Experiment  10:  Analysis  of  Iodine  Sterilized  Drinking  Water    

Iodine  is  an  effective,  simple  and  cost  effective  means  of  disinfecting  contaminated  water  to  provide  clean  drinking  water  in  regions  of  extreme  poverty  or  those  devastated  by  natural  disasters.  Considering  some  recent  natural  disasters  where  clean  water  has  become  scarce,  iodine  purification  is  a  practical  application  to  demonstrate  the  use  of  chemistry  in  a  real-­‐world   situation.   At   neutral   to   slightly   acidic   pH,   three  major   iodine   species   exist   in  solution:  free  iodine,  triiodiode  and  hypoiodous  acid,  with  the  amount  of  each  species  being  pH  dependent.   The   species   that   have   the   largest   sterilizing   effect   are   aqueous   elemental  iodine   and   hypoiodous   acid,   with   the   former   having   the   greatest   effect.     Although   the  recommended   dose   may   vary   depending   on   the   microorganisms   present   in   the   water  supply,   a   dose   of   2mg/L   of   aqueous   iodine   is   generally   considered   effective.     There   are  many  commercial  aqueous   iodine  sources  but  the  two  most  common  forms  are  an   iodine  tincture   solution   and   a   tablet   containing   tetraglycine   hydroperiodide   (Figure   1).   The  tincture  solution  is  a  mixture  of  2–7%  elemental  iodine,  with  sodium  or  potassium  iodide,  dissolved   in  a  mixture  of  ethanol  and  water.  The   iodine   solution  does  not  use  a  buffer   to  control   the   amount   of   aqueous   iodine   released  but   rather   relies   on   ethanol   to   lower   the  solubility  of  triiodide  to   increase  the  amount  of  aqueous  iodine.     In  the  case  of  the  tablet,  tetraglycine  buffers  the  water  to  pH  5.5  to  ensure  a  high  level  of  aqueous  iodine  is  formed.  As  a  consequence,  an  equal  dose  of  the  tincture  solution  only  generates  about  half  as  much  aqueous   iodine  as   the   tablet.    Due   to   the   lack  of  buffering,   the   tincture   solution   is  not  as  effective  as  the  tablet;  hence  the  experiment  will  focus  on  he  iodine  sterilization  tablet.  The  tablets   that   will   be   used   in   this   experiment   are   Coghlan's   Emergency   Drinking   Water  Germicidal  Tablets.    According  to  the  included  documentation,  one  tablet  is  effective  in  the  disinfecting  0.5L   of  water   and   each   tablet   contains   20mg  of   tetraglycine  hydroperiodide,  which  produces  8mg  of  aqueous  iodine.    

   

 Figure  1:  structure  of  Tetraglycine  Hydroperiodide  

In  Brief…    Under  the  conditions  relevant  to  this  experiment,  the  chemistry  of  aqueous  iodine  

solutions  can  be  described  by  the  following  chemical  equations:    

I2 + H2O HOI + I − + H +

I2 + I− I3

−

HOI OI − + H +

3HOI IO3− + 2I − + 3H +

IO3− + 5I − + 6H + 3I2 + 3H2O  

During  this  experiment  each  lab  section  will  be  working  together  to  analyze  a  water  sample  treated  with  a  Coghlan's  Emergency  Drinking  Water  Germicidal  Tablet.  Each  group  will  analyze   the  water  sample  using  UV-­‐visible  spectroscopy,   two  redox   titrations  and  an  Iodide   selective   electrode.     Using   the   listed   instrumental   techniques   and   the   iodine  equilibrium   equations   above   it   is   possible   to   determine   the   amounts   I2,   I⁻,   I3⁻   and   HOI  present  in  the  tablet-­‐sterilized  drinking  water.    Before  the  Lab  Period:    1. To  use  ascorbic  acid  and  iodate  as  primary  standards  they  must  first  be  dried  for  at  

least  two  hours.    The  week  before  you  perform  experiment  10  make  sure  you  prepare  dried  potassium  iodate  and  ascorbic  acid.    

• Use  a  top  loading  balance  to  dispense  ~5%  more  than  the  calculated  amount  of   potassium   iodate   and   ascorbic   acid   into   separate   clean,   dry,   labelled  weighing  bottles.  Dry  the  bottles  in  a  100°C  oven  for  at  least  2  hours  with  the  lid  open.    After  the  2  hours  transfer  the  weighing  bottles  into  a  desiccator  to  cool    

 2. Determine  how  to  make  all  stock  solutions  and  calibration  standards  required  in  the  

experiment  and  record  this  information  in  your  laboratory  notebook.    

