Abstract solids instead of small crystals. This is

Abstract

The goal of this
experiment was to determine the amount of calcium in an unknown sample by
converting the calcium into CaC2O4H2O through gravimetric analysis.

Ca2+(aq) + C2O42-(aq)
+ H2O(l) ® CaC2O4×H2O(s)

Three portions of
0.4347g, 0.3982g and 0.3938g of the unknown sample were used. The mass of the calcium oxalate
monohydrate crystals formed were 0.5022g, 0.4554g and 0.4551g. The percentage
of calcium by mass in the three samples were 31.69%, 31.37% and 30.70%. The
average percentage of calcium by mass in the unknown sample was found to be
31.58% with a standard deviation of 0.149 and a relative error of 2.27%. The lab
techniques of weigh by difference, quantitative transfer, gravimetric analysis
and homogeneous precipitation were used in this lab.

 

Introduction

The primary purpose of
this lab is to determine the content of calcium in an impure sample of calcium
carbonate by converting the calcium into CaC2O4H2O through gravimetric analysis.

Ca2+(aq) + C2O42-(aq)
+ H2O(l) ® CaC2O4×H2O(s)

The lab techniques of
weigh by difference, quantitative transfer, homogeneous precipitation and
gravimetric analysis were used in this lab. Gravimetric analysis is the method
in which the percent composition of a substance can be calculated by analyzing
the masses of reactants and products. For gravimetric analysis, an insoluble
solid of the desired compound must form; the precipitate must be pure; and this
insoluble compound must be easily weighed and handled. Due to the insolubility
and the ease of weighing of CaC2O4×H2O, it is chosen as the desired
product of the experiment. This allows the filtration process to be successful
(as CaC2O4×H2O can exist as large crystals that are easy to be filtered)
and so the final measurements will be accurate and precise. Homogeneous
precipitation is the steady and slow precipitation that ensures the formation
of large solids instead of small crystals. This is done by heating the
reactants at a relatively low temperature (near boiling point) which allows the
reaction to occur very slowly. The calcium ions in the unknown sample must be
converted slowly into calcium oxalates to ensure maximum yield and purity.

Using urea as a weak base of ammonia which reacts slowly can successfully slow
down the formation of the CaC2O4×H2O crystals.

The overall reactions
are listed as below:

CaCO3(s) + HCl(aq) ® CaCl2(aq) + H2O(l) + CO2(g)

CaCl2(aq) ® Ca2+(aq) + 2Cl-(aq)

(NH4)2C2O4(aq) + HCl(aq)
® HC2O4-(aq)
+ 2NH4+(aq) + Cl-(aq)

(NH2)2CO(s) + H2O(l)
® CO2(g) + 2NH3(aq)

NH3(aq) + H+(aq) ® NH4+(aq)

HC2O4-(aq) ? C2O4-(aq)
+ H+(aq)

Ca2+(aq) + C2O4-(aq)
+ H2O(l) ® CaC2O4×H2O(s)

 

Excess 1M HCl need to
be added at an early stage of the experiment to ensure the pH is under 2. A
solution with a low pH will allow the successful formation of HC2O4-
instead of C2O42-. C2O42-
can immediately combine with Ca2+ and form small CaC2O4
crystals which is not desired in the early stage of the experiment. Another
reason for adding excess HCl is that a lot of HCl is required to make sure that
all of the calcium in the calcium carbonate solid are changed into free Ca2+.

By doing this, the results of the experiment will be more accurate. The pH of
the solution must be increased later in the lab. This is because a high
concentration of C2O42-
is needed to precipitate out all of the Ca2+ in the solution.

HC2O4-(aq) ? C2O4-(aq)
+ H+(aq)

As the pH of the solution increases, there are more OH- ions
than H+ ions in the solution. Based on Le Chatelier’s principle, the
system would favor the forward reaction to produce more H+, thus more
C2O42- can be produced.

 

Gravimetric analysis is
an important method in the world of science as it is one of the most accurate
method of macro quantitative analysis. For example, thermo-gravimetric analysis
was used in the field of biology to study the mass losses of biomass and the
percent of evolved species through biomass pyrolysis1.

Thermo-gravimetric analysis was used to obtain data sets to calculate the yield
of liquids and yields of pyrolyzed gas and compare them1. Gravimetric
analysis is also applied to a gas-liquid chromatography study2.

Fatty acid methyl esters were streaked on to prepared chromatograph plates and
examined. Groups of saturated, tans-monounsaturated, cis-monounsaturated,
di-unsaturated and poly-unsaturated esters formed and the proportion of these
groups were determined gravimetrically, hence the first three groups can be
identified and separated through gas-liquid chromatography2.

