12.1 Experimental Techniques

• Time can be measured using a stopwatch or clock accurate to one or two decimal places
• Units of time commonly used are seconds or minutes
• Temperature is measured with a thermometer or digital probe
• Laboratory thermometers usually have a precision of half or one degree
• Digital temperature probes are more precise than traditional thermometers
• Units of temperature are degrees Celsius (ºC)
• Mass is measured using a digital balance accurate to two decimal places
• Balances must be tared (set to zero) before use
• The standard unit of mass in kilograms (kg) but in chemistry grams (g) are most often used
• 1 kilogram = 1000 grams
• Volume of liquids can be determined using various apparatus
• For approximate volumes, measuring (or graduated) cylinders are used
• Graduated cylinders are available typically in a range of sizes from 10 cm to 1 litre (1 dm)
• Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid
• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm and 50 cm
• Whichever apparatus you use, you may see markings in ml (millilitre) which is the same as cm
• Volume of gases is measured by collecting it in a graduated measuring apparatus
• A gassyringe is usually the apparatus used
• If the gas is heavier than air and colored, the cylinder can be used upright
• A graduated cylinder inverted in water may also be used, provided the gas isn't water-soluble
• Be careful when recording time not to mix up seconds and minutes in the same table
• If time intervals are less than a minute, it's best to change the recorded units to seconds
• For example, 1 minute and 30 seconds should be recorded as 90 seconds
• Titration is a method of analyzing the concentration of solutions
• Titration can determine exactly how much alkali is needed to neutralize a quantity of acid
• You may be asked to calculate the moles present in a given amount, the concentration or volume required to neutralize an acid or a base
• Titration can also be used to prepare salts
• The typical materials needed for performing a titration are:

  • 25 cm volumetric pipette
  • Pipette filler
  • 50 cm burette
  • 250 cm conical flask
  • Small funnel
  • 0.1 mol/dm sodium hydroxide solution
  • Sulfuric acid - concentration unknown
  • A suitable indicator
  • Clamp stand, clamp & white tile
• Steps in performing a titration:

  • Use the pipette and pipette filler and place exactly 25 cm sodium hydroxide solution into the conical flask
  • Place the conical flask on a white tile so the tip of the burette is inside the flask
  • Add a few drops of a suitable indicator to the solution in the conical flask
  • Perform a rough titration by taking the burette reading and running in the solution in 1-3 cm portions, while swirling the flask vigorously
  • Quickly close the tap when the end-point is reached (sharp color change) and record the volume, placing your eye level with the meniscus
• • Now repeat the titration with a fresh batch of sodium hydroxide
  • As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes color
  • Record the volume to the nearest 0.05 cm
  • Repeat until you achieve two concordant results (two results that are within 0.1 cm of each other) to increase accuracy
• Indicators are used to show the endpoint in a titration
• Wide range indicators such as litmus are not suitable for titration as they do not give a sharp enough color change at the endpoint
• Methyl orange and phenolphthalein are very suitable indicators
• Some of the most common indicators with their corresponding colors are shown below:

  • Common Acid-Base Indicators