3. Determine   the   physical   and   hazardous   properties   of   the   chemicals   used   in   this  experiment  and  record  this  information  in  your  laboratory  notebook.  

 4. Answer   the   pre-­‐lab   questions   and   submit   these   one   business   day   before   your   lab  

period.    Chemicals  and  Supplemental  Equipment:  

 • 500mL  of  Coghlan's  Emergency  Drinking  Water  Germicidal  Tablet  sterilized  water  

o One  tablet  dissolved  in  500mL  of  water  • 0.4mM  Glycine  Buffer  (pH=5.5)  • 0.4mM  Glycine  –  50g/L  Potassium  Iodide  Buffer  (pH=5.5)  • Iodine-­‐TISAB  Solution  

o 5.5M  NaNO3  • Deionised  water  • 2  matched  pairs  of  10.0  mm  pathlength  1mL  UV  cuvettes  (TA)  • A  variety  of  volumetric  transfer  pipettes  in  volumes  ranging  from  1  –  50mL  • In  addition  to  the  volumetric  flasks  located  in  the  drawers  you  will  also  be  provided  

with  10mL  and  25mL  volumetric  flasks    Pre-­‐Lab  Questions:    1. Give   balanced   chemical   equations   for   the   preparation   of   the   triiodide   and   iodine  

standards,  as  well  as  the  two  redox  reactions  performed  in  the  titration  experiments.  

 2. At  what  UV-­‐visible  wavelengths  do  triiodide  and  iodide  absorb  in  aqueous  solution?    3. Why  must   the   ascorbic   acid   standard   solution   be   used   immediately,   and   not  made  

ahead  of  time?    UV-­‐VIS  Spectrophotometer  Operating  Instructions:    1.   Handle  the  UV  cuvettes  with  care,  handling  them  in  such  a  way  as  to  not  touch  the  

front  and  rear  clear  windows.    Pick  up  and  move  the  cuvettes  by  touching  only  the  opaque  surfaces.  

    When  filling  a  cell,  first  rinse  it  at  least  three  times  with  small  portions  (~0.5  mL)  of  

the  solution  using  a  Pasteur  pipette:  position  the  tip  of  the  pipette  near  the  bottom  of  the  cell  against  one  side  and  slowly  expel  the  pipette  contents.    This  helps  avoid  air  bubble  formation,  which  can  affect  the  reading  if  the  bubbles  stick  to  the  faces  of  the  cell  in  the  light  path.  

    Wipe  the  outside  surfaces  carefully,  as  not  to  scratch,  with  Kimwipe™  before  placing  

the  cell  in  the  spectrophotometer  with  the  clear  windows  in  the  light  path.    2.   When   performing  multiple  measurements,   it   is   usually   a   good   idea   to   retain   one  

Pasteur   pipette   for   each   solution,   just   in   case   you   have   to   repeat   some  measurements  with  fresh  portions  of  your  samples.    Do  not  mix  the  pipettes  up  or  use  the  same  pipette  for  different  samples!    Note  which  side  of  the  cell  is  the  front,  it  will  be  marked  by  an  arrow.  Always  place  the  cell  in  the  instrument  the  same  way,  and  make  sure  it  is  fully  seated  in  the  cell  holder  before  taking  a  measurement.  

 3.   When  emptying  a  cell,  always  pour  the  contents  into  a  waste  container,  never  back  

into  your  sample  container.    Rinse  the  cell  thoroughly  with  deionised  water  from  a  wash-­‐bottle,  and  place  the  cell  on  a  Kimwipe™  in  a  safe  place  while  not  in  use.  

 4.   The  spectrophotometers  and  connected  computers  should  already  be  on  when  you  

come  into  the  lab;  if  not,  either  your  demonstrator  or  the  laboratory  instructor  will  start  them  for  you.    The  method  should  be  loaded  and  the  spectrophotometer  ready  to  start  the  experiment,  if  this  is  the  case  move  on  to  no.  5,  if  not  continue  reading.    