Gravimetric analysis was also used to study moisture diffusivity3.

The mass loss of a sample through a non-isothermal procedure was determined
using thermo-gravimetric analysis to program a heating profile which were
compared to original isothermal procedures to interpret the temperature
dependence of the moisture diffusivity3.

 

Experimental

The procedures of this lab were adapted
from the procedures in the chem 203 lab manual4. 0.3-0.5 g of the
unknown were weighed out in each 3 labeled 250 mL beakers using the technique
of weigh by difference. 100 mL of deionized water were added to each 3 samples.

And 1M HCl were then added to the beakers until the pH reached 2. The amount of
HCl added were recorded
to be 7.0 mL, 7.0 mL and 7.0
mL respectively. 4 drops of Methyl red indicator were then added to
each of the solution. 3 portions of 20 mL of ammonium oxalate were measured out
and poured into three 50 mL beakers. 3 portions of 1.0 mL of 1M HCl were then
added to each of the ammonium oxalate in the beaker and the pH were checked and
ensured to be less than 2. The solutions were then added to the samples that
were pH adjusted and contains the methyl red indicator. Enough solid urea was
then added to the solution. The amount of urea added were 11.1179g, 11.0474g
and 11.2856g respectively. Swirling was applied to dissolve all the urea. The 3
samples were then heated to near boiling point on a hot plate. 3.0045g, 3.0225g
and 3.0024g of additional urea were added in the first half hour. As the
indicator did not change color, more urea were added – 5.0190g, 5.0116g and
5.0121g. Then, 5.0484g, 5.0253g, 5.0223g and 5.0560g, 5.0145g, 5.0139g of
additional amount of urea were added to fully react. After the solution turned
from pink to yellow, the liquids on the watch glass and the stirring rod were
rinsed into the beakers.

Vacuum filtration
apparatus was set up and the hot solutions were filtered through the sintered
glass crucibles to isolate the calcium oxalate monohydrate crystals. Small
amounts of deionized water were used to transfer all precipitations into the
crucible. After filtration, the crystals were washed using 2 portions of 15 mL
ice cold deionized water and rinsed with another 2 portions of 10 mL acetone
over the vacuum filtration apparatus. Air were drawn through the crystals to
dry the crystals and remove the traces of acetone. Then the samples were dry in
air for another half hour. Then the crucibles were put in a beaker and dried at
105°C. After drying, the crucibles were cooled
down and weighed. After first weighing, the crucibles were put back into the
dessicator for 15 minutes and reweighed. This was repeated until a constant
weight was obtained.

 

Results

The mass of the unknown
samples that were measured using weigh by difference are 0.4347±0.0002g, 0.3982±0.0002g and 0.3938±0.0002g
(Table 1).

Table 1. Mass of the unknown sample (g)

 

#1

#2

#3

Mass

0.4347±0.0002g

0.3982±0.0002g

0.3938±0.0002g

 

The 3
sintered crucibles that used for vacuum filtration was measure to have masses
of 31.4510±0.0002g, 31.4015±0.0002g and 32.8162±0.0002g (Table 2).

Table 2. Mass of Crucibles (g)

 

#1

#2

#3

Mass

31.4510±0.0002g

31.4015±0.0002g

32.8162±0.0002g

 

The volumes of HCl
added were recorded (Table
3).

Table 3. Volumes of HCl added

 

#1

#2

#3

The amount of HCl added to dissolve the sample

7.0 mL

7.0 mL

7.0 mL

The amount of HCl added to react with (NH4)2C2O4

1.0 mL

1.0 mL

1.0 mL

 

Using the amount of the
1 M HCl that were added to dissolved the sample, the percentage of the mass of
calcium in the unknown sample can be predicted. Since 7 mL of HCl were added to
dissolved the sample, the number of moles of HCl can be calculated from its
concentration and volume.

n(HCl) = HCl  V(HCl) = 1 mol L-1  0.007 L = 0.007 mol

This shows that the number of moles of CaCO3 is also 0.007
mol from the balanced equation.

CaCO3(s) + HCl(aq) ® CaCl2(aq) + H2O(l) + CO2(g)

n(HCl) = n(CaCO3) = 0.007 mol

Therefore, the mass of
CaCO3 and the percentage of calcium by mass in the sample can be
calculated:

m(CaCO3) = molar mass(CaCO3) n(CaCO3) = 100.09 g
mol-1  0.007 mol = 0.70063 g

m(Ca) = m(CaCO3)  = 0.28056 g

Ca % =  = 64.54%

Table 4. Prediction of Ca% based on the amount of HCl added.

