Micro-chemical testing

<bibimport/> Author:Christine Bläuer
back to Analysis of Salts

The analysis of salts can be carried out by micro-chemical testing on very small amounts of sample(s). The analysis consists in the qualitative detection of single ions in more or less concentrated salt solutions. The reaction products are either visible to the naked eye (formation of a precipitate or color change of the solution) or through a microscope where the specific crystal shapes can be identified using a magnification of about 100 times.

Abstract

Micro-chemical salt analysis consists in the detection of single ions in more or less concentrated salt solutions

Analytical procedure

The first step of every material analysis is sampling. In this procedure, the samples to be analyzed are pure salt efflorescence. These can be either fluffy or form a crust on the surface of the material.

Fluffy salt efflorescence on brick wall

Epsomite crust on gypsum stucco

It is therefore important to make sure that the samples only contain the salt efflorescence and that particles or grains of the substrate were not included during the sampling. Adequate sampling procedures are critical as otherwise the test results will be distorted.

• The samples need to be kept and transported in air-tight containers.
• Suitable containers are: small, closable plastic bags; glass tubes with stopper; etc.
• Unsuitable containers are: gelatine capsules.

The samples should be analyzed as soon as possible. If the samples have to be kept for a time before they are analyzed, they should be kept in a refrigerator.

Types of salt samples

In principle there are three types of salt samples that can be analysed micro-chemically with the methods described here:

• Pure single salt phases (minerals of a single salt)
• Salt mixtures containing two or more salt minerals - when it is not possible to separate the individual salt phases
• Water extractions from samples of salt containing materials

The testing methods are the same for the tree types of samples with only minor differences, but the interpretation of the results varies. The procedure is based on the detection of ions in a water extraction. Therefore, it is a prerequisite for the analysis that the ions be in solution. However, during dissolution of the sample chemical reactions may take place that will falsify the analytical results.

Necessary equipment and chemicals

Microscope with transmitted or reflected light with magnification of about 100-fold. Some of the chemical reactions may release corrosive gases that can attack the metallic parts of the objectives. To somewhat protect these, they can be covered with a thin layer of Vaseline or other vacuum grease. This protective layer needs to be renewed regularly. Under no circumstances should a precious polarized light microscope be used to investigate the reaction products of micro-chemical reactions.

Further equipment:

• Wash bottle containing demineralized water.
• 2 waste containers for the rinsing water produced during the analytical procedure - one container for innocuous and anonther for toxic waste (details see individual tests). Very suitable for this purpose are large 0.5 Liter plastic beakers.
• Glass Pasteur capillary pipettes with suction cups.
• Micro slides with and without cavities.
• Spot plate.
• Cotton wool.
• Thin glass rods.
• Micro-pipettes.
• Spatula and/or scalpel.
• Small laboratory heating plate or alcohol burner to evaporate sample drops.
• Absorbent tissue paper
• pH-Paper strips (pH-values 1-14)

The required reagent solutions and chemicals for comparison are listed in the following tables.

Note: All chemicals used need to be pure p.A . grade

Table 1: Reagent solutions

Solution	Chem. formula / concentration	use / test method
De-mineralized water	H2Odemin.	Dissolution of the salts
Diluted hydrochloric acid	2M HCl	acidifying / gypsum
Diluted nitric acid	2M HNO3	Na+, K+ ; acidifying / Cl-, gypsum
Concentrated sulfuric acid	conc. H2SO4	Na+, K+
Diluted sulfuric acid	1M H2SO4	Ca++
Concentrated ammonia solution	NH3 aq.	Mg++
diluted ammonia solution	2M NH4OH	Mg++
sodium hydroxide solution	2M NaOH	Mg++
Silver nitrate solution	1% aqueous solution of AgNO3	Cl-
Calcium chloride solution	1% aqueous solution of CaCl2	SO4=
Di-ammonia hydrogen phosphate solution	10% aqueous solution of (NH4)2HPO4	Mg++
Di-sodium hydrogen phosphate solution	10% aqueous solution of Na2HPO4	Mg++
Titan yellow solution	10 mg Thiazol Yellow dissolved in 10 ml de-mineralized water	Mg++
Barium chloride solution	BaCl2, 0.5M aqueous solution	SO4=

Table 2: solid reagents

substance	Chem. formula	use / test method
Bismuth nitrate	Bi(NO3)3	Na+, K+
Cesium chloride	CsCl cryst.	Mg++
Diphenylamine	(C6H5)2NH cryst.	NO3-