   

A  Perkin-­‐Elmer  UV/visible  spectrophotometer  -­‐  the  Lamda11       Within   the   main  UVWinLab   window   should   be   a  Methods   window;   if   this   is   not  

visible,   select   Methods   from   the   Window   menu.     The   desired   method   can   be  launched   simply   by   double-­‐clicking   on   the   corresponding   entry   in   the   Methods  window.    Once  the  method  file  opens,  you  will  see  three  tabs  labelled  Scan,  Inst.,  and  Sample.     The   scan   tab   should   show   the   correct   wavelength   range;   note   that   the  instrument  scans  from  long  to  short  wavelength.  

 5.   In   the   Sample   tab,   enter   a   unique  Result   Filename,   and   type   in   suitable   Sample  

Identity  values  in  the  spreadsheet-­‐like  area.    Once  satisfied,  click  on  the  Setup  button  in  the  toolbar.    The  software  will  configure  the  instrument;  once  it  is  ready,  the  Start  button  should  be  highlighted   in  green.    When  you  are  ready  to  start  collecting  data,  click  the  Start  button!  

 6.   When  prompted  to  Insert  Blank,   fill   the  cuvette  with  your  blank  solution,  open  the  

sample  compartment,  and  carefully  slide  the  cuvette  into  the  appropriate  holder  (see  below).    Make  sure  the  cuvette  is  properly  seated  in  the  bottom  of  the  holder,  but  do  not  force  it  into  position.    Gently  lower  the  lid,  then  click  the  OK  button  in  the  dialog  box.    

   

Single  beam  (left)  and  double  beam  (right)  spectrophotometer  sample  compartments.    The  sample  cuvette  is  placed  in  the  position  marked  ‘S’  in  the  photographs.    The  double  beam  instrument  requires  a  second  cuvette  filled  with  the  solvent  or  reagent  blank  to  be  placed  

in  the  ‘reference’  beam.  The  light  path  is  from  left  to  right  through  the  sample  compartment.  

 7.   Once   the   blank   has   been   scanned   to   obtain   the   background   reading,   you   will   be  

prompted   for   each   solution   in   turn,   in   the   order   they   appeared   in   the   method’s  sample  list.    Note  that,  depending  on  how  this  is  set  up,  you  may  be  asked  to  scan  the  blank  a  second  time,  so  that  a   ‘blank’  spectrum  appears  in  the  graphics  window.    As  each  spectrum  is  obtained,   it  will  be  added  to  the  graphics  window.    Once  you  have  acquired  all  the  data,  ask  your  TA  how  to  print  a  copy  of  the  spectra.  

     

 During  The  Lab  Period:    During   this   experiment   each   lab   section   will   work   together   to   analyze   iodine   tablet  sterilized   drinking   water   using   UV-­‐VIS   spectroscopy,   redox   titrations   and   an   iodide  selective   electrode.   Each   student   should   be   prepared   to   know   and   understand   how   to  perform  any  part  of  the  experiment,  as  the  whole  demo  group  will  be  working  together  as  one   team.     No   one  may   leave   the   lab   until   the   entire   experiment   is   finished.   If   you   are  finished   the   section   of   the   experiment   you   were   assigned,   move   on   and   help   other  members  of  your  group  who  are  still  working.      *** Be  careful  and  pay  attention  to  ensure  the  correct  buffer   is  used   in  each  part  of   the  

experiment!    

*** Be  sure  to  mix  the  iodine  tablet  sterilized  water  sample  thoroughly  before  dispensing  into  a  beaker  or  aliquoting  for  analysis!    

*** As   with   experiment   5   all   solutions   are   to   be   transferred   to   Nalgene   bottles   after  preparation.    

*** Retain   all   your   solutions   until   you   have   acquired   all   your   data   and   plotted   your  calibration  curves;  you  may  need  to  make  additional  calibration  solutions  before  you  are  finished!  

 *** Calculate   the  actual   concentrations   of   each   solution.   Record   this   information   along  

with  all  the  data  your  group  has  acquired  in  your  lab  notebooks  before  the  end  of  the  lab  period,   this  will   contribute   to   your  performance  grade.  All   team  members  must  have  the  data  from  every  section  recorded  in  their  lab  book,  no  exceptions!  

 *** DO  NOT  FORGET  to  provide  the  TA  with  a  copy  of  the  data  as  well.    Use  excel  on  one  

of   the   lab  computers   to  make  a  data   table   for   the  TA  and  provide  him/her  with  the  original  labeled  printouts  from  the  UV-­‐VIS  and  auto-­‐titration  experiments.    

