7 Application

8/11/2019 7 Application

1/16

Titration

Ion Selective

Electrodes

Density

Balances

Determination of

Salt in Food

The Ultimate Salt Guide

Proven Analytical Methods and Results

Food

Ana

lysis


2/16

2METTLER TOLEDOSalt Guide

Salt and humans go a long way together. In earlier times, before mining of rock salt had

started, salt was a high-priced and much sought after commodity. Nowadays, with cheaper

salt prices, salt is a key ingredient in processed foods. This guide will:

Give insights into methods of salt determination in food

Provide a short overview of the history of salt

Show and explain measurement in selected samples

Present a few tips and hints to improve operator technique on titrators and balances.

Content 2

1. Introduction 3

2. Solutions Overview 43. Argentometric Titration 4

4. Ion Selective Methods 5

5. Determination of Salt Content Based on Density 7

6. Ash Content 8

7. Conclusions 9

8. More Information 9

9. Appendix 9

Ti-Note 17 10

Ti-Note 12 12

Ti-Note 19 14


3/16


1. Introduction

Salt From White Gold to Commodity

Salt has a long standing culture and can be traced back to ancient times in most civilizations. Babylonians and

Sumerians have used salt for conservation of food items. Salt was always in high demand and rare in many

regions. Because of salt, many cities have become rich and influential, for instance Luneburg within the Hanse

region.

No wonder that salt has been called 'the white gold'. Interestingly though, even the word salary originates from

the original meaning of a 'soldier's allowance for salt'. Salt was especially high-priced in the Middle Ages and

became affordable in the Germanophone regions only when harvesting the several 100 meters thick, about 250

million year old, salt layers of the Zechsteinmeer in Northern Germany, was made possible.

Table salt, consisting mainly of sodium chloride, is the most commonly used salt in our food. Even after the

production process of customary table salt, either from rock salt or sea salt, 1-3% of other salts are remaining;

unprocessed sea salt contains up to 5% of water. Table salt is a cleaned and refined salt. To improve attributes

such as pourability and hygroscopy, small amounts of other substances are later on added to the salt. In table

salt, sea salt and stone salt are often distinguished. Both are harvested in different processes

Rock Salt

Rock salt is often a product that is mined from underground. Harvesting is assured either through digging the

rock salt out or by solving out. Table salt is then separated from other substances in salines, through selected

solving and concentrating and then condensation or boiling.

Different cultures around the globe have developed different techniques to harvest salt. Some South American

Indian tribes gain a potassium chloride rich salt from leeching plant ash. In the area around the Chad Lake,

earth containing salt is leeched, filtered and then boiled down. In earlier times, in some areas of Northern

Germany, peat which had been flooded by the sea was utilized to extract salt.

Sea Salt

Harvesting salt from sea water is probably the oldest method of harvesting salt. Sea water is channeled into

salt gardens, where the water slowly evaporates. All dissolved ions, depending on their solubilities, crystallize

one after another in different layers. Sodium chloride is in the top layer that is harvested before the water entirely

evaporates. Contamination with other salts cannot be prevented in this way of salt harvesting, however often is

a marketing feature when sold. Nowadays about 20% of the global consumption of salt is extracted from sea

water.

Salt Today

Salt is an essential ingredient of processed food and the salt content of products often needs to be determinedas accurately as possible. Considering the past glory of the 'white gold', salt nowadays often has something of a

bad reputation. Too much of it mainly sodium ion has adverse effects on our health. Currently WHO and FAO

have taken initiatives that aim at capping the salt consumption by defining maximum values for food products.

It is expected that this trend will continue and maximum admissible values of salt content will come into place.

Pressure on the food manufacturers to reduce the salt content in their products is likely to increase. Thus, many

recipes have undergone reformulation or are still waiting to be reworked. Such tasks require quite a lot of testing

including salt content determination.

The following sections will give insights into the different methods of salt determination, e.g. titration, loss on

drying, then give insights into the determination of the salt content in selected food items and finally offer some

tips and hints on how to improve measurement techniques for easier working procedures and more accurate

results.