Table 3: Useful reagents to compare reaction products

Salt for control reactions	Chem. Formula	use / test method
Sodium nitrate, Nitratine or nitratite	NaNO3	Na+, NO3-
Potassium nitrate, Niter	KNO3	K+, NO3-
Gypsum	CaSO4 • 2H2O	Ca++, SO4=
Sodium chloride, Halite	NaCl	Na+, Cl-
Magnesium nitrate	Mg(NO3)2 • 6 H2O	Mg++, NO3=
Magnesium sulphate, Epsomite	MgSO4 • 7H2O	Mg++, SO4=
Sodium bicarbonate or sodium hydrogen carbonate, Nahcolite	NaHCO3	Na+, HCO3-, slightly alkaline
Sodium carbonate	Na2CO3	Na+, CO3=, strongly alkaline
Ammonium nitrate	NH4NO3	NH4+, NO3_, very slightly acid

Micro-chemical tests

General remarks

When working with chemicals safety precautions have to be taken. For example, no smoking, eating or drinking should be allowed in the laboratory or certainly not during the testing. It is advised to wear protective clothes, such as a lab coat and goggles. As gases may be released during some of the tests, the work should be carried out under appropriate ventilation, if possible in a fume hood. Hands should be washed before start of the testing, and at any point in which an interruption may occur. When leaving the laboratory, the lab coat and goggles should be removed and hands washed again. Upon resuming work in the laboratory, the same initial procedures should be taken, i.e., protective clothing donned, hands washed.

Used pipettes, glass rods, object slides, scalpels, etc. need to be thoroughly washed or cleaned before every test and before dipping them into reagent solutions. If necessary, they have to be replaced with clean new ones.

Each test described here has to be carried out on a separate test drop or salt grain.

Important information about micro-chemical analysis can be found in the book by [Chamot.etal:1946]Title: Handbook of chemical microscopy - Chemical methods and inorganic qualitative analysis. Vol. II
Author: Chamot, Emil Monnin; Mason, Clyde Walter

Test methods

Sample preparation and general tips

Some salt grains are separated from the sample and dissolved in a few drops of de-mineralized water in the small cavity of a micro slide. The sample should be observed during the dissolution process using a microscope. This allows observing if the sample contains insoluble constituents and, in some cases, it is possible to deduce from their different dissolution rate if the sample contains more than one salt phase.

If an insoluble residue is observed this has to be removed by filtration as described below. One part of the sample should always remain untouched to perform tests that have to be executed on non-dissolved material (tests for ammonia, carbonates and hydrogencarbonates).

Micro-chemical test for the following ions are described in detail:

• pH value
• Micro-chemical test for sodium and potassium (Na⁺, K⁺).
• Micro-chemical test for ammonia (NH₄⁺).
• Micro-chemical test for calcium (Ca²⁺).
• Micro-chemical test for magnesium (Mg²⁺).
• Micro-chemical test for chloride (Cl^-).
• Micro-chemical test for nitrate (NO₃^-).
• Micro-chemical test for sulfate (SO₄^2-).
• Micro-chemical test for carbonate (CO₃^2-).
• Micro-chemical test for hydrogen carbonate (bicarbonate) (HCO₃^-).

Micro-chemical test procedure

Dissolution of the sample

Some grains of the salt sample are put in the cavity of the glass slide and one or two drops of demineralized water added. The dissolution behavior of the sample is observed using a microscope. During this process, usually three possible situations can be observed

1. All particles are more or less quickly dissolved and a limpid clear solution produced.
2. All particles are dissolved but after a moment a precipitate is formed. (see Interpretation / micro-chemical investigation of salt mixtures).
3. Not all particles are dissolved, because the sample contains other particles than salt.

In the first case the micro-chemical testing can take place using the solution without further treatment. In the other two cases the solution must be separated from the insoluble parts by filtration.

Filtration

A rubber suction cup is mounted on the Pasteur pipette. A small piece of cotton wool is rolled between finger and thumb into a compact strand and inserted into the tip end of the pipette. Hold the cotton wool in place while pressing the suction cup then dip the free end of the strand into the suspension and filter the solution through the cotton wool by releasing the suction cup. The cotton wool retaining the insoluble particles is discarded and the filtered solution is placed in the cavity of a fresh glass slide. Should this solution still be unclear or milky the filtering procedure has to be repeated using a fresh pipette and cotton wool.

It is worth going though the effort of making a good filtration, because the reaction products of the test reactions can hardly be detected in a milky or unclear solution.

pH-value and micro-chemical tests

For each micro chemical test place a drop of the solution onto a plain glass slide (without a cavity) using a glass rod.

The pH of the solution is tested by applying a drop with the glass rod onto an appropriate pH-paper strip.

Detailed test procedures are given for each individual ion. It is best to make each test on a fresh clean object slide, because this avoids all possible contamination by vapors emitted during tests performed on neighboring drops.

Interpretation of the results

Micro-chemical analysis of single salts

When single salts are present the analysis is especially helpful to verify or complement the results of microscopic analysis. For example, the salt minerals Arcanite (K₂SO₄) and Aphthitalite (K₃Na(SO₄)₂) can only be distinguished from one another microscopically using a microscope that allows conoscopy and the salt grains of the sample are big enough to allow producing a reasonable conoscopic image. Micro-chemically the two salts can however be distinguished quite easily.