 Part  A  –  Determination  of  Triiodide  Using  UV-­‐VIS  Spectroscopy:    *** In  this  section  of  the  experiment  be  sure  to  rinse  all  your  glassware  and  make  up  all  

your  solutions  using   the  provided  0.4mM  glycine-­‐50g/L   potassium   iodide   buffer  (pH=5.5).  

 A.1: Verify  that  the  UV-­‐visible  spectrophotometer  is  turned  on  when  you  arrive  in  the  lab.    

If  you  are  unsure  what  instrument  you  will  be  using,  consult  your  TA.    

A.2: Prepare  a  ~1x10-­‐3  M  triiodide  stock  solution  by  combining  0.0650g    (±10%)  of  iodine  with  0.8500g  (±10%)  of  Potassium  Iodide  in  a  250mL  volumetric  flask.    

NOTE:  Iodine  can  be  troublesome  to  dissolve  in  aqueous  solutions  however;  iodine  dissolves  more  readily  in  concentrated  KI  solutions.  Therefore,  first  dissolve  the  KI  in  ~50mL  of  the  glycine-­‐potassium  iodide  buffer  within  the  volumetric  flask,  then  dissolve  the  iodine  in  that  solution.  Once  the  iodine  is  dissolved  fill  the  volumetric  flask  to  the  mark  with  buffer.  

 A.3: Accurately  prepare  a  series  of  triiodide  calibration  standards  from  the  stock  solution  

with  the  approximate  concentrations  of  1x10-­‐5M,  1x10-­‐6M,  1x10-­‐7M.  You  can  use  small  volume  volumetric  flasks  (i.e.  10mL)  for  this  as  the  cuvettes  you  will  be  using  to  make  your  UV  measurements  only  require  1mL  of  sample.  Transfer  all  your  standards  and  stock  solution  into  Nalgene  bottles.    NOTE:  Be  sure  to  calculate  the  actual  concentrations  for  your  lab  report    

A.4: Dispense   a   sample   of   the   iodine   tablet   disinfected   drinking   water   into   a   Nalgene  bottle.  Be  sure  to  mix  the  water  sample  well  prior  to  dispensing.  Consult  your  TA  for  instructions   on   how   to   run   the   UV-­‐VIS   spectrophotometer   experiments   to   analyze  your  samples.    Be  sure  to  use  glycine  buffer  as  your  blank  (Not  the  glycine-­‐iodide  buffer  you  used  to  prepare  your  samples)  

 NOTE:  Make  sure  to  obtain  the  absorbance  of  the  sterilized  drinking  water  sample  in  triplicate.    

 A.5: After   obtaining   the   UV-­‐VIS   data,   plot   a   calibration   curve   in   excel.   Determine   the  

concentration   of   triiodide   in   the   sterilized   drinking  water   to   then   prepare   another  two   calibration   standards   to   bracket   around  your   sample,   so   you   end  up  with   a   5-­‐point  calibration  curve.      

A.6: Acquire  the  UV-­‐VIS  data  of  the  new  standards.      Part  B  –  Determination  of  Iodine  Using  UV-­‐VIS  Spectroscopy:    *** In  this  section  of  the  experiment  be  sure  to  rinse  all  your  glassware  and  make  up  all  

your  solutions  using  the  provided  0.4mM  glycine  buffer  (pH=5.5).    B.1: Verify  that  the  UV-­‐visible  spectrophotometer  is  turned  on  when  you  arrive  in  the  lab.    

If  you  are  unsure  what  instrument  you  will  be  using,  consult  your  TA.    

B.2: Prepare   a   ~1x10-­‐2   M   Iodine   stock   solution   by   combining   0.2500g     (±10%)   of  potassium   iodate  with  1.3500g   (±10%)  of  Potassium   Iodide   in   a  250mL  volumetric  flask.   First   dissolve   the   solids,   within   the   volumetric   flask   with   ~50mL   of   glycine  buffer   and   ~5mL   of   3M   hydrochloric   acid   (use   a   graduated   cylinder   for   the   acid).  After  the  solids  have  dissolved  fill  to  the  mark  with  glycine  buffer.  

