Preface

Cautions have been raised about excessive salt intake because it is considered to be related to hypertension, strokes, and many other diseases. On the other hand, it has recently been said that salt should be taken with water to prevent heat stroke during hot summers.
According to the “Dietary Reference Intakes for Japanese (2010)” (the Ministry of Health, Labour and Welfare) revised for the first time since 2005, the recommended daily allowance of salt for adult males is less than 9 g and less than 7.5 g for females, in terms of salt equivalent. The recommended daily allowance of salt for adults with hypertension is less than 6 g according to the “Japanese Society of Hypertension Guidelines for the Management of Hypertension (2009)” (the Japanese Society of Hypertension).
Similarly, the guidelines developed jointly by the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) of the UN in 2003 recommend that the daily salt intake for adults be less than 5 g (less than 2 g in terms of sodium equivalent). In 2006, the American Heart Association recommended that the daily sodium intake should be less than 2,300 mg (2.3 g). Thus, it has recently been recommended to further reduce daily salt intake.

So, how is salt content measured?

Methods for Measuring Salt Contents

Given below are typical methods for measuring salt content, and they all have both advantages and disadvantages:

  1. Sodium ion electrode method
    This method detects sodium ions (Na+) and converts them into the salt concentration.
  2. Chloride ion electrode method
    This method detects chloride ions (Cl-) and converts them into the salt concentration.
  3. Conductivity method
    This method converts conductivity (electric conductivity) into the salt concentration.
  4. Refractive index method
    This method measures the reflective index of a sample and converts it into the salt concentration.
  5. Coulometric titration method
    This method uses the reaction between silver ions (Ag+) produced by electrolysis of silver (anode decomposition) and chloride ions (Cl-) in a sample. It is called a chloride counter (this method is widely used in clinical examinations).
  6. Precipitation titration method
    This method uses precipitation formation following the reaction between silver ions in a silver nitrate solution and chloride ions in a sample, and is subdivided into the Mohr method (F. K. Mohr), the Fajans method, etc. depending on the endpoint indicator used.
  7. Dry methods
    Flame photometry, high-frequency plasma emission spectrometry, etc.

Of these methods, the ion electrode methods, the reflective index method, and the conductivity method are suitable for use with a small, simple measuring device. However, the conductivity method is disadvantageous in that measurements by this method are affected by all ions present. The reflective index method is not selective for salt dissolved in a sample, and measurements by this method are affected by sugars, various kinds of organic acid, and other components that affect the reflective index of the sample. Thus, this method can only be used in certain limited applications such as measurement of the salt content in pickle brine.
Measurements by the sodium ion electrode method are affected by potassium ions (K+) and lithium ions (Li+), but this method is highly selective for other ions. The physiological activity of salt comes from sodium ions, and this method is meaningful in that it detects sodium ions. For example, this method also detects sodium ions from monosodium glutamate (a flavor enhancer) in foods and indicates them in terms of salt equivalent. Based on the molecular weight ratio between salt and monosodium glutamate (to be precise, formula weight: 58.4 and 169.1, respectively), 1 g of monosodium glutamate is equivalent to about 0.35 g of salt.
We sell salt meters based on the sodium ion electrode method and the conductivity method. The LAQUAtwin series Salt Meter, which was recently released, also uses the sodium ion electrode method.