Volumetric analysis

• If we add copper sulfate to water, the copper sulfate is called the solute and the water is called the solvent which is called a solution.
• The solute and solvent together form a perfect mixture. A solution is a completely perfect mixture of a solute and a solvent. In a solution the particles of the solute are uniformly distributed throughout the solvent, i.e. the mixture is homogeneous
• For example, when common table salt is added to water a solution is formed. It is no longer possible to distinguish between the ions of the salt and the molecules of the water. There is no boundary between the particles, e the particles (salt ions and water molecules) are in the same phase-we say that the mixture is homogeneous
• If there is a large amount of solute relative to the quantity of solvent, the solution is said to be concentrated. If there is only a small amount of solute relative to the quantity of solvent, the solution is said to be dilute
• Chemists often need to know the exact amount of solute present in a certain volume of solution. A chemist who is analysing polluted water, for example, must very accurately measure the quantity of oxygen dissolved in the water. The precise amount of solute in a given amount of solution is referred to as the concentration of the solution
• Concentration of a solution

  The amount of solute that is dissolved in a given volume of solution
• Ways of expressing the concentration of a solution

  • Percentage of Solute
  • Parts Per Million (ppm)
  • Moles of Solute Per Litre of Solution (Molarity)
• Weight per weight (w/w)

  A solution labelled 10% w/w NaCl means that there are 10 g of sodium chloride per 100 g of solution
• Volume per volume (v/v)

  A solution marked 10% v/v has 10 cm³ of ethanol per 100 cm³ of solution
• Parts Per Million (ppm) is a method of expressing the concentration of a solution that is only used for very dilute solutions. It is particularly useful in water analysis (Chapter 19) where you are dealing with very low concentrations of certain substances, perhaps only a few milligrams of a substance in a litre of water.
• Molarity

  The number of moles of solute per litre of solution
• Whenever the symbol M is used, always make sure that it is used in conjunction with a chemical formula, eg. a 1 M NaOH solution means a sodium hydroxide solution that has a concentration of 1 mole per litre.
• However, for most laboratory experiments, these reagents must be used in their diluted state, e.g. the concentration of "bench" dilute hydrochloric acid and "bench" dilute ammonia is only 3 M approx.
• When a solution is diluted, more solvent is added but the quantity of solute is unchanged. Since the volume of the solution increases and the same number of moles is present, then the concentration of the solution must decrease, Fig. 13.7.
• Note: it does not matter what units are used to express the concentration as long as the same units are used on each side of the equation.
• Dilute a concentrated solution

  The number of moles of solute in the diluted solution is the same as the number of moles of solute in the concentrated solution
• It does not matter what units are used to express the concentration as long as the same units are used on each side of the equation
• Standard solution

  A solution whose concentration is accurately known
• Primary standard

  • It must be available in a highly pure state
  • It must be stable in air
  • It must dissolve easily in water
  • It should have a fairly high relative molecular mass
  • It should undergo complete and rapid reaction
  • It should not be hydrated
• Preparing a standard solution of sodium carbonate

  1. Accurately weigh sodium carbonate

  2. Dissolve in deionised water

  3. Transfer to volumetric flask

  4. Add deionised water to graduation mark

  5. Invert flask 20 times to mix
• Titration


  A laboratory procedure where a measured volume of one solution is added to a known volume of another solution until the reaction is complete
• The stage when the two solutions just react completely with each other is called the equivalence point of the titration</b>
• When the equivalence point is shown by an indicator it is called the end point of the titration
• Apparatus used in volumetric analysis

  • Graduated cylinder
  • Volumetric flask
  • Pipette
  • Burette
  • Conical flask
• Equivalence point


  The stage when the two solutions just react completely with each other
• End point

  When the equivalence point is shown by an indicator
• Apparatus used in volumetric analysis

  • Graduated cylinder
  • Volumetric flask
  • Pipette
  • Burette
  • Conical flask
• Graduated cylinder