4/16


2. Solutions Overview

Salt Content Determination in Solid and Liquid Samples

Several techniques for the determination of salt, chloride, sodium and potassium in almost any kind of samples

are available at METTLER TOLEDO. Liquid samples may undergo direct determination. Solid samples however,

may require a preparation step to release the salt and dissolve the ions.

Titration Ion selective

methods

Density Analytical

Balance

Precision

Balance

Salt content

Chloride content

Sodium content

Potassium content

Ash content (total salts)

3. Argentometric Titration of Salt

Titration

Titration is one of the oldest chemical quantitative analyses. Still today, this very reliable method of high

accuracy and precision is well appreciated and frequently applied. Its linearity, i.e. whether a certain method

produces correct results over the concentration range of interest, is unsurpassed. Hence, very low up to very

high concentrations are safely determined. For the calibration of the method, simply the titer determination is

carried out.

Argentometry

A very common method of the salt content determination is the argentometric titration of the chloride ion. Based

on the chloride content, the amount of sodium chloride, i.e. salt, is then calculated.

The argentometric titration is a precipitation reaction: The sparingly soluble silver chloride is formed from the

chloride ions contained in the sample and the added silver nitrate of the titrant.

Ag++ Cl-AgCl

With precipitation titrations, several characteristics should be noted.

The titration reaction may be slower compared e.g. with an acid/base titration in aqueous samples. Thus,

apply a medium titration speed as referenced in many METTLER TOLEDO applications.

At the start of the titration, the sample solution may become supersaturated before the precipitate is formed.Hence, the electrode cannot indicate the proceeding of the titration reaction. In order to avoid super saturation,

we recommend adjusting the pH of the sample solution to the required value. For the chloride tit ration in

general, the sample solution is slightly acidified with nitric acid to 4.5 pH.

With highly concentrated sample solutions, inclusions of sample and/or titrant may occur in the precipitate,

thereby falsifying the result. Rapid stirring during titration is an effective countermeasure.

ResultsSample Mean RSD % n

Mustard (low salt) 0.142% 1.19 8

Tomato juice 0.50% 0.13 5

Ham pie 1.69% 3.12 6Ketchup 2.67% 0.17 15

Seasoning 17.24% 0.18 8

Mixed spices 55.66% 0.18 6


5/16


4. Ion Selective Methods

Ion Selective Electrodes

Ion selective electrodes (ISE) are an alternative method to measure the concentration of ions in solutions. It is a

simple setup consisting of the respective ISE, a suitable ion meter, titrator or similar instrument and a stirrer. An

ISE is built of a sensing element, a membrane, and the electrode body. There are 4 types of sensing membranes

in use for the various ISEs.

Type Application examples

Glass membrane pH, sodium

Crystalline membrane (solid state) Fluoride, iodide, cupper

Polymer membrane (liquid membrane) Potassium, calcium, lithium

Gas sensing Dissolved oxygen, carbon dioxide

Types of membranes

Measurement

The ISE responses to the concentration - or more precisely the activity - of the determinant ion. The response

follows the Nernst equation which is well known from pH measurement. If the influence of interfering ions is

included as well, this equation is extended to the Nicolsky equation.

However, both equations describe a linear relation between the potential readings (in mV) and the logarithm of

the ion concentration (or activity respectively). At the detection limit and in high concentrations, linearity is no

longer achieved and determinations deteriorate. For most ions, the applicable concentration range is specified

from 10-1to 10-5mol/L.

Response TimeISEs reach a stable potential reading typically within 1 to 3 minutes. The response time usually is at the shorter

end in concentrated solutions and at the longer end in diluted solutions respectively.

Storage of the ISE

For short periods of time, it is recommended to store the electrode in 0.01 mol/L standard solution of the

respective ion.

For longer periods, i.e. more than 1 week, store dry. For dry storage see the ISE instructions. In general, drain

electrode and flush with deionized water. Then protect the sensing element.

Interferences

ISEs are selective to one ion but not specific for it. Thus, other ions present in the sample solution also contribute

to the reading of the electrode. The selectivity coefficient describes the preference of the ISE over the interfering

ions. Selectivity coefficients are typical for an ISE and specified in the instruction manual.


