3. ChemAparatus pt 1

• Reagents


  Chemicals used in analytical procedures
• Selecting and handling reagents and other chemicals

  • Cleaning and marking of laboratory ware
  • Evaporating liquids
  • Measuring mass
  • Equipment and manipulations associated with weighing
  • Filtration and ignition of solids
• Lesson outline


  • Classifying chemicals
  • ACS (Reagent) grade
  • Primary-standard Grade
  • Laboratory Grade
  • Technical Grade
  • Special-purpose reagent chemicals
• ACS (Reagent) grade


  Meets or exceeds purity standards set by the American Chemical Society. Can be used for general procedures that require stringent quality specifications and a purity of ≥95%.
• Primary-standard Grade


  A highly purified compound that serves as a reference material and have been carefully analyzed by the supplier, and the results are printed on the container label.
• Laboratory Grade


  The most popular grade for use in educational applications but its exact level of impurities are unknown.
• Technical Grade


  Used for commercial and industrial purposes; suitable for non-critical tasks such as dissolving and rinsing.
• Special-purpose reagent chemicals


  Chemicals that have been prepared for a specific application are also available. Information pertinent to the intended use is supplied with these reagents.
• Rules for Handling Reagents and Solutions

  1. Select the best grade of chemical available for analytical work

  2. Replace the top of every container immediately after removing reagent

  3. Hold the stoppers of reagent bottles between your fingers

  4. Never return any excess reagent to a bottle

  5. Never insert spatulas, spoons, or knives into a bottle that contains a solid chemical

  6. Keep the reagent shelf and the laboratory balance clean and neat

  7. Follow local regulations concerning the disposal of surplus reagents and solutions
• Cleaning and Marking of Laboratory Ware

  1. Mark each vessel that holds a sample so its contents can be positively identified

  2. Thoroughly clean every beaker, flask, or crucible before use

  3. Dry the interior surface of glassware is seldom necessary

  4. Use an organic solvent to remove grease films
• Evaporating liquids


  1. Remove chlorides and nitrates by adding sulfuric acid and evaporating

  2. Remove urea by adding it to acidic solutions

  3. Remove ammonium chloride by adding concentrated nitric acid and evaporating

  4. Eliminate organic constituents by adding sulfuric acid and heating (wet ashing)
• Analytical Balance


  An instrument for determining mass with a maximum capacity that ranges from 1 g to a few kilograms with a precision of at least 1 part in 105 at maximum capacity

  
• Types of Analytical Balances

  • Macrobalances
  • Semimicroanalytical balances
  • Microanalytical balances
• Macrobalances


  • Maximum capacity ranging between 160 and 200 g, measurements can be made with a standard deviation of 0.1 mg
• Semimicroanalytical balances


  • Maximum loading of 10 to 30 g with a precision of 0.01 mg
• Microanalytical balances


  • Capacity of 1 to 3 g and a precision of 0.001 mg (1 μg)

  
• General rules for working with an analytical balance

  1. Center the load on the pan

  2. Protect the balance from corrosion

  3. Observe special precautions for weighing liquids

  4. Consult your instructor if the balance appears to need adjustment

  5. Keep the balance and its case scrupulously clean

  6. Allow heated objects to return to room temperature before weighing

  7. Use tongs, finger pads, or glassine paper to handle dried objects
• Buoyancy error


  The weighing error that develops when the object being weighed has a significantly different density than the masses

  
• Buoyancy Correction methods


  Equation to correct for buoyancy effects

  
• Buoyancy error is less than 0.1% for objects that have a density of 2 g/cm3 or greater, so it is seldom necessary to correct the masses of most solids

  
• Temperature Effects on weighing


  • Convection currents within the balance case exert a buoyant effect
  • Warm air trapped in a closed container weighs less than the same volume at a lower temperature
• Both temperature effects cause the apparent mass of the object to be low, so heated objects must always be cooled to room temperature before being weighed
• 0°C drying oven


  Used to dry samples
• Porcelain filtering crucible


  Used to filter and collect precipitates
• Weighing bottle


  Contains about 7.5 g of KCl
• This error can amount to as much as 10 or 15 mg for typical porcelain filtering crucibles or weighing bottles
• Heated objects must always be cooled to room temperature before being weighed
• Buoyancy Correction


  Corrects the mass of a sample for the effects of buoyancy

  
• Static charges on glass and porcelain wares can be treated with a wipe containing trace amounts of a radioactive material like polonium
• Regular checking of the apparatus can make sure that the accuracy is constant
• Top-loading auxiliary balances


  Can accommodate 150 to 200 g with a precision of about 1 mg, some tolerate loads as great as 25,000 g with a precision of 0.05 g
• Triple-beam balance


  A single-pan balance with three decades of masses that slide along individual calibrated scales, precision may be one or two orders of magnitude less than a top-loading instrument but can be adequate for many weighing operations
• The mass of many solids changes with humidity because they tend to absorb weighable amounts of moisture
• Drying a sample, a precipitate, or a container

  1. Heating (usually for one hour or more) at an appropriate temperature

  2. Cooling

  3. Weighing, repeat until successive masses agree within 0.2 to 0.3 mg
• Desiccators and Desiccants


  The base section contains a chemical drying agent, such as anhydrous calcium chloride, calcium sulfate (Drierite), anhydrous magnesium perchlorate (Anhydrone or Dehydrite), or phosphorus pentoxide
• Notes for working with a desiccator

  • Minimize the uptake of moisture by dried materials
  • Use a sliding motion to open/close the lid to minimize disturbing the sample
  • Allow some cooling before seating the lid to relieve any excessive vacuum
  • Lock the lid in place when moving the desiccator
  • Very hygroscopic materials should be stored in containers with snug covers
• Weighing bottles


  Convenient for drying and storing solids, plastic bottles are rugged but abrade easily and are not as easily cleaned as glass
• Manipulating weighing bottles


  1. Heat at 105 °C to 110 °C for 1 hour to remove moisture

  2. Contain in a labeled beaker with a cover glass to protect from contamination

  3. Avoid touching dried objects with fingers, use tongs, gloves or paper to handle
• Weighing Hygroscopic solids


  1. Use a weighing bottle for each sample

  2. Heat, quickly cap and cool in a desiccator

  3. Weigh one bottle, empty contents, reweigh bottle
• Weighing liquids


  1. Obtain mass by difference

  2. Noncorrosive, nonvolatile liquids can be transferred to weighed containers

  3. Volatile or corrosive liquids should be sealed in a weighed glass ampoule