Question

9 Definie standard solution What are caliberation Curves 1) Provide an example of Caliberalion Curve to delermine the caliberalion of Hg in water samples » Define primary standards oranide 5 characteristics of birimary Standards Pronde 5 examples of primary standards

Answer 1

1. Standard solutions play a central role in all titrations. Therefore, we must consider the desirable properties for such solutions, how they are prepared, and how their concentrations are expressed. The ideal standard solution for a titrimetric method will

• be sufficiently stable so that it is necessary to determine its concentration only once;
• react rapidly with the analyte so that the time required between additions of reagent is minimized;
• react more or less completely with the analyte so that satisfactory end points are realized;
• undergo a selective reaction with the analyte that can be described by a balanced equation.

Few reagents completely meet these ideals. The accuracy of a titration can be no better than the accuracy of the concentration of the standard solution used.

2. Generally, a plot of instrument response versus known analyte concentrations is used to produce a calibration curve, sometimes called a working curve. It is often desirable that the calibration curve be linear in at least the range of the analyte concentrations. A linear calibration curve of absorbance versus analyte concentration is shown below.

Residual = y; - (mx; +b) y, Absorbance 0.000 1.000 2.000 5.000 6.000 7.000 8.000 9.000 3.000 4.000 x, Concentration

3. Analytical methods for the determination of mercury play an important role in monitoring the safety of food and water supplies. One of the most useful methods is based on the atomic absorption by mercury of 253.7 nm radiation. The calibration curve for Hg in water samples is given below (The analytical method used is Cold-Vapor atomic emission spectroscopy and the entity plotted in y axis is absorbance**).

Hg y= 0.1752x -0.0279 31 R2 = 0.9997 10 20 Concentration (ug/l)

4. A primary standard is a highly purified compound that serves as a reference material in titrations and in other analytical methods. The accuracy of a method critically depends on the properties of the primary standard.

In other words, a primary standard is an ultra pure compound that serves as the reference material for a titration or for another type of quantitative analysis.

5. Important characteristics for a primary standard are the following:

• High purity.
• Atmospheric stability.
• Absence of hydrate water so that the composition of the solid does not change with variations in humidity.
• Modest cost.
• Reasonable solubility in the titration medium.
• Reasonably large molar mass so that the relative error associated with weighing the standard is minimized.

6. Examples for primary standards include

• Potassium bromate (KBrO₃)
• Sodium carbonate (Na₂CO₃)
• Mohr’s Salt (FeSO₄(NH₄)₂ SO₄.6H₂O)
• Potassium hydrogen phthalate (KHP)
• Potassium dichromate (K₂Cr₂O₇)