We measured the pH, conductivity, sodium ion and potassium ion concentrations, and salt content of commercially available salted and salt-free tomato juices by using our LAQUAtwin series compact meter (using the sodium ion liquid membrane electrode method). The conductivity measurements were converted into their salt content equivalents. The tomato juices used as the samples were measured directly with no pretreatment. Measurement was performed at room temperature (25ºC). The following table shows the measurement results:

【Talbe A】Salt Content and Other Measurements of Tomato Juices Obtained by the LAQUAtwin

Salted tomato juice pH Conductivity(mS/cm-1) Na+(ppm) K+(ppm) Salt(% conductivity conversion) Salt(% sodium conversion)
Display value - - 1421 2526 0.42
Measured value 4.27 12.6 1100 2300 0.29 0.71
Salt-free tomato juice pH Conductivity(mS/cm-1) Na+(ppm) K+(ppm) Salt(% conductivity conversion) Salt(% sodium conversion)
Display value - - 60 2500 0
Measured value 4.27 7.5 61 2200 0.02 0.42

As shown in this table, the measurements of sodium ion and potassium ion concentrations in both the salted and salt-free tomato juices are almost the same as the values indicated on the labels. The measurements of salt content in the salted and salt-free tomato juices were 0.29% and 0.02%, compared to 0.42% and 0% indicated on the labels. On the other hand, the salt content equivalents calculated from the conductivity of the salted and salt-free tomato juices were 0.71% and 0.42%, which are a bit larger than the values indicated on the labels. In particular, the salt content equivalent calculated for the salt-free tomato juice, which is much larger than the indicated value, shows that a salt meter that uses the conductivity method is disadvantageous in that the method is affected by all the ions present in the sample.


Conductivity and Salt Concentrations and Points to Remember in Using a Salt Meter Using the Conductivity Method

Let us explain conductivity and salt concentrations here. Cheap salt meters convert the measurement of conductivity of a sample into its salt content equivalent.
The following table shows the relationship between saltwater concentrations and conductivity.

Table: Saltwater Concentrations and Conductivity (Solution Temperature: 25ºC)

NaCl density
(W/V)%

Conductivity
(mS/cm)

NaCl density
(W/V)%

Conductivity
(mS/cm)

0.1

2.0

1.1

19.2

0.2

3.9

1.2

20.8

0.3

5.7

1.3

22.4

0.4

7.5

1.4

24.0

0.5

9.2

1.5

25.6

0.6

10.9

1.6

27.1

0.7

12.6

1.7

28.6

0.8

14.3

1.8

30.1

0.9

16.0

1.9

31.6

1.0

17.6

2.0

33.0

Source: IEEE.J.Ocean.Eng.,OE-5(1),3-8(1980).


A salt meter using the conductivity method uses these values to convert conductivity into salt content equivalents. Intended to be used with brackish waters (mixtures of seawater and fresh water), some salt meters use the conductivity of standard seawater and dilute solutions of standard seawater.

Conductivity refers to how easily current flows in a sample, which is affected by the ions present in it. Thus, the conductivity method lacks selectivity for salt, and cannot be expected to be accurate with samples containing ionic compounds (ions) other than salt. This is evident from the measurement result of the salt-free tomato juice (the salt content calculated from the conductivity was 0.42%, compared to 0% indicated on the label). When using a salt meter that uses this method, you should be aware of this. In particular, a salt meter using the conductivity method is not suitable for the measurement of salt content in samples containing large amounts of organic acids such as acetic acid and citric acid, which significantly affect conductivity values. Specifically, this type of salt meter is not suitable for the measurement of salt content in tomato juice, citrus juice, or foods containing a large amount of vinegar. A salt meter using the sodium ion electrode method should be used for these kinds of samples.