Dilution, Dosing and At-Use Concentration: What Are You Actually Testing?

The 30-second explanation

A concentrate does not have one universal test strength.

Consider a liquid laundry detergent. The package contains a concentrated formula, the user adds a dose, and the machine distributes that dose into a changing volume of water. The dose may be easy to record, but the nominal concentration in the wash liquor also depends on water volume. The amount that reaches a particular fabric or stain may be affected by the load, machine, cycle, and timing of the addition.

A spray cleaner creates a different situation. There may be no large wash bath at all. The relevant conditions may be the product dilution and the amount applied per unit of surface area. A dishwasher tablet is different again: the unit dose is fixed, but the wash-water volume and cycle sequence affect the concentration profile.

This is why “we tested both at the same concentration” is not a complete description until the concentration basis and application conditions are stated.

Start with the language

Four quantities that should be kept separate.

1. Dilution ratio

A dilution ratio describes the relative amount of concentrate and diluent. It is a preparation instruction—not necessarily the best way to report the final solution. Ratio notation is often interpreted inconsistently, so the actual preparation should be written in unambiguous terms.

2. Product dose

Dose is the quantity of finished product delivered per use: for example, grams per laundry load, millilitres per dishwasher cycle, or millilitres placed in a dilution-control bottle. Dose is often the most consumer-relevant basis because it reflects what a user is instructed to dispense.

3. At-use product concentration

At-use product concentration is the amount of finished product relative to the final working solution or test system. It may be expressed as a mass fraction, volume fraction, mass per volume, or another clearly defined basis.

4. Active or ingredient concentration

Active concentration considers how much of a defined ingredient or ingredient group is present at use. This can be helpful when investigating surfactant efficiency or another formulation variable, but it does not automatically make two complete formulas equivalent. Builders, solvents, chelants, enzymes, pH, and surfactant type may still differ.

For some applications, a fifth value matters just as much: application loading, such as millilitres per square metre, grams per fabric mass, or total product applied to a test panel. A correctly diluted spray can still be under-applied or over-applied.

The ratio trap

“1:10” is not a complete instruction.

Ratio language is used in more than one way. Some people mean one part product plus ten parts water. Others mean one part product in ten total parts of prepared solution. Those preparations are not the same.

On an additive, same-basis calculation, one part product + ten parts water produces eleven total parts: nominally 9.09% product—not 10%.

The numerical difference may be small in some applications and important in others. More importantly, the ambiguity makes the work harder to reproduce.

A better laboratory instruction states the actual quantities and the intended final concentration. For example: “Add 10.0 mL of product and dilute with water to a final volume of 1,000 mL, producing a nominal 1.00% v/v product solution.”

This is different from adding 10.0 mL of product to 1,000 mL of water. In the second preparation, the nominal final volume is approximately 1,010 mL and the product concentration is approximately 0.990% v/v, assuming volumes are additive.

If the ratio can be interpreted two ways, report the preparation—not just the ratio.

Three useful calculations

Report the number that matches the question.

Finished product in the working solution

If 10.0 mL of product is diluted to a final volume of 1,000 mL, the nominal concentration is 1.00% v/v. The basis must be stated: volume per volume, mass per mass, and mass per volume are not interchangeable.

Defined active matter at use

A 1.00% finished-product solution made from a product that is 20.0% active contains a nominal 0.200% active matter, provided both percentages use compatible bases. This calculation can be useful for surfactant work, but “total active” must be defined. It should not silently combine unrelated ingredients simply because they are non-water components.

Nominal concentration in a wash system

A 5.0 g dose distributed through 10.0 L of water corresponds to 0.50 g/L of finished product. If the product is 20.0% active by mass, the nominal active concentration is 0.10 g/L.

Real appliances may fill, drain, recirculate, or retain water during a cycle, so this can be a useful nominal value rather than a claim that concentration remains constant at every moment.

When a liquid is dosed by volume but its composition is stated by mass, density may be required to convert between the two. NIST notes that liquid density is required when converting a solution from mass-fraction to volume-fraction terms. For viscous concentrates, gravimetric dosing is often easier to reproduce than a casual volume measurement. See the NIST overview of liquid-density determination.

Choosing the test condition

The fairest comparison depends on the decision.

There is no single concentration basis that is automatically fair for every study. The test condition should follow the question being asked.

A dose-response study is often more informative than a single point. Product rankings can change with concentration. A product that performs well at a high dose may lose its advantage when diluted, while another may reach a useful performance plateau earlier.

More product is not guaranteed to improve the whole cleaning result. Overdosing may increase residue, filming, foam, rinse demand, cost, or material exposure without producing a proportional improvement in soil removal.

The chosen concentration also has to leave room for useful differentiation. If every product performs perfectly, the condition may create a ceiling effect. If every product fails, it may create a floor effect. As explained in Sinner's Circle, concentration is only one part of a process that also includes mechanical action, time, and temperature.

Common mistakes

Small preparation choices can change the conclusion.

• Reporting a ratio without stating whether it means product to water or product in final solution
• Using the amount of water added as the final volume without acknowledging the difference
• Mixing mass-, volume-, and mass-per-volume percentages without density or a defined conversion
• Comparing equal doses while the products are intended for different wash volumes or application rates
• Comparing formulas at equal active matter without defining which ingredients count as active
• Eyeballing viscous products or using dispensing devices that have not been checked for delivered quantity
• Changing concentration while also changing dwell time, temperature, water hardness, or mechanical action
• Selecting a single test concentration that causes every product to pass—or every product to fail

A good report records both the preparation and the basis of comparison. That usually means identifying the product dose, diluent or water condition, final solution quantity, resulting product concentration, relevant active concentration, and application amount.

A note on regulated products

Cleaning comparisons and antimicrobial directions are not interchangeable.

This article addresses cleaning-performance study design. For disinfectants, sanitizers, and other regulated biocides, the use concentration, application rate, method, surface, and contact time may be conditions of an authorized claim—not variables to adjust casually.

Health Canada's current biocide labelling guidance requires directions to identify conditions such as contact time, application rate, method of application, and applicable surfaces. Testing or using a product outside those directions can answer a different question and may not support the labelled claim. See Health Canada's guidance on biocide labels.

The common misconception

“Both products were tested at the same dose, so the comparison was fair.”

Maybe—but only if equal dose matches the decision. It may be appropriate for a fixed-dose efficiency comparison. A label-dose or cost-per-use comparison may instead require each product's own recommended dose. Equal dose may be inappropriate when products have different concentrations, densities, active contents, application rates, or intended use systems.

The useful question is not simply, “What dilution did you use?” It is, “What quantity reached the test system, how was it expressed, and why was that condition the right one for the decision?”