1 Chapter 2 Steps in a chemical analysis

2 2.1 Plan of analysis Before doing any quantitative analysis, the following questions should be answered: 1- what analytical information is needed ! 2- what analyses are necessary to provide these ! information 3-which analytical methods should be used ! - limits - interferences - instruments and equipment

3 2.2 Sampling What is the aim of sampling! To have a representative, measurable, sample can be analyzed Difficulties in sampling results from non-homogenous sample particles size Liquid samples i.e. water, are more homogenous than solid ones i.e. soil

4 Water samples are more homogenous than wastewater samples Types of samples - Grab: one single - Composite: multi grab

5 2.3 Drying the sample What is drying mean Removing the water content What is the goal of drying 1-Make the sample anhydrous 2- evaluating the water content (moisture) from the sample by: - Moisture weight loss - Collection of the moisture from the sample

6 2.4 Measuring the sample The results of quantitative analysis are usually reported in 1- weight % 2- volume % 3- mole % 4- other units, i.e Molarities, Normality,..PPM….

7 In measuring it is important for analytical steps to use the most appropriate tool i.e - analytical balance in weighing small amounts - The use of volumetric tools for liquid samples

8 2.5 Dissolving the sample The good solvent for a sample have to dissolve the amount of the sample completely in as short time as possible The types of solvents used most commonly: ( principle like dissolve like ) 1- Water: suitable for ionic polar substances inorganic ones and some non-organic ones

9 2- organic solvents: such as alcohols, ketones, acetonitriles,… 3- mineral acids: concentrated or slightly diluted acids (i.e. HCl, H 2 SO 4 ) specially for metals. 4- Fusion: some samples fail to be dissolved in any other ways, so they can be brought into solution by fusing them with a high temperature acid or base. i.e Potassium pyrosulfate K 2 S 2 O 7 Sodium carbonate Na 2 CO 3

separating interfering substances The ideal quantitative method for a substance is that can measure the maximum desired substance in the presence of possible foreign substances. (selectivity) Interferences: species that present and prevent the direct measurement or increase or decrease the exact one of the analyte Solution to interferences! !

11 Separation of interferences: Is important step to minimize the interferences It can be achieved by: 1- precipitating: the analyte or the interferences 2- solvent extraction 3- ion exchange 4- adding inhibition substances

measuring the desired substance In this step, the quantity of the substance being determined is actually measured The measuring step is different from method to another. i.e. -In gravimetric: isolation and weighing the precipitate -In titremetric: measuring volumes of standard solution required to react with substance.

13 The Measuring step usually needs some adjustment of several conditions ( pH ), and within a specific range ( detection limit ) 2.8 calculating and evaluating the results The measurement step provide the data needed to calculate the quantity of each species in the sample, Then expression of results with the appropriate unit is needed and the final result expression is reported after averaging when duplicates or replicates used, or after correction by dilution factors when dilution is performed.