Since high blood pressure and stroke have a lot to do with overconsumption of salt (sodium ions), many doctors advise not consuming too much salt. (It is said that not more than 10 grams a day should be ingested.)

Also, there are a number of meters to measure the quantity of salt contained in various foods are on the market. Those meters work on various principles, a partial list of which follows:

1. A meter that measures concentration of chloride ions and converts it to NaCl
2. A meter that measures concentration of sodium ions and converts it to NaCl
3. A meter that measures conductivity and converts it to NaCl
4. A meter that measures refractivity of water and converts it to NaCl

As discussed in Chapter I, salinity can be measured by measuring the conductivity of water (if the water contains only NaCl). (The Twin conductivity meter comes with a salinity conversion display function.) For the purpose of reducing salt intake, we consider the method whereby Na+ is measured the most appropriate.

The density of ions in a solution can be determined from the measured conductivity, and this is often used in industry.

Examples: HCl, HNO₃, H₂SO₄, NaCl, CNaOH

Generally, electrolyte solutions take the maximum value, indicating existence of two densities of a solution against a certain conductivity, and we must be careful.

When air is absorbed as-is, or after pretreatment, in a proper reagent, the air causes chemical changes, and the conductivity of the reagent changes. By detecting this change, the density of ions in the air can be measured. The tester to measure air pollution by sulfur dioxide uses this system.

Examples: CO₂, CO, CH₄, HCl, H₂S, NH₄

When a solution of a substance B (with known density) that reacts with a substance A is gradually added to a solution containing substance A to be measured, substances A and B react, and the conductivity changes. When B is further added up to a point beyond which no further reaction occurs (point of equivalence), change will occur according to the increase in conductivity of B that has been added. The method by which one measures the quantity of A from the quantity of B added to the point of equivalence is referred to as titrimetric measurement of conductivity.

As mentioned earlier, when water contains only one kind of substance (NaCl, for example), the density of ions can be determined from the measured conductivity. The density of ions cannot be determined, however, from the measured conductivity of a solution that contains various substances. Where there is a will, however, there is a way to make it possible. The way is ion chromatography. This is a method for measuring concentration of ions in a solution, which distinguishes among different kinds of ions, and density can be determined from the conductivity of those ions. Actually, however, a solution to be measured is passed together with a specific liquid in a substance called an ion-exchange medium. This temporarily collects the ions in the liquid, and they exit as time passes. At this point, the time ions are collected varies with each kind of ion, and the ions come out separated by the kind ion. The time it takes for ions to exit is known for each different kind of ion. Therefore, the density of a specific kind of ion can be determined from conductivity and time. Since ion chromatography measures minute quantities of ions of numerous kinds, it is being used in many fields, including pollution, foods, clinical studies, industries and pharmaceuticals.