CHM 421 ANALYTICAL CHEMISTRY

EXPERIMENT 6 :

ANALYSIS OF BLEACH AND COPPER (II)UNKNOWN

NAME : OMMY MADINA BINTI ABDUL HALIM

STUDENT NUMBER : 2019295426

GROUP NUMBER : RAS2451A

LECTURER’S NAME : MOHD HAFIZ BIN YAAKOB

DATE EXPERIMENT : 11 NOVEMBER 2019

DATELINE EXPERIMENT : 25 NOVEMBER 2019

Abstract :

This experiment have 3 main objectives which are to standardise the sodium
thiosulphate solution,to determine the percentange of NaClO in bleach sample and to
determine the percentange of copper in an uknown copper sample.In the standardise of sodium
thiosulphate is which are need to calculate so that will get the exact concentration of sodium
thiosulphate.The exact concentration is 0.0377 M.Next,analysis of bleach which used 10 mL and
then need to diluted with distilled water in the 100 mL of volumetric flask.10.0 mL aliquot of
bleach solution with KI and HCl was titrated against the standardizationsodium thiosulphate.
The sample was titrated with the standardised sodium thiosulphate solution above until the
amber iodine colour begins to fade to light yellow . 2 mL of the starch solution was added and
the titration process was continued until dark colour of the starch-iodine complex just
disappears.The last part which analysis of Copper (II) Unknown with KI and HCI which used
oxidation reduction titration concept.The calculation which percentage of NaClO is 0.61 % and
the percentage of copper in an unknown copper sample is 27.60% that show is the bleach
sample is Copper (II) sulphate pentahydrate.

Objectives :

1)To Standardise the Sodium Thiosulphate Solution.

2)To Determine the Percentange of NAClO in Bleach Sample.

3) To Determine the Percentange of Copper in an Uknown Copper Sample.

Introduction:

The active ingredients in bleach is hypochlorite ion (ClO-) which has oxidation number
+1.Chlorine is more stable with oxidative state is -1 so that ClO- is strong oxidizing agent which
has ability to remove electrons from coloured compounds to its effectiveness as bleaching
agent.

When hypochlorite ions reacts with iodide ion it will produced iodine and chlorine ion
are produced thus also will further react to form the triiodide compkex.Triiodide is reduced back
to iodide by titration with standardised thiosulphate solution.

H2O + ClO + 2S2O32- S4O62- + Cl- + 2OH-

This experiment will determine of NaClO and generic brand bleach using the reaction and
technique.Molarity and volume thiosulphate solution used to reach the endpoint and the
volume of bleach solution titrated.

The percent by mass copper in Copper (II) unknown can be determined using bleach
sample.Firstly it will dissolved the dilute acid with excess potassium iodide to produce triiodide
and then it will titrated the triiodide ion with standadides thiosulphate solution .

Cu2+ 2I- + 2S2O32- 2CuI + S4O62-

Method :

A.Standardisation of the Sodium Thiosulphate Solution.

A burette thoroughly was washed with detergent followed by tap water. Then,the
burette was rinsed with distilled water and finally the sodium thiosulphate provided. Next,the
cleaned burette was filled with fresh thiosulphate solution.About 2g of solid KI into a 250 mL
conical flask. 50.0 mL of 0.010 M KIO 3 solution was pipetted into same flask and then 10 mL of
1.0 M H2SO4 was added.The colour of the solution in flask should turn to a deep brown
colour.The reaction that occurs in the flask is :

IO3-(aq) + I- (aq) + H+ (aq) I2 (or I3-) + H2O (aq)

The solution immediately was titrated in the flask with thiosulphate solution until the
colour of the solution is light yellow. At this point, 2 mL of starch indicator was added using a
cylinder. The solution should turn turn blue-black once the starch is added. If there is no colour
change, titrated pass the end point in step 3 and will need to repeat the titration.

