Analytical chemistry LEC quiz 2(multiple choices)

• Volumetric Methods are quantitative analysis

  where a well-defined reaction is caused to take

  place, wherein the reagent is added from an

  apparatus so designed that the volume of the

  solution employed to complete a reaction can

  be accurately measured.
• Volumetric Methods

  -The concentration of this solution is accurately

  known, and the volume employed serves,

  therefore, as a measure of the amount of

  substance acted upon.
• The essence of volumetric determination is the chemical reaction between a standard solution (of known concentration), which is usually added, and a given volume of a substance to be determined using a “titration” process.
• We identify this point by using:

  *indicators (subjective method)

  devices (objective method), eg. potentiometry,

  conductimetry etc.
• DIVISIONS OF VOLUMETRIC ANALYSIS

  *Neutralization Methods

  *Oxidation – Reduction (“Redox”) Methods

  *Precipitation or Saturation Methods.

  *Complex Formation Methods.

  *Diazotating Methods
• Neutralization Methods

  -An acidic substance is titrated with a standard solution of an alkali (acidimetry) or a basic substance is titrated with a standard solution of

  an acid (alkalimetry).
• Oxidation – Reduction (“Redox”) Methods.

  -A reducing substance is titrated with a standard solution of an oxidizing agent, or an oxidizing substance is titrated with a standard solution of a reducing agent.
• Examples:

  1. oxidimetry - standard solution is an oxidizer

  2) manganometry - titration agent is KMnO4

  3) bichromatometry - titration agent is K2Cr2O7

  Other examples:

  4) iodometry - titration agent is Iodine, I2

  5) bromatometry - titration agent is KBrO3

  6) reductometry - is based on effects of substances having reducing properties, eg.

  7) titanometry - reactions using TiCl3
• Precipitation or Saturation Methods.

  -A substance is titrated by precipitating it with a standard solution of a precipitating agent.

  -are based on formation of poorly soluble substances.

  -E.g. Argentometry – where a standard solution of

  AgNO3 is used.
• Complex Formation Methods.

  -A substance is titrated with a standard reagent to the

  formation of a complex ion.

  -based on the formation of less dissociable soluble

  compounds.

  1. chelatometry - uses standard solutions of chelatones

  or chelating reagents.

  2.) mercurimetry - uses Hg(NO3)2 as titration agent.
• Diazotating Methods

  -are used for determination of certain organic compounds.

  -Sodium nitrate (NaNO3) is the volumetric agent used as standard solution.
• The determination of the concentration of acidic

  solutions of unknown normality by means of standard

  basic solutions is known as ACIDIMETRY.

  
  

  If an acid is added to a base, a neutralization reaction

  occurs.

  -The acid unites with the base to form salt and water:

  Acid + Base Salt + Water
• -The use of standard acid solutions to determine

  the concentration of basic solutions of unknown

  normality is called ALKALIMETRY.

  -The most commonly used indicator in running a

  titration is phenolphthalein because of its ability

  to change color at a pH near neutrality.
• An indicator which changes color at the point of neutralization or END POINT

  END POINT - is used to indicate that equivalent amounts of acid and base have been brought together.
• A pH indicator is a material, usually an organic dye, that is one color above a characteristic pH and another color below that pH.
• -Litmus and phenolphthalein are common pH

  indicators.

  -Litmus is red in acid (below pH 4.7) and blue in

  base (above pH 8.1).

  -Phenolphthalein is clear in acid (below pH 8.4)

  and pink-purple in base (above pH 9.9).
• These ranges may seem large, but near the EQUIVALENCE POINT, the point at which the materials are equal, there is a large change in pH.
• The difference is the titration error which can be determined experimentally is called the indicator blank.
• -When a STANDARD BASE or ACID is added drop by drop to the UNKNOWN SAMPLE SOLUTION from a BURET, this process is called TITRATION.
• TITRATION

  ➢ Measures the amount of analyte that

  quantitative reacts with titrant

  ➢ Reaction must be:

  ❑ Fast

  ❑ Complete

  ❑ Possess Known Stoichiometry
• Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant.
• A standard solution - is a solution in which the

  concentration is known to a high degree of accuracy.

  -Standard acid solutions are usually prepared from hydrochloric acid, although nitric, sulfuric, and oxalic acids are occasionally used.