 

n(HCl)

m(CaCO3)

m(Ca)

Ca %

#1

0.007 mol

0.70063 g

0.28056 g

64.54%

#2

0.007 mol

0.70063 g

0.28056 g

70.45%

#3

0.007 mol

0.70063 g

0.28056 g

71.23%

 

Average prediction of percentage of Ca by mass = 68.74%

 

A lot of solid urea
were measured out and added to the solution to fully react, the amount of urea
added to the solution are recorded below (Table 5).

Table 5. Mass of urea added

 

#1

#2

#3

Mass of urea added before the solution was heated

11.1179±0.0002g

11.0474±0.0002g

11.2856±0.0002g

Mass of urea added while heating (1st time)

3.0045±0.0002g

3.0225±0.0002g

3.0024±0.0002g

Mass of urea added while heating (2nd time)

5.0190±0.0002g

5.0116±0.0002g

5.0121±0.0002g

Mass of urea added while heating (3rd time)

5.0484±0.0002g

5.0253±0.0002g

5.0223±0.0002g

Mass of urea added while heating (4th time)

5.0560±0.0002g

5.0145±0.0002g

5.0139±0.0002g
 

Total amount of urea added

29.2458±0.0002g

29.1213±0.0002g

29.3363±0.0002g

 

The mass of the
crucibles with the crystals after heated in the oven and reweighed every 15
minutes after cooling down are recorded below (Table 6).

Table 6. Mass of crucibles after heating

 

#1

#2

#3

Mass of crucibles after heating

31.9491±0.0002g

31.8537±0.0002g

33.2683±0.0002g

Mass of crucibles reweighed after 15 mins

31.9529±0.0002g

31.8565±0.0002g

33.2708±0.0002g

Mass of crucibles reweighed after 20 mins

31.9532±0.0002g

31.8569±0.0002g

33.2713±0.0002g

 

To calculate the
percentage by mass Ca in the unknown sample, the mass of the calcium oxalate
monohydrate that produced during the lab need to be determined. This is done by
finding the difference between the mass of the crucible containing the crystals
and the mass of the sintered crucibles.

Sample calculation:

m(CaC2O4H2O) = m(crucibles with CaC2O4H2O) – m(crucibles)

= 31.9532 g – 31.4510 g

= 0.5022 g

Then, the number of
moles of Ca can be determined using the mass and the molar mass of CaC2O4H2O.

n(Ca) = n(CaC2O4H2O) =  =  = 3.43710-3 mol

The mass of Ca in the
sample can then be determined.

m(Ca) = n(Ca) molar mass(Ca) = 3.43710-3 mol  40.08 g mol-1 = 0.1378
g

And the percentage by
mass of Ca in the unknown sample can be calculated (Table 7).

%Ca =  100 =   100 = 31.69%

Table 7. Percentage by mass of Ca

 

Mass of CaC2O4H2O

Moles of Ca

Mass of Ca

Ca percentage by mass

#1

0.5022 g

3.43710-3 mol

0.1377 g

31.67%

#2

0.4554 g

3.11710-3 mol

0.1249 g

31.37%

#3

0.4551 g

3.11510-3 mol

0.1248 g

31.70%

 

And the average
percentage by mass of Ca in the unknown sample is  = 31.58%.

To evaluate the quality
of this data, a Q-test must be taken. To carry out the Q-test, the difference
between the questionable value and the nearest value in the data set is
determined and divided by the range of the set.

for crucible #2, 

 0.909

The Qepx is
then compared to a value listed on a Q-test table corresponding to the number
of values in the data set and the degree of confidence desired in the rejection
process. For a data set of 3 observations, the Qcrit is 0.940. The Qepx
for each trial are calculated below in Table 8.

Table 8. Qepx for each trial and comparison of Qepx with
Qcrit

 

#1

#2

#3

Qepx

0.091

0.909

0.091

 

Since all of the Qepx
values are under 0.940, none of them should be rejected with a 90%
confidence level. Thus, the mean percentage by mass of Ca in the unknown is
31.58%.

The actual percentage
by mass of Ca in the unknown sample is 30.88%, so the relative error and
standard deviation of the data collected must be calculated.

 

 

= = 0.149

 

Discussion

The determination of
the mass percentage of calcium in an impure sample of CaCO3 was the
goal of this lab. This is done by using the method of gravimetric analysis.

Gravimetric analysis is the technique in which the percent composition of a
substance can be calculated by analyzing the masses of reactants and products.