 

B.3: You  will  need  to  accurately  prepare  a  series  of  iodine  calibration  standards  from  the  stock   solution   with   the   approximate   concentrations   of   1x10-­‐3M,   1x10-­‐4M,   1x10-­‐5M.  However   prepare   them   one   at   a   time,   and   run   them   immediately   on   the   UV-­‐VIS  spectrophotometer.  Before  you  start  preparing  the  calibration  standards  consult  your  TA   for   instructions   on   how   to   set   up   and   run   the   UV-­‐VIS   spectrophotometer  experiments.     Set   up   the   method   to   include   all   your   samples,   and   run   the   sample  blank.  What  should  the  blank  be?    

 B.4: Using  small  volume  volumetric  flasks  (i.e.  10mL,  you  will  only  need  1mL  of  sample  to  

take  a  measurement)  prepare  your  calibration  standards  one  at  a  time.  To  prepare  a  calibration  standard  add  the  correct  volume  aliquot  of  stock  or  higher  concentration  standard   into   your   10mL   volumetric   flask,   then   add   ~1mL   of   3M   HCl   using   a  graduated  cylinder,  mix  and  fill  to  the  mark  with  glycine  buffer.  After  the  standard  is  prepared,   take   the  UV-­‐VIS  measurement   of   that   standard.     Repeat   until   you   have   a  measurement  for  all  your  standards.    

 B.5: Dispense   a   sample   of   the   iodine   tablet   disinfected   drinking   water   into   a   Nalgene  

bottle.  Be  sure   to  mix  the  water  sample  well  prior  to  dispensing.  Analyze  the  water  sample  using  the  UV-­‐VIS  spectrophotometer  in  triplicate.  

 B.4: After   obtaining   the   UV-­‐VIS   data,   plot   a   calibration   curve   in   excel.   Determine   the  

concentration   of   triiodide   in   the   sterilized   drinking  water   to   then   prepare   another  two  calibration  standards  to  bracket  your  sample.  Prepare  the  standards  as  above,  if  using  a  different  size  volumetric  flask  be  sure  to  maintain  the  same  ratio  of  3M  HCl.    Acquire  the  UV-­‐VIS  data  immediately  after  each  standard  is  prepared.    

 Part  C  –  Iodometric  Titration  Using  Ascorbic  Acid:    *** The   group   members   performing   this   part   of   the   experiment   should   also  

simultaneously  prepare  the  samples  and  standard  solutions  required  for  part  D.    *** In  this  section  of  the  experiment  be  sure  to  rinse  all  your  glassware  and  make  up  all  

your  solutions  using  the  provided  0.4mM  glycine  buffer  (pH=5.5).    

C.1: Obtain  three  100mL  beakers  and  rinse  thoroughly  with  glycine  buffer.  To  each  beaker  accurately   pipette   25mL   of   the   sterilized   drinking  water   then   add   a  mini  magnetic  stir-­‐bar   to   and   fill   approximately   to   the   60mL  mark   of   the   beaker   the  with   glycine  buffer.      NOTE:    Be   sure   to   thoroughly  mix   the  sterilized  drinking  water  prior   to  dispensing  into  the  beaker  from  which  you  will  take  your  25mL  aliquots.        

 C.2: Accurately   prepare   a   ~5x10-­‐4M   ascorbic   acid   standard   solution   in   a   500mL  

volumetric  flask  using  the  ascorbic  acid  that  was  dried  during  the  previous  lab  period.  You  may  opt  to  prepare  this  via  serial  dilution  of  a  more  concentrated  stock  solution;  

either  method  is  fine.    Transfer  the  solution  to  the  provided  bottle  that  can  mount  to  the  auto  titrator.  If  you  cannot  find  the  bottle  consult  your  TA.    NOTE:   The   titrations   must   be   performed   immediately   after   preparation   of   the  

ascorbic  acid  standard  solution.    Why?    

C.3: After  the  ascorbic  acid  standard  solution  is  prepared,  consult  your  TA  on  how  to  set  up  and  run   the  auto-­‐titrator.  Then   titrate  each  water   sample  with   the  ascorbic  acid  standard  solution,  be   sure   to  allow  enough   time   for  your   sample   to  mix   thoroughly  before  starting  the  titration.  

