## 3 Chemical Tests
### Typical Methods of Chemical Testing  
#### 3.2.1 Determination of the Chloride and Sulfate Ions  
Generally, chloride and sulfate ions content tests should be carried out on powdered samples of concrete taken during drilling or from core drilling or other concrete fragments obtained from the structure. Test samples should be taken from representative places (in concrete cover, especially at the reinforcement surface) where there is a risk of pollution. It is recommended that samples be taken from different depths. The minimum required amount of sample is 1 g, however, when collecting powdered samples it is recommended to collect about 20 g of the material, so as to ensure the representativeness of all concrete components. The test drilling should have a diameter of 30 to 50 mm; they can be cut into slices or crushed at different depths.  
There are two similar chemical laboratory tests that allow direct determination of chloride content in a concrete sample (mass percentage of chloride referred to total concrete or to binder)--the Mohr or Fajans method [28,29]. In both cases, the principle of the method is the reaction of precipitation from a solution containing Cl\({}^{-}\) ions (concrete power) of sediment (AgCl) using a silver nitrate solution (AgNO\({}_{3}\)) of an exactly known concentration. In the first method, potassium chromate (K\({}_{2}\)CrO\({}_{4}\)) is used as an indicator, which causes the solution to become reddish-brown. The test environment must be neutral or weakly alkaline (pH from 7 to 10.5). In the case of the Fajans method, a different indicator is used. This is fluorescein in the form of an aqueous solution of fluorescein sodium, which is an adsorption indicator. This causes color changes that are observed, not in the solution (as in the Mohr method), but on the surface of the sediment. In this method, the chlorides can only be determined in neutral solutions. To calculate the chloride content, an equation is used that is identical in both methods.  
There is also a standard way of determining the chloride content in hardened concrete described in the PN-EN 14629:2008--Products and systems for the protection and repair of concrete structures; Test methods--determination of chloride content in hardened concrete [30]. The procedure for the determination of chloride ions by the Volhard method (Method A) and the potentiometric method (Method B) are described here. Method A is quite similar to the described above two (Mohr or Fajans) with a different type of indicator. This is an iron-ammonium alum, which forms a red complex with the first drop of excess thiocyanate solution. Method B is based on potentiometric titration [31] of a silver nitrate solution (AgNO\({}_{3}\)). The test consists of adding small multiples of the solution to the powered concrete sample, recording potential changes, and determining the first derivatives from the obtained data. In both cases (Methods A and B), the standard provides a formula for calculating the chloride content as a percentage of chloride ions in relation to the sample mass. The Volhard method is the most commonly used in practice for the determination of the chloride content in concrete [32]. The main Rilem recommendations are based on this method [33,34].  
To determine the content of sulfate ions in the concrete sample, a typical chemical laboratory test--the gravimetric method [28]--is used. The determination is based on the precipitation of sulfates from the concrete power (on hot) in acidic solution (HCI), using barium chloride (BaCl\({}_{2}\)). Then, the hardly soluble barium sulfate (BaSO\({}_{4}\)) is precipitated, which after roasting is weighed and converted into the content of SO\({}_{4}\)\({}^{2-}\) ions.