An instrument that measures salt content is called a salt meter.
There are a few kinds of salt meters that differ in their measurement principles and methods.
4) The refractometer method uses the fact that the refractive index increases with solubility in an aqueous solution to measure the salt concentration by measuring the refractive index.
ATAGO offers salt meters that use 3) the electrical conductivity method and 4) the refractometer method.
The Advantages of an Electrical Conductivity Salt Meter
Salt is an electrolyte, so there is a correlation between electrical conductivity and salt concentration.
Electrical conductivity is a scale for measuring how much electricity is flowing through a medium, and it is the inverse of electrical resistance (electrical conductivity = 1/electrical resistance). The electrical resistance of a distance between electrodes of 1 cm is called an ohm centimeter (Ωcm), and the inverse is called the specific conductivity.
That value multiplied by 1 million is the commonly-used electrical conductivity (microsiemens: μS/cm).
If the electric resistance for 1 cm is 1 million Ωcm, then the specific conductivity is 1 millionth, and electrical conductivity is calculated by multiplying that by 1 million, resulting in a value of 1 μS/cm.
The Advantages of an Electrical Conductivity Salt Meter
＊Electrical conductivity has an especially large advantage over the Mohr method (silver nitrate titration).
・ The instrument is portable.
・ Measurements can be taken anywhere.
・ Measurement takes only 3 seconds.
・ A reagent is not necessary.
Reagents usually cannot be brought onto the production floor. In addition, as a consumable a reagent adds another running cost to production, and the disposal of the reagent is not environmentally friendly.
・ No need for a titrator or scientific tools such as pipettes.
・ The measurement is automatic, so there is no discrepancy between testers.
In the Mohr method, the end point is determined by a change in color, so inexperienced testers can make mistakes when measuring. With the electrical conductivity method, measurement is automatic, causing no discrepancies between users and requiting no experience.
・ Anyone can easily take measurements.
・ Only 0.3 mL of liquid is needed for measurement.
・ Just wipe the electrode with water or alcohol.
・ Easy to clean and store.
・ Lower cost than automatic titration equipment.
Most salt meters using the electrical conductivity method use gold plate electrodes, but a major feature of ATAGO's electrical conductivity salt meters is that they use titanium electrodes. Gold plate electrodes are easily scratched after years of use, and once the electrode is scratched, the measurements fluctuate and are no longer precise. ATAGO's Salt Meters, however, use titanium electrodes that are scratch-resistant and extremely durable—the failure rate of ATAGO's Salt Meters is only 0.1% within 3 years after purchase.
Caution must be taken when using electrical conductivity salt meters when measuring liquid solutions that contain other electrolytes. For example, if there is 1 g of monosodium glutamate in 100 g, it will add about 0.16%. Actual foods, however, do not have as much as 1 g in 100 g, so it can be disregarded.
The following are the effects on electrolytes containing salt (sodium chloride) and electrical conductivity.
① Weak acid・・・(Examples) glutamic acid, phosphoric acid, acetic acid, ascorbic acid, etc.
② Weak base・・・(Examples) ammonium, etc.
③ Strong acid・・(Examples) hydrochloric acid, nitric acid, etc.
④ Strong base・・(Examples) sodium (soda), potassium, calcium, etc.
The combinations of ①/②, ①/④, ②/③, and ③/④ are electrolytes.
Among these, the ①/② combination (for example, ammonium phosphate) has almost no effect on electrical conductivity. The combinations of ①/④ and ②/③ (for example, monosodium glutamate) does not have much of an effect on electrical conductivity when there are other strong electrolytes (when the sample contains sodium chloride).
Conversely, the ③/④ combination (for example, sodium chloride [sodium hydrochloride] or sodium nitrate) has a major effect on electrical conductivity.
Features of Salt Meters using the Refractometer Method
A refractometer is an instrument that measures the concentration of liquid by using the principle of light refraction.
ATAGO's lineup of refractometers includes a salt meter for measuring salt water.
For more information on the principles of refractometers, see "Data book: Refractometers."
The salt meters using refraction specialize in measuring the salt concentration of salt water. Salt meters using the electrical conductivity method do not just measure the sodium chloride (electrolytes) in salt water, they can also measure the sodium chloride in liquids containing other components, but salt meters using refraction can only measure pure salt water.
So what advantages does the refraction method have over the electrical conductivity method?
First, for strong salt water with a concentration of 10% to 28%, the electrical conductivity method requires dilution, but refractometers can directly measure the salt concentration without dilution. For example, salt water used in the salt-making process or when making 'udon' (wheat noodles) has a concentration of higher than 10%.
Second, for the food and seafood industries, salt water is widely used—from the brine used for soaking pickled vegetables or dried fish, to the salt water used for stock when boiling crab or whitebait. For a long time, Baumé hydrometers have been widely used at these kinds of sites. Although Baumé hydrometers have the advantage of a low price, there are many disadvantages. With Baumé hydrometers, errors can occur from person to person when reading the measurement, and there is no temperature correction function, which means that precise measurements are impossible. Also, they are made of glass, making them undesirable for use on the food production floor.
ATAGO's refractometer Salt Meters include Baumé scale marks to make it easy for customers using the Baumé scale to replace their Baumé hydrometers.
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