 Part  D  –  Iodometric  Back  Titration:    *** In  this  section  of  the  experiment  be  sure  to  rinse  all  your  glassware  and  make  up  all  

your  solutions  using  deionized  water.    D.1: Accurately   prepare   a   ~4x10-­‐5M   potassium   iodate   standard   solution   in   a   500mL  

volumetric   flask  using   the  potassium   iodate   that  was  dried  during   the  previous   lab  period.  You  may  opt  to  prepare  this  via  serial  dilution  of  a  more  concentrated  stock  solution;  either  method  is  fine.    Transfer  the  solution  to  the  provided  bottle  that  can  mount  to  the  auto  titrator.  If  you  cannot  find  the  bottle  consult  your  TA.  

 D.2: Accurately   prepare   a   ~1x10-­‐4   sodium   thiosulfate   solution   in   a   500mL   volumetric  

flask.   You   may   opt   to   prepare   this   via   serial   dilution   of   a   more   concentrate   stock  solution;  the  choice  is  yours.  

 NOTE:  You  will  be  using  sodium  thiosulfate  pentahydrate  make  sure  you  use  the  right  molecular  weight.  

 D.3: To   a   100mL   beaker   accurately   dispense   10mL   of   the   sterilized   water   sample   and  

10mL  of  the  sodium  thiosulfate  solution.  The  colour  of  the  solution  should  turn  clear,  why  is  this?  After  the  solution  turns  clear  add  deionized  water  up  to  the  60mL  mark  on  the  side  of  the  beaker.  

 D.4: Consult  your  TA  on  how  to  set  up  and  run  the  auto-­‐titrator.      D.5: Just  before  you  titrate  your  samples,  add  2mL  of  0.5g/ml  potassium  iodide  solution  

and  2mL  of  3M  hydrochloric  acid  to  your  100mL  beakers  using  a  graduated  cylinder.  Allow   the   samples   to   mix   thoroughly   on   the   auto-­‐tirator   stirring   plate,   and   then  titrate  the  water  sample  with  the  iodate  standard  solution.      

 NOTE:  You  must  only  add  the  acid  and  potassium  iodide  just  before  the  titration  and  not   before,   if   allowed   to   sit   the   acid   may   prevent   the   titration   from   proceeding  properly.  Also  these  components  must  not  be  added  before  the  thiosulfate  is  allowed  to  react  with  the  tablet  sterilized  water.    

 

D.6: Repeat   the   titration   with   two   more   sterilized   water   samples   to   obtain   triplicate  results.  

 D.7: Once   the   titrations   of   the   water   sample   are   complete,   you   must   standardize  

thiosulfate   solution   (Why   is   this?).   Prepare   a   thiosulfate   titration   sample   by  accurately  dispensing  10mL  of   the  sodium  thiosulfate  solution   into  a  100mL  beaker  then  add  deionized  water  up  to  the  60mL  mark  on  the  side  of  the  beaker.      

 D.8: As  outlined  in  section  D.5,  just  before  you  are  ready  to  titrate  the  sample,  add  2mL  of  

0.5g/ml  potassium  iodide  solution  and  2mL  of  3M  hydrochloric  acid  to  your  100mL  beaker  using  a  graduated  cylinder.  Allow  the  samples  to  mix  thoroughly  on  the  auto-­‐tirator  stirring  plate,  then  titrate  the  sample  with  the  iodate  standard  solution.      

 D.9: Repeat   the   thiosulfate   standardization   titration   two  more   times   to   obtain   triplicate  

results.    

Part  E:    Determination  of  Iodide  by  Ion  Selective  Electrode:    *** In  this  section  of  the  experiment  be  sure  to  rinse  all  your  glassware  and  make  up  all  

your  solutions  using  the  provided  0.4mM  glycine  buffer  (pH=5.5).    E.1: Accurately   prepare   a   ~1x10-­‐2   M   potassium   iodide   stock   solution   in   a   250mL  

volumetric  flask.    

E.2: Accurately   prepare   a   series   of   iodide   calibration   standards   from   the   stock   solution  with  the  approximate  concentrations  of  1x10-­‐3  M,  1x10-­‐4  M,  1x10-­‐5  M,  1x10-­‐6  M,  1x10-­‐7  M,   1x10-­‐8  M.   (Be   sure   to   accurately   calculate   the   actual   concentrations   for   your   lab  report)  

 E.3: To  prepare  a  sample  for  ISE  measurement,  accurately  pipette  15mL  of  the  standard  to  

be   analyzed   into   a   50mL   Nalgene   beaker.     To   the   same   beaker   accurately   pipette  15mL  of  iodide-­‐TISAB  solution,  and  add  a  mini-­‐magnetic  stir  bar.  