The titration was continued after the addition of starch,being careful as you are already
nearing the end point. The titrant was added drop wisely until the solution in the conical flask is
clear and colourless. The volume of titrant used was recorded. The reaction that occurs in the
flask between thiosulphate and triiodide to produce iodide and tetrathionate is :

S2O32- (aq) + I2(aq) (or I3-) I- (aq) + S4O62-(aq)

The whole process was repeated at least two more times .

B.Analysis of Bleach

A clean and dry 100 mL volumetric flask was obtained.Weighed to the nearest 0.001g.
The weight of the empty 100 mL volumetric flask wa recorded into the notebook.Thoroughly a
10.0 mL pipette was cleaned and rinsed with the first bleach solution. Accurately, 10.0 mL of the
sample was pipette into pre-weighed 100 mL volumetric flask above. The flask was reweighed
and the weight was recorded. This will give the density of the bleach solution. The sample to the
100 mL mark was with diluted water. Stopper the flask and mixed well. The pipette out with
distilled water was rinsed followed with a little of the diluted bleach solution and then a fresh
10.0 mL aliquot of was pipette into a 250 mL conical flask. Next, 1.0g of potassium iodide was
added and the resulting mixture was swirled. Then 5.0 mL of 6M HCl was added to the mixture .

The sample was titrated with the standardised sodium thiosulphate solution above until
the amber iodine colour begins to fade to light yellow . 2 mL of the starch solution was added
and the titration process was continued until dark colour of the starch-iodine complex just
disappears. The ½ drop quantities as you near the endpoint by swiftly turning stopcock 180°. An
abrupt colour changes from dark blue to colourless marks the endpoint. The titration was
repeated with two more 10.0 mL aliquot of diluted bleach solution.

C.Analysis Of Copper (II) Unknown

Weigh out about 0.3g of the copper (II) unknown was accurately into a 250 mL conical
flask.The solid with 35 mL 0.05 M sulphuric acid was dissolved. 1.0g of potassium iodide was
added and the mixture was swirled. Carefully,the mixture was titrated with the standardised
sodium thiosulphate until the amber colour of the triiodide begins to fade.At this point, 2 mL
was added of the starch solution. Continued to titrate until grey starch -iodine colour disappear
to produce a milky white-pinkish colour that marks the end point.The process for two more
sample was repeated.
Results

A.Standardisation of the Sodium Thiosulphate Solution.

ROUGH 1

Final reading of thiosulphate solution 29.10 38.60

Initial reading of thiosulphate solution 0.00 0.00

Volume of thiosulphate solution (mL) 29.10 38.60

Table 1 : Standardisation of the Sodium Thiosulphate Solution

B.Analysis of Bleach

Brand of Bleach I : Ultra Kiraw Bleach

1.Molarity of Na2S2O3 solution = 0.0377 M

2. Mass of volumetric flask + 10 mL bleach = 88.6158 g

3.Mass of volumetric flask = 77.8421 g

4.Mass of 10 mL bleach = 10.7737 g

5. Density of bleach = 1.07737g

Sample 1 Sample 2
Final reading of thiosulphate solution 5.00 3.30

Initial reading of thiosulphate solution 0.00 0.00

Volume of thiosulphate solution (mL) 5.00 3.30

Moles of Na2S2O3 1.685 x 10-4 1.112 x 10-4

Moles of NaClO in 10 mL aliquot1 8.425 x 10-5 5.650 x 10-5
Moles of NaClO in 10 mL aliquot2 8.425 x 10-5 5.650 x 10-5
Moles of NaClO in 10 mL bleach sample 8.425 x 10-4 5.650 x 10-4
Average moles of NaClO in 10 mL bleach sample 7.0375 X 10-4 mol
Gram/ L of NaClO in bleach 0.0545 g
Mass percent of NaClO in bleach 0.51 %
Mass percent of NaClO from manufacturer’s label 5.25 %
Percent deviation between experimental and label 4.74 %
Table 2 : Analysis of Bleach