The techniques of quantitative transfer, weigh by difference, homogeneous
precipitation and vacuum filtration were also applied to this lab. Quantitative
transfer is important for this lab, it ensured all the precipitates to be
rinsed and collected. The technique of quantitative transfer and weigh by
difference allowed the data collection to be very accurate and precise. Since
there are 3 special requirements for gravimetric analysis, calcium oxalate
monohydrate was chosen as the desired product. The requirements are: 1. The
compound formed must be an insoluble solid; 2. This solid precipitate must be
pure; 3. The insoluble compound must be easy to be weighed. Calcium oxalate
monohydrate was chosen because it exists as large crystals with a very low solubility,
so filtration can be successful (all the calcium can be collected) and the data
can be accurate. Homogeneous precipitation is the steady and slow precipitation
to ensure the formation of large crystals instead of small crystals. This is
done by heating the reactants at a relatively low temperature (near boiling
point) which allows the reaction to occur very slowly.

 

The pH played a crucial
role in this lab. The pH was checked to be less or equal to 2 to ensure the
formation of HC2O4-, otherwise crystals of CaC2O4
would form right after the (NH4)2C2O4
were added. Additionally, a low pH means there was an excess of HCl in the
solution. This is favored because an excess amount of HCl is needed to dissolve
all the CaCO3 and change all of the calcium into free calcium ions. In
the second part of the experiment the pH of the solution was increased to above
6. This is because a high concentration of C2O42-
were needed to fully react with all of the Ca2+. If the pH of the
solution is increased, according to Le Chatelier’s principle, the system would
react to produce more H+ to reverse the change. Therefore, the
system favors the forward reaction and so more C2O42-
can be produced.

HC2O4-(aq) ? C2O42-(aq)
+ H+(aq)

Also, to ensure maximum
yield and purity, the crystals must be produced very slowly. This is done by
using urea as a source of ammonia. Ammonia is a weak base and it can only react
slowly. Therefore, the formation of the calcium oxalate monohydrate crystals
can be slowed down.

 

The percentage of Ca by
mass in the unknown sample of CaCO3 was determined to be 31.58% in
this lab while the actual percentage is 30.88%. This percentage of 31.58% is
different from the prediction of 68.74% because a lot of HCl were added on
purpose at that point to keep the pH of the solution under 2. Therefore, there was
an excess amount of HCl in the solution. Hence it is unreasonable to use this
amount of HCl to predict the amount of Ca in the unknown sample.

 

The final numbers of
31.67%, 31.37% and 31.70% are quite alike with a standard deviation of 0.149 and
the relative error is only 2.27%. The percent of calcium was found to be 0.7%
more than the actual percentage. This error can be explained by some accidents
that occurred during the lab. The crucibles were not handled properly using the
kimwipes some times and this can cause dirt to stick on to the crucibles and
causing the final measured mass to be bigger than it should be. The mass of the
sintered crucibles and the crucibles with the calcium oxalate monohydrate were
not measured using the same electronic balance – this can cause the final data
to be a little bit off than expected. Also, calcium oxalate monohydrate is
hydroscopic – it is very likely that the crystals absorbed some water from the
surroundings during the weighing and drying process and result in a larger mass
to be measured and recorded.

 

Overall, this lab was very
successful as the standard deviation of the data is only 0.149 and the relative
error of the data collected is only 2.27% which is very small. The techniques
of gravimetric analysis and homogeneous precipitation were studied and
successfully utilized in this lab. The percentage of calcium (31.58%) in an
unknown sample of impure calcium was successfully determined by converting the
calcium into CaC2O4H2O through gravimetric analysis.

 

Acknowledgements

I discussed the role of
pH in this experiment with Yingjie Ji from BB1.

 

Reference

1.    
Seo, D. K., Park, S. S., Hwang, J., & Yu, T. (2010). Study of the
pyrolysis of biomass using thermo-gravimetric analysis (TGA) and concentration
measurements of the evolved species. Journal of Analytical and Applied
Pyrolysis, 89(1), 66-73. doi:10.1016/j.jaap.2010.05.008

2.    
Dunn, E., & Robson, P. (1965). Quantitatively gravimetric analysis
of fatty ester mixtures by thin-layer chromatography. Journal of
Chromatography A, 17, 501-505. doi:10.1016/s0021-9673(00)99901-1

3.    
Li, Z., & Kobayashi, N. (2005). Determination of Moisture
Diffusivity by Thermo-Gravimetric Analysis under Non-Isothermal Condition. Drying
Technology, 23(6), 1331-1342. doi:10.1081/drt-200059523

4.    
Chem 203 lab manual.