given as a plain ratio of solvent to solute. For large factors, this confusion makes only a minor difference, but in precise work it can be important to make clear whether dilution ratio or dilution factor is intended.