  -Standard alkali solutions are usually prepared from sodium hydroxide, although potassium hydroxide and barium hydroxide are occasionally used.
• The Kjeldahl method first came into existence in 1883 and was developed by a Danish chemist named Johan Kjeldahl.

  -This method was specifically developed for determining the nitrogen contents in organic and inorganic substances.
• -In today’s world, Kjeldahl nitrogen determinations are used on several samples like wastewater, soil, fertilizers, meat, feed, grain, and many other substances.

  -The method is also used for estimating the protein content in foods.
• Even with new changes, the basic principles introduced by Johan Kjeldahl is still relevant, the main objective or purpose of this procedure is the oxidation of organic compounds using concentrated sulfuric acid.
• Kjeldahl method is divided into three main steps.The method has to be carried out in proper sequence.

  -The steps include: digestion, distillation, and titration.
• 1. DIGESTION

  -In this method, a certain substance or sample is heated in the presence of sulfuric acid.

  -The acid breaks down the organic substance via oxidation and reduced nitrogen in the form of ammonium sulphate is liberated.

  -Potassium sulphate is usually added to increase the boiling point of the medium.
• -Catalysts like mercury, selenium, copper, or ions of mercury or copper are also used in the digestion process.

  -The sample is fully decomposed when we obtain a clear and colorless solution.
• 2. DISTILLATION

  -The distillation of the solution now takes place and a small quantity of sodium hydroxide is added to convert the ammonium salt to ammonia.
• -The distilled vapors are then trapped in a special trapping solution of HCl (hydrochloric acid) and water.
• 3. TITRATION

  -The amount of ammonia or the amount of nitrogen present in the sample is then determined by back titration.

  -As the ammonia dissolves in the acid trapping solution some HCl is neutralized.

  -The acid that is left behind can be back titrated with a standard solution of a base such as NaOH or other bases.
• -In an indirect titration the substance being determined is not titrated directly but is allowed to react with a measured quantity of a given reactant which is added in excess of the amount required for complete reaction.

  -Then the amount of reactant remaining is determined by titration.

  -Instead of titrating a substance directly with a standard solution, it is frequently more feasible to allow the substance to react with a measured amount of a given reagent and then to titrate that part of the reagent left over from the reaction.
• -In an indirect titration the substance being determined is not titrated directly but is allowed to react with a measured quantity of a given reactant which is added in excess of the amount required for complete reaction.

  -Then the amount of reactant remaining is determined by titration.

  -Instead of titrating a substance directly with a standard solution, it is frequently more feasible to allow the substance to react with a measured amount of a given reagent and then to titrate that part of the reagent left over from the reaction.
• Oxidation-Reduction Reaction (REDOX) - is a chemical reaction in which the oxidation state of one or more substances changes.

  Oxidation state – also referred to as “Oxidation number (O.N.)” is the charged an atom have if “electrons were not shared but were transferred”.

  An O.N. has the “sign” BEFORE the number (e.g. +2), whereas the “IONIC CHARGE” has the “sign” AFTER the number (e.g. 3+)
• • OXIDATION – refers to the “LOSS” of Electrons (LEO).

  • REDUCTION – refers to the “GAIN” of Elrectrons (GER).

  • OXIDIZING AGENT (O.A.) or “OXIDANT” – is the substance that makes it possible for another substance to be “oxidized”.

  • It “REMOVES” Electrons from another substance by “ACQUIRING” them itself.

  • In oxidizing another substance, the O.A. is itself reduced.
• • REDUCING AGENT (R.A.) or REDUCTANT – is the substance that “GIVES-UP” Electrons, thereby causing another substance to be reduced.

  • The R.A. is “OXIDIZED” in the process.
• -Even with new changes, the basic principles introduced by Johan Kjeldahl is still relevant, the main objective or purpose of this

  procedure is the oxidation of organic compounds using concentrated sulfuric acid.
• The most common method used for the determination of nitrogen content in organic materials is the KJELDHAL METHOD
• The Kjeldahl method is widely used because it is simple, rapid, accurate, and reliable. However, its major drawback is that it cannot determine total nitrogen present in the sample since some forms of nitrogen are not converted into ammonium salts under these conditions.