C.Analysis of Copper (II) Unknown

Molarity of Na2S2O3 Solution = 0.03770 M

Sample 1 Sample 2
Mass of Copper (II) compound taken (g) 0.3077 0.3114
Final reading of Na2S2O3 solution 19.50 18.90

Initial reading of Na2S2O3 solution 0.00 0.00

Volume of Na2S2O3 solution (mL) 19.50 18.90

Moles of Na2S2O3 1.3143 x 10-3 1.2786 x 10-3

Gram of Na2S2O3 0.0835 0.0812
Weight percent of copper in sample taken 27.83 % 27.36%
Average weight percent of copper in sample 27.60%
Theoretical mass percent of copper in compound 25.45 %
Percent Error 2.15%
Name unknown copper (II) unknown Copper (II) Sulphate pentahydrate
Chemical Formula of copper (II) unknown CuSO4.5H2O

Table 3 : Analysis of Copper (II) Unknown

Calculations :

A.Standardisation of the Sodium Thiosulphate Solution.

IO3-(aq) + I- (aq) + 6 H+ (aq) I2 (or I3-) + 3 H2O (aq)

2S2O32- (aq) + I2(aq) (or I3-) 2I- (aq) + S4O62-(aq)

 Rough

No of moles of IO3- = (0.010 ) (50.0 X 10 -3)

= 0.0005 mol

1 mole of IO3- = 1 mole of I2

Therefore , 0.0005 mole of IO3- = 0.0005 mole of I2

No of moles of S2O32- = 0.0005 mol of I2 x 2 mol of S2O32-

= 0.001 mol

Therefore , 1 mole of 2 S = 0.001 mole of S2O32-

Molarity of S2O32- = No of moles S2O3 / Volume

= 0.001 mol / 29.70 x 10-3 L

= 0.0337 M

So,The concentration of sodium thiosulphate is 0.0337 M .

B.Analysis of Bleach

H2O + ClO + 2S2O32- S4O62- + Cl- + 2OH-

Sample 1 Sample 2
2- -3
No of moles of S2O3 = (0.0337 )(5.00 x 10 ) No of moles of S2O32- = (0.0337 )(3.30 x 10-3)
= 1.685 x 10-4 mol = 1.112 x 10-4 mol
1 mol of ClO = 2mol of S2O32- 1 mol of ClO = 2mol of S2O32-
Therefore = 1.685 x 10-4 / 2 Therefore = 1.112 x 10-4 / 2
= 8.425 x 10-5 mol (in 10 mL aliquot ) = 5.650 x 10-5 mol(in 10 mL aliquot )
1 mol of ClO = 2mol of S2O32- 1 mol of ClO = 2mol of S2O32-
Therefore = 1.685 x 10-4 / 2 Therefore = 1.112 x 10-4 / 2
= 8.425 x 10-5 mol (in 100 mL aliquot ) = 5.650 x 10-5 mol(in 100 mL aliquot )
Moles of ClO- in 10 mL bleach sample = Moles of ClO- in 10 mL bleach sample =
= 8.425 x 10-5 X 10 mL = 5.650 x 10-5 X 10 mL
= 8.425 x 10-4 mol = 5.650 x 10-4 mol

Average of moles of NaClO in 10 mL bleach sample = ( 8.425 x 10 -4 mol + 5.650 x 10-4 mol ) / 2

= 7.0375 X 10-4 mol

Mass of NaClO in bleach sample = (7.0375 X 10-4 mol) (77.44 g mol-1 )

= 0.0545 g

Mass percent of NaClO in bleach sample = Mass solute

X 100 %
Mass solution

= ( 0.0545 / 10.7737 ) x 100

= 0.51 %

Mass percent of NaClO from manufacturer’s label = 5.25 %

Percent deviation between experimental and label = 5.25 % - 0.51 % = 4.74 %

C.Analysis of Copper (II) Unknown

Cu2+ 2I- + 2S2O32- 2CuI + S4O62-

Sample 1 Sample 2
2+ -3
No of moles of Cu = (0.0337 )(19.50 x 10 ) No of moles of Cu2+ = (0.0337 )(18.90 x 10-3)
 = 6.5717 x 10-4 mol = 6.3693 x 10-4 mol
2 mol of Cu2+ = 6.5717 x 10-4 mol x 2 2mol of Cu2+ = 6.3693 x 10-4 mol x 2
Therefore = 1.3143 x 10-3 mol Therefore = 1.2786 x 10-3 mol

