Give examples of primary standards used in different types of titrimetry
Answers
Explanation:
Examples of primary standards used in redox titrations include pure iron, NaC2O4 (sodium oxalate), As2O3 (arsenic trioxide), K2Cr2O7 (potassium dichromate), KBrO3 (potassium bromate), KIO3 (potassium iodate) and KH(IO3)2 (potassium hydrogen iodate).
A primary standard is a reference chemical used to measure an unknown concentration of another known chemical. It can be used directly when performing titrations or used to calibrate standard solutions. Primary standards exhibit distinctive characteristics that ideally suit them for making accurate and reliable assessments of concentration without the need for specialized equipment. These characteristics include high purity, low reactivity with the air or surrounding environment, low hygroscopicity, large molar mass and a predictable reactivity.
Balancing Act
In a typical titration, an indicator -- often a color-changing chemical -- is used to mark the appearance of the stoichiometric point, or equivalence point, of a reaction between the primary standard and the reactant chemical, which is a known chemical with an unknown concentration. Using the balanced equation for the reaction, the molar equivalents of the reactant chemical with the standard is determined. Careful control of the volumes of each chemical dispensed into the reaction vessel, therefore, allows calculation of the concentration of the reactant chemical.
Pure Standards
The purity of primary standard must be very high, as impurities in the preparation could alter the stoichiometric point of the reaction and therefore skew the concentration calculation of the reactant chemical. Likewise, the primary standard chemical should not be reactive with or take in moisture from the surrounding air, as this would also alter the stoichiometric point of the reaction. Chemicals used as primary standards naturally exhibit low reactivity with air and low hygroscopicity, or ability to attract and absorb moisture from the air.
Standard Properties
Primary standards should also have high molar masses that allow them to be easily and accurately weighed on a balance with minimal error. This property facilitates accurate calculation of the molar concentrations of solutions when they are prepared in volumetric flasks. Ideally, the standards should also be inexpensive, commonly available, nontoxic and soluble in water or other common solvents. These properties are not necessary, but they do make standards easier and safer to work with in the laboratory.
Acid-Based Titrations
Acid-based titrations depend on mixing acids with bases to achieve neutralization. Some examples of primary standards used in acid-based titrations include potassium hydrogen iodate, sodium borate, sodium carbonate and pure metals and salts such as zinc, copper and sodium. Oxidizing-reducing, or redox, titrations involve the mixing of oxidizing and reducing agents to determine the concentration of the reactant chemical. Examples of primary standards used in redox titrations include pure iron, NaC2O4 (sodium oxalate), As2O3 (arsenic trioxide), K2Cr2O7 (potassium dichromate), KBrO3 (potassium bromate), KIO3 (potassium iodate) and KH(IO3)2 (potassium hydrogen iodate).