  • Not particularly accurate, should be used only for measuring out approximate volumes of liquids
  • Should always be rinsed out with deionised water before use
• Volumetric flask

  • Designed to contain a definite volume of solution
  • Has an engraved graduation mark on the narrow neck to allow the volume of solution to be accurately brought up to this mark
  • The capacity of the flask and the temperature at which the stated volume is most accurately measured are usually marked on the flask
  • Should always be rinsed out with deionised water before use
  • The last few cm³ of water must be added carefully so that the bottom of the meniscus rests on the calibration mark
  • The contents must be well mixed by inverting the stoppered flask about 20 times to ensure a homogeneous mixture
• Pipette


  • Designed to deliver only an exact volume of solution
  • Filled by drawing the solution into it by suction using a pipette filler
  • The liquid is allowed to run into a beaker until the bottom of the meniscus is level with the graduation mark
  • The tip of the pipette is touched against the side of a glass beaker to remove any drop adhering to it
  • The pipette is then allowed to discharge slowly into a conical flask, keeping the tip in contact with the side of the flask
  • A small amount of liquid remains in the tip and is not blown out, as it has been taken into account when the pipette was calibrated
  • Should always be rinsed with deionised water and then with the liquid it will contain before use
• Burette

  • Consists of a long narrow glass tube closed at the bottom by a tap
  • Graduated in divisions of 0.1 cm³, with the volume of liquid normally read to 0.1 cm³
  • Should be rinsed out with deionised water and then with the liquid to be used in it before use
  • A small funnel should be used when filling the burette to avoid spillages
  • The tap should be checked to ensure it turns freely
  • The burette should be clamped vertically to ensure accuracy in reading the volume of liquid delivered
  • The eye should be level with the bottom of the meniscus when reading the volume
  • After use, the burette should be rinsed out with deionised water and clamped upside down with the tap open
• Conical flask

  • Designed so that it may be swirled without the liquid splashing out
  • The shape allows any droplets adhering to the inside to be easily washed down
  • Should always be washed out with deionised water before use, but it is not necessary to dry the inside
  • Should not be rinsed with the solution it is to contain, as this would alter the known volume of solution added from the pipette
• Sodium carbonate is a base that reacts with hydrochloric acid according to the equation: Na2CO3 + 2HCl → 2NaCl + H2O + CO2
• By titrating the given hydrochloric acid solution against a known volume of sodium carbonate solution whose concentration is accurately known, we can determine the concentration of the hydrochloric acid solution
• An indicator (methyl orange in this case) is used
• Carrying out a titration

  1. Wash pipette, burette and conical flask with deionised water

  2. Rinse pipette with sodium carbonate solution

  3. Rinse burette with hydrochloric acid solution

  4. Pipette 25 cm³ of sodium carbonate solution into conical flask

  5. Add a few drops of methyl orange indicator

  6. Clamp burette vertically and fill with hydrochloric acid solution

  7. Adjust acid level to zero graduation mark

  8. Place white tile under conical flask

  9. Slowly add acid from burette while swirling conical flask

  10. Wash down sides of conical flask with deionised water

  11. Stop titration when indicator changes colour

  12. Note volume of acid added

  13. Repeat two more times for accurate results
• Carrying out a titration (continued)

  1. Stop titration when indicator changes from yellow to red (pink)

  2. Note volume of acid added

  3. Repeat two more times for agreement within 0.1 cm³

  4. Use average of two agreeing titrations as result
• Preparing and standardising sodium hydroxide solution

  1. Dissolve about 1 g NaOH in 250 cm³ water

  2. Rinse pipette and burette

  3. Pipette 25 cm³ NaOH solution into conical flask

  4. Add methyl orange indicator

  5. Fill burette with HCl solution

  6. Titrate NaOH against standard HCl

  7. Repeat two more times for agreement

  8. Use average titration result

  9. Evaporate solution to isolate NaCl sample
• Reaction equation: HCl + NaOH → NaCl + H₂O