Mass of Cu2+ = (1.3143 x 10-3)(63.54) Mass of Cu2+ = (1.2786 x 10-3)(63.54)

= 0.0835g = 0.0812 g

Weight percent of copper in sample taken = Mass solute

X 100 %
Mass solution

= (0.0835/0.3) x 100

= 27.83 %

Weight percent of copper in sample taken = (0.0812/ 0.3) x 100

= 27.36 %

Average weight percent of copper in sample = ( 27.83 % + 27.36% ) / 2

= 27.60 %

Theoretical mass of copper in compound :

Copper (II) acetate monohydrate = 31.80 %

Copper (II) chloride dehydrate = 20.90 %

Copper (II) sulphate pentahydrate = 25.45 %

Percentage error = 27.60 %- 25.45 %

= 2.15 %

Name of copper (II) unknown = Copper (II) Sulphate Penthydrate

Chemical Formula = CuSO4.5H2O

Discussion :

Oxidation-Reduction titration can determined the amount of hypochlorite ion present in

the solution of bleach.Iodine ion will added with hypochlorite ion solution which iodine will
iodized.Iodine is slightly soluble in water and it also dissolve well in iodide ion which is can form
the complex ion that is triiodide ion (I 3-).Triiodide ions is yellow in colour which dilute solution
and dark-brown in concentrated so when it titrated with standard solution it will reduces the
iodine back to iodide ions.The last reaction is being when the red-brown colour of triiodide ions
turns to yellows and fastly turn clear colour of iodide ions.Strach will added which contain
iodide or triiodide ionthat can form reversible blue complex.To calculate copper (II)
unknown,the first step which to know the molarity of thiosulphate that is 0.0377 M from the
H2O + ClO + 2S2O32- that will produce S4O62- + Cl- + 2OH- .Then the mass of hypochlorite should be
obtained so that it can proceed to calculate unknown copper (II).The mass that been calculated
will multiply with 100 to get the percentage of ClO - which is 0.16% and the from the
manufacture’s label is 5.25 %.The percentage deviation is 4.74 %.In the last part of the
experiment which analysis of copper (II)unknown in the sample is 27.60% which show the
unknown copper is Copper (II) sulphate pentahydrate.

There are some error that make the result of this experiment become less
accurate.Firstly,the parallex errors which is the eye’s level is not perpendicular with the burette
reading during record the observation.Next,before adjusting the initial volume and titration
there are air bubble in the burette.Next,swirl the flask not completely react until the colour it
fully react.
The weaknesses of the experiment may affected the result so it have some suggestion as
a preventation to be improve for the next experiment.Firstly,make sure that the eye’s level
perpendicular with the burette reading to avoid parallex error during
experiment.Furthermore,turn the stopcocks 360° a few time until the air bubble are
removed.Lastly,make sure that swirled the flask completely until the colour is fully react.

Conclusion :

The molarity of sodium thiosulphate in the solution is 0.0377M .The determination of the
percentage of NaClO in bleach sample is 0.16% while the percentage of copper in an unknown
Copper Solution id 27.60 % which show that the bleach is Copper (II) Sulphate Pentahydrate.

References :
La Dolce Aisya Ain (March 13,2013),Experiment 6, Retrieved November 11,2019 from
https://www.scribd.com/doc/130817277/Experiment-6

Uzumakein (n.d), Experiment 6A : Analysis of Bleach and Copper (II) Unknown, Retrieved
mmmmNovember 11,2019 from

https://www.coursehero.com/file/21551433/EXPERIMENT-6A-Analysis-of-Bleach-and-CopperII-
Unknown/