 E.4: Using   the   same  method   as   in   experiment   5   use   the   ISE   to  measure   the   calibration  

standards.    Preparation  Sterilized  Water  Sample  for  ISE  Measurement:    E.5: Accurately   prepare   a   ~1x10-­‐4   sodium   thiosulfate   solution   in   a   250mL   volumetric  

flask.   You   may   opt   to   prepare   this   via   serial   dilution   of   a   more   concentrate   stock  solution;  the  choice  is  yours.    NOTE:  You  will  be  using  sodium  thiosulfate  pentahydrate  make  sure  you  use  the  right  molecular  weight.  

 

E.6: Prepare   a   sterilized   water   sample   for   ISE   measurement   by   accurately   dispensing  15mL  of   the  water   sample   followed  by  15mL  of   iodide-­‐TISAB   solution   into   a  50mL  Nalgene  beaker.    Add  a  mini-­‐magnetic  stir  bar  to  the  beaker  as  well.    

 E.7: Using   the   same   method   as   in   experiment   5,   use   the   ISE   to   measure   the   sterilized  

water  sample.        E.8: After   recording   the   value   in   your   lab   notebook   DO   NOT   remove   the   ISE   from   the  

sample,   instead,   dispense   15mL   of   the   thiosulfate   solution   into   the  Nalgene   beaker  containing   the   sterilized  water   sample.  Allow   the   reading   to   stabilize   and   record   in  your  lab  notebook.    DO  NOT  FORGET  to  account  for  the  dilution  factor  of  adding  the  thiosulfate   solution   to   your   sample.   (What   is   the   difference   between  measuring  the   sample   before   and   after   the   addition   of   the   thiosulfate?   What   does   the  thiosulfate  do  to  the  water  sample?)  

 E.9: Repeat  two  more  times  to  obtain  triplicate  results.      Post-­‐Lab  Assignment:    

Although  you  will  be  using  the  data  from  the  entire  group,  you  should  each  submit  your  own  written  report  for  this  experiment.    There  is  no  report  form  for  this  experiment;  instead  you  will  be  preparing  a  formal  lab  report  consisting  of  the  following:    Title  page:     This  should  clearly  state  the  title  of  the  formal  report,  as  well  as  your  name,  student  number,   and  demo  group  number.    Departmental  policy   also   requires   that   you   include  a  signed  academic  honesty  pledge:    “I  certify  that  this  submitted  laboratory  report  represents  entirely  my  own  efforts.    I  have  read   and   understand   the   University   of   Toronto   policies   regarding,   and   sanctions   for,  plagiarism”    Abstract  (1  mark):     This  should  be  a  ~1/2  page  (50–125  words)  summary  of  what  was  done  in  the  lab  and  what  was   found,   including  but  not   limited   to   the  concentrations  and  uncertainties  of  the   different   iodine   species   found   in   the   iodine   tablet   sterilized   water.   Don’t   include  procedural  details.    Introduction  (2  marks):     This   should   outline   the   purpose   of   the   experiment   and   give   the   reader   sufficient  scientific  background  necessary  to  understand  what  you  are  writing  about  in  the  report.    In  this  case  you  should  assume  that  the  target  audience  of  your  report  have  a  background  in  analytical   chemistry,   so   the   scientific   background   of   the   introduction   should   focus   on  aqueous  iodine  chemistry.    

Experimental  (1  marks):     Include  a   short   journal   style  outline  of   the  experimental  procedures  used.   Include  sufficient  detail  so  that  another  scientist  can  reproduce  your  experiments.    Be  concise,  do  not  include  vessel  sizes  or  masses  of  the  components  used  to  make  solutions,  just  state  the  final   concentrations   of   the   samples   you   made.     You   also   do   not   have   to   explain   the  bracketing  process,  just  list  the  concentrations  you  decided  to  use  as  brackets  when  you  list  the  concentrations  of  the  standards  used.    Do  not  include  results  in  this  section.    For  example,  a  partial  summary  of  part  A:          A   9.9x10-­‐4   M   triiodide   stock   solution   was   prepared   by   dissolving   iodine   and   potassium  iodide  in  a  0.4mM  glycine  –  50g/L  iodide  buffer  (pH=5.5).    The  stock  triiodide  solution  was  serially  diluted  into  standards  with  concentrations  of  9.9x10-­‐6M,  5x10-­‐6M,  9.9x10-­‐7M,  5x10-­‐7M   and   9.9x10-­‐8M.     UV-­‐visible   spectra   of   the   triiodide   standards   and   neat   Coghlan's  Emergency  Drinking  Water  Germicidal  Tablet  sterilized  water  were  acquired.      Results  (8  marks):       Within  the  report   include  a  results  section  that  consists  of  a  summary  table  of   the  results   obtained   and   sample   calculations   of   each   experiment.   The   results   table   should  consist  of  two  columns,  one  with  a  description  of  each  experiment  and  the  other  with  the  results  of  each  experiment  including  units  and  uncertainties.    The  experiment  description  should  be  more  descriptive   than,   for   example,   “part  A”.     A  more   appropriate   description  would  be,  “Part  A:  determination  of  the  triiiodide  concentration  of   iodine  tablet  sterilized  water”.      