6/16


7/16


5. Determination of Salt Content Based on Density

The salt content of a solution made of salt and water (i.e. 2 components only) can also be determined by

density measurements. The more salt is dissolved in the water (or a solvent) the higher is the density. Based on

a conversion table, the salt content is evaluated from the density measurement.

Traditional techniques for the density measurement are hydrometers and pycnometers. However, these methodsare time consuming, need considerable sample volumes and are prone to reading errors.

Sodium chloride, 25C, in water

Salt content g/L 25 20 10 5 1

Density g/cm3 1.1887 1.1478 1.0707 1.0340 1.0053

Density Measurement and Density Meters

Current density meters apply the physical principle of the oscillating U-tube. The oscillation frequency depends

on the mass of the U-tube which is the contents of the U-tube respectively. Thus, a few milliliter of sample are

applied by syringe or an automatic sample changer to the density meter instrument.

Schematic of density cell

The measurement takes 2 - 3 minutes, needs no reagents and allows the sample to be recollected for further

uses.

Rinsing and drying of the U-tube before the next sample can be done automatically. This adds result safety to the

fast speed of the measurement.

Because density depends on temperature, modern density meters are electronically thermostated and

compensate the density results to 20C or any other temperature.

Influence of temperature on the density of water


8/16


9/16


7. Conclusions

There are several methods available to determine the salt content of food items. Depending on the consistency

of the sample (liquid vs. solid), legal and labeling requirements, level of accuracy and precision as well as

opportunities of the lab, the most suitable method is selected. Argentometric titration surely is the most accurate

method to determine the salt content and very frequently applied. However, the use of ISEs to determine

selectively sodium or potassium may be needed. Salt content determination via density might prove to be fasterbut less accurate.

METTLER TOLEDO supplies food laboratories with the right instruments, for the corresponding method of choice.

Find out more about our products, and get in touch with our experts, for suggestions on how you can benefit

from METTLER TOLEDO's expertise in the food industry.

8. More Information

Find more about solutions from METTLER TOLEDO

More about potentiometric titrators: www.mt.com/titrat ion

More about density meters: www.mt.com/Liquiphysics

More about Excellence balances: www.mt.com/excellence

If you liked this guide, we are proudly presenting the METTLER TOLEDO series of guides for the food industry.

Please click on the below links to get access to the respective food guides.

The Ultimate Sugar Guide www.mt.com/lab-sugar

The Ultimate Acidity Guide www.mt.com/lab-acidity

The Ultimate Formulation Guide www.mt.com/lab-formulation

The Ultimate Edible Fats and Oils Guide www.mt.com/lab-oil

The Ultimate Moisture and Water Content Guide www.mt.com/lab-water

9. Appendix

Ti-Note 17 Chloride Content in Ketchup

Ti-Note 12 Potassium Content Detemination by Direct Measurement

Ti-Note 19 Sodium Determinationwith ISE


10/16



11/16



12/16


13/16



14/16



15/16



16/16

For more information

www.mt.com

Mettler-Toledo AGLaboratory Division

Im Langacher

CH-8606 Greifensee, Switzerland

Subject to technical changes

09/2012 Met tler-Toledo AG

G C S

Good Measuring Practices by METTLER TOLEDO is a global program sup-

porting you in laboratory and production environments with quality assur-ance measures for balances, scales, pipettes and analytical instruments.

The ve steps of all Good Measuring Practices guidelines start with an

evaluation of the measuring needs of your processes and their associated

risks. We also take into account regulatory requirements and norms

relevant to your industry.

With this information, Good Measuring Practices provide straight forward

recommendations for selecting, installing, calibrating and operating of

weighing and measuring instruments.

www.mt.com/gwp for weighing

www.mt.com/gtp for titration

www.mt.com/gpp for pipetting

www.mt.com/gdrp for density and refractometry

Good Measuring Practices

Five Steps to Improved Measuring Results

GoodMeasuring

Practices

1Evaluation

2Selection

3Installation /

Training

5Routine

Operation

4Calibration /

Qualification

7 Application

Documents

Transcript of 7 Application