Micro-chemical analysis of salt mixtures

It is not uncommon that different salt minerals are found present within a single efflorescence. When analyzing such mixtures it is important to be aware of the possible chemical reactions that can take place during the analytical procedure, especially during dissolution. In some cases the ions can react with one another to produce another salt than the ones present originally. Such reactions can occur within building materials [Arnold:1981]Title: Salzmineralien in Mauerwerken
Author: Arnold, Andreas
or they can happen only later during the micro-chemical testing procedure. In general, it is impossible to separate individual salt minerals for micro-chemical testing and salt mixtures have to be dissolved together. In particular, reactions that produce water insoluble residues need to be taken into account, because the precipitation reduces the amount of one or more ions present in the original salt mixture.

Example

The problems encountered when analyzing such salt mixtures are described here for the case of mixtures of calcium sulfate and sodium carbonate.

Mixtures containing gypsum (CaSO₄ • 2H₂O) as well as natron (Na₂CO₃ • 10H₂O) or thermonatrite (Na₂CO₃ • H₂O) are frequently found on monuments. If such a mixture is dissolved in water, calcite (CaCO₃) is precipitated immediately, which often can be observed by the appearance of a white veil within the drop. When this drop is acidified the calcite is dissolved and the carbonate ion disappears from the solution as CO₂ gas visible as an effervescence while the related calcium ion remains in solution. If the salt solution is separated without acidifying from the precipitate by filtration, the filtrate solution is depleted with regard of the ions that had precipitated with the calcium ion to form calcite.

The analytical results depend on the relative quantities of gypsum and sodium carbonate in the original sample:

1) If the stoichiometrical amounts of gypsum and sodium carbonate are exactly the same (a most unlikely case) the filtered solution will be neutral (pH = 7) and contain Na⁺ and SO₄^2-. In the acidified solution only Na⁺-, SO₄^2-- and Ca²⁺ ions will be found.

2) If more gypsum than sodium carbonate is present, the filtrate will contain Na⁺, SO₄^2- and Ca²⁺ with a neutral pH. In the acidified solution, besides Na⁺ and SO₄^2-, somewhat more Ca²⁺ will be present than in the filtered solution.

3) If less gypsum is present than sodium carbonate the filtrate contains not only Na⁺ and SO₄^2-but also CO₃^2- ions, and therefore the pH of the solution is alkaline: The acidified drop contains Na⁺, SO₄^2- and Ca²⁺.

It is important to be aware of the possible reactions to avoid misinterpretation of the results. In these cases a careful mineralogical investigation helps to observe the salt minerals that are really present on the building. The described precipitation reactions can also take place within the building materials if the diverse salts are dissolved together. Therefore it is possible that the efflorescing salts on a given object may differ at different times.

Micro-chemical analysis of water extractions from salt containing material

If the presence of salts is presumed within stone or mortar, the micro-chemical tests can be performed on concentrated water extractions of these mixed materials. In this case, no analysis of salt minerals is possible and only the water extractable ions are detected.

Within the material these ions are present as ions in the pore solution or they are crystallized in the pore space. If the sample contains salt mixtures precipitations of insoluble phases, such as calcite, occurring during dissolution, these cannot be detected because they cannot be extracted by water or distinguished from the same compound that may be present in the stone or mortar powder.

Example

The problems arising during the analysis of such samples will be discussed considering samples containing sodium carbonate and gypsum.

During dissolution calcite is precipitated from the sample. As nearly all mortars and many building stones contain calcite this precipitate cannot be distinguished from the calcite already present in the material. In relation to the relative quantities of gypsum and sodium carbonate the analytical result will be as follows:

1. If stoichiometrically equal amounts of gypsum and sodium carbonate are present, the filtrate solution of the sample will contain Na⁺, SO₄^2- and its pH will be neutral. This means that nothing indicates that the sample initially contained gypsum and sodium carbonate!
2. If more gypsum than sodium carbonate is present, the filtrate will contain Na⁺ und SO₄^2- as well as Ca²⁺; the pH will neutral as in the former case. In this case nothing indicates the initial presence of sodium carbonate.
3. If less gypsum than sodium carbonate is present the filtrate contains Na⁺ and SO₄^2- and the pH is alkaline. This means there is no indication that the sample originally contained gypsum.

The described problems can also occur when analyzing salts by means of water extractions of drill cores or other salt containing materials. As shown above, the results can be distorted.

Literature

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Gy, Pierre 1996. L’échantillonnage des lots de matière en vue de leur analyse. Masson, Paris, Mailand, Barcelona; Gy, Pierre 1998. Sampling for analytical purposes. Wiley & Sons Ltd. West Sussex

crystallization of a dissolved salt