The  sample  calculations  should  be  subdivided  into  each  experiment  with  the  use  of  sub-­‐headings.   Each   section   should   first   include   an   equation  derivation.   If   the   equation   is  generated  from  a  calibration  curve,  show  the  curve  and  the  equation  of  the  line,  and  then  the  expression  based  on  the  curve.  If  the  equation  is  based  on  a  chemical  equation  or  series  of   chemical   equations,   first   show   the   balanced   chemical   equations   and   then   the   derived  expression.    At  this  level  you  should  not  be  inserting  hand  written  or  ASCII  based  equations  or  calculations,  but  rather  you  should  be  generating  your  own.  Software  such  as  Microsoft  Equation   Editor   (included   in   most   versions   of   Microsoft   office)   or   the   freely   available  MathType  software  package  make  it  easy  to  generate  your  own  equations.  

You  will   also  be   required   to  had   in   an   annotated   excel   spreadsheet  using   the  one  handed  in  with  experiment  5  as  a  template.  This  spreadsheet  will  include  all  the  raw  data  from  each  experiment  and  all  the  calculations,  statistical  analysis,  regression  analysis  and  error  analysis.    In  the  cell  next  to  a  calculation  clearly  annotate  the  excel  equation  used  to  generate  that  result.    

Attach  this  to  the  end  of  your  report  as  an  appendix.    Always  use  the  correct  number  of  significant  figures/decimal  places.      Discussion  (10  marks):     This   should   be   a   detailed   exploration   of   the   findings   from   your   experiment.   Your  discussion  should  include  but  not  be  limited  to  discussing  the  following  questions:  

• What  do  the  results  outlined  in  the  preceding  section  indicate?    

• Can  you  determine  if  any  HOI  is  present  and  estimate  a  concentration?    • Part  C  and  D  determine  the  concentration  of  reducible  iodine  species,  are  the  results  

of   each   section   statistically   different?    Which   experimental   design   introduces   less  error?  

• Is  the  concentration  of  I2  determined  in  part  B  the  same  as  part  C  and  D,  why  or  why  not?    

• Find   the   literature   values   for   the   equilibrium   constants   of   the   different   iodine  species  under  acidic  conditions.    Are  the  concentrations  of  the  different  species  you  determined   in   agreement  with   the   literature   equilibrium   constant?     Assume   each  tablet  releases  a  max  of  8mg  of  I2.  

• Is  there  any  evidence  of  systematic  error  (bias)  in  any  of  the  techniques  used?    • If   you   encountered   problems   in   performing   the   experiment,   what   were   they   and  

what   did   you   do   to   address   them?   Or   how   did   they   adversely   affect   your  experiment?  

 Conclusion  (3  marks):     Provide   a   final   summary   of   your   findings   and   conclusions   from   your   discussion,  what  you  learned  from  the  experiment  overall,  and  any  recommendations  as  to  what  you  might  do  differently  if  you  were  to  repeat  this  experiment.      At  the  end  of  your  conclusion  it  is   a   good   idea   to   relate  your   findings  back   to  how   they   relate   to   real  world  problems  or  issues.    A  real  world  problem  related  to  iodine  drinking  water  sterilization  tablets  are  the  health   effects   associated   to   long-­‐term   exposure   to   iodine.     What   is   the   maximum  recommended  daily   ingestion   level  of   iodine  and  based  on   this   are   these   tablets   a  viable  long-­‐term  water  purification  solution?