A brine percentage is usable only when its denominator is stated. Weight by weight (w/w) uses the finished solution mass. Weight by volume (w/v) uses the finished solution volume. A salt-to-water ratio compares salt with the water charged before mixing. The same number on these three bases describes different instructions.
Compare the three bases
| If it says | Use this basis |
|---|---|
| % w/w | Grams of solute per 100 g of finished solution. |
| % w/v | Grams of solute per 100 mL of finished solution. |
| Salt per 100 kg water | Kilograms of salt charged per 100 kg of starting water. |
For w/w, divide solute mass by total solution mass in matching units and multiply by 100. For w/v, report the number of grams per 100 mL of finished solution. Writing that unit in full helps prevent a reader from treating it as a mass fraction. The water-based ratio can be useful for describing how an operator charges a tank. Identify it on the batch sheet as an addition ratio so it can be distinguished from finished concentration.
Work two batches with the same salt charge
These examples illustrate the arithmetic for a two-component solution. Assume the salt dissolves and all material remains in the batch. The numbers serve as teaching examples.
- Batch A
10 kg salt + 90 kg water -
Finished mass · 10 kg + 90 kg = 100 kg.
w/w · (10 ÷ 100) × 100 = 10% w/w.
Water basis · (10 ÷ 90) × 100 ≈ 11.11 kg salt per 100 kg starting water. - Batch B
10 kg salt + 100 kg water -
Finished mass · 10 kg + 100 kg = 110 kg.
w/w · (10 ÷ 110) × 100 ≈ 9.09% w/w.
Water basis · (10 ÷ 100) × 100 = 10 kg salt per 100 kg starting water.
The second batch contains the same mass of salt spread across a larger finished batch. For a fixed finished batch mass, multiply that mass by the target mass fraction to find the required solute mass. If the only remaining ingredient is water, subtract the solute mass from the finished batch mass to find the water charge. In a multi-ingredient formulation, account for every ingredient retained in the finished mass. Keep a premix charge and the mass of a named constituent separate.
Volume example · 25 g in 500 mL of finished solution
Divide the salt mass by the final liquid volume, then scale to 100 mL. (25 ÷ 500) × 100 = 5 g per 100 mL, or 5% w/v. The 500 mL is the volume measured after the salt dissolves.
When the tank is marked in litres
A volume scale gives you litres. A mass specification requires kilograms. The connection is the density of the finished solution under stated conditions. To calculate batch mass, multiply the finished volume in litres by the density in kilograms per litre. To convert a numerical w/v value to w/w, divide the w/v value by the finished solution density expressed in grams per millilitre. That works because 100 mL of solution has a mass of 100 times that density in grams.
Use a density applicable to the actual composition and temperature, or weigh the finished batch if the process is controlled by mass. Starting water volume alone does not establish finished solution volume. The tank fill mark and the batch mass can each be useful, provided the record identifies what each value measures.
Put the basis on the specification
A line such as “10 kg sodium chloride per 100 kg finished solution, by mass” identifies both quantities. A volume-based line should name the solute in grams and the finished solution in millilitres or litres. An ingredient charge instruction should name the starting water amount explicitly. When purchasing a brine preparation tank, distinguish usable liquid volume from how the plant will establish the batch mass and verify the concentration.
Keep the calculated recipe basis and the verification method in separate fields. A recipe calculation starts from known quantities. An instrument reading depends on the instrument and sample. Use the guide to Brix and salt measurements to decide what a reading actually represents.
Frequently asked questions
Is 10 kg of salt per 100 kg of water 10% w/w?
The water-based addition ratio is 10 kg per 100 kg water. The finished solution weighs 110 kg, so the salt concentration is approximately 9.09% w/w.
Does 1% w/v use 100 mL of starting water?
It means one gram of the stated solute per 100 mL of finished solution. The denominator is the final volume after preparation.
Can I use tank litres as batch kilograms?
Use the finished solution density to convert litres to kilograms, or obtain the batch mass directly by weighing. A water-density assumption can change the result.
Is a premix addition percentage the same as salt concentration?
A premix percentage tracks the mass of that blend on the stated basis. To calculate the amount of salt supplied by the blend, use its documented salt content and account for other salt sources.
Is brine concentration the same as injector pickup?
Concentration describes the liquid composition. Injection pickup compares the change in product mass with its starting mass. They answer different production questions.
For a batch with visible grains, clumps or a deposit, see the brine tank residue troubleshooting guide.
Sources and scope
The university references define concentration units. The batch arithmetic illustrates those units for plant records.
- Oregon State University, CH 461 Handbook of Useful Information, 29 September 2019, page 3. Weight percent, mass per finished volume and the density distinction.
- Purdue University, Concentrations of Solutions, undated. Mass of the solute relative to total solution mass.