Investigating Chloride Content Using Gravimetric Analysis: A Comprehensive Method and Analysis

Purpose

To familiarize and show the technique of the gravimetric analysis by determining the chloride content of an unknown soluble salt.

Theory

Some solutions completely dissolve within the water, while others need a boost by a precipitation reaction. By using this theoretical knowledge to determine the chloride ion concentrations by forming the silver chloride. The reaction equation for the precipitation of the chloride ion by silver ion is (Ag〗_((aq))^++〖Cl〗_((aq))^- →〖AgCl〗_((s) ). This equation describes that silver ions in the aqueous solution and chloride ion in the aqueous solution goes to completion to form the silver chloride which is a solid precipitate.

The reaction equation for the solubility of AgCl is 〖AgCl〗_((s) )→ 〖Ag〗_((aq))^++ 〖Cl〗_((aq))^-. The assumption that was made earlier that the 〖AgCl〗_((s) ) is very insoluble thus the reaction goes to completion is inaccurate because, the 〖AgCl〗_((s) ) does have small amount of solubility, that is, the K_sp which is the solubility product for this particular reaction.

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The K_sp of AgCl in water is very small number which suggests that even though silver chloride can dissolve in water it is very small amount thus, it could be negligible, K_sp=[〖Ag〗_((aq))^+ ]∙[〖Cl〗_((aq))^- ]=1.6*10^(-10), which is very small number therefore, there would not be much loss in the amount of AgCl received as there would not be much dissolved salt in the aqueous solution.

Thus, water can be used to obtain AgCl as the solubility of silver chloride (AgCl) is approximately zero.

The purpose of precipitation in acid media is in order to avoid the co-precipitation of silver with anions of weak acid. If the reaction is performed in the neutral media than the silver ion would react with the anion of the weak acid and would result in co-precipitates that is not needed to obtain, the AgCl precipitates thus, the experiment is performed in acid media. The purpose of heating the reaction mixture was to form the precipitate. Before heating the particles are repulsive in nature so all the AgCl particles are away from one another upon heating the particles aggregates and as a result the precipitate formation occurs.

Thus, the precipitate is heated in order to coagulate so, it doesn’t go through the filter paper easily and is collected in proper amount. Photodecomposition refers to the decomposition of AgCl crystals in presence of light. The equation of decomposition of AgCl in presence of light is 〖AgCl〗_((s) ) → 〖Ag〗_((s) )+1/2 〖Cl〗_2(g) . If the photodecomposition of AgCl occurs in air, the chlorine gas is released and the results would be very low. If the photodecomposition of AgCl occurs in the presence of excess silver ion in the solution then there would be an additional reaction, 〖3Cl〗_2(g) + 〖3H_2 O〗_((l) )+5〖Ag〗_((aq))^+ → 〖5AgCl〗_((s) )+ 〖ClO〗_3(aq)^-+ 6H_((aq))^+ , the results would be much higher since the mass of the precipitate is increased by chlorine gas converting AgCl and 〖AgCl〗_((s) ) staying the same in the reaction, thus the mass is increased resulting in a much higher value. The loss of precipitate due to washing it by 100mL of water would be 〖[Ag〗^+]= 〖[Cl〗^-] and let both the concentrations be x,

K_sp=[〖Ag〗_((aq))^+ ]∙[〖Cl〗_((aq))^- ]=1.6*10^(-10),

[x]∙[x]=1.6*10^(-10),

x=1.3〖∙10〗^(-5) moles poer litre.

Precipitate lost due to 100mL of fresh water can be calculated as,

1.3∙10^(-5)=x ÷0.10L

x= 1.3∙10^(-5)*143mol ÷0.10L

x=0.001859 g=1.8*10^(-3) g . Thus, the results would be lower. The ions of weak acid interact with chloride ion which results in co-precipitates, some such ions are 〖CO〗_3^(2-),and 〖OH〗^-, thus the result would be much higher than expected.

Procedure

The code number for an unknown salt was recorded as 354, and the salt was obtained, this salt was kept until the experiment was finished. Using an analytical balance 0.1149 g of the salt was weighed by the difference and was placed in a 250mL, clean, numbered beaker. 100mL of distilled water was added in the beaker as well as, approximately, 1mL of diluted (6M) HNO_3 that was obtained from a TA, the solution was stirred until it completely dissolved. The 23mL of (0.1M) AgNO_3 was added in the beaker with gentle stirring. The solution was then placed on a hot plate and stirred until the solution was almost to boiling.

The solution was checked for completeness by adding a few drops of silver nitrate, there was no additional formation of silver chloride thus the solution was placed in the drawer to prevent the photodecomposition of the salt. The vacuum filtration apparatus was setted up and a filter paper was obtained from a TA, the filter paper was placed in a 50mL beaker and was weighed on an analytical balance with the filter paper inside. The filter paper was placed in a Buchner Funnel and both of them were placed on the filtration flask. The solution was poured gradually through the funnel and 5mL of 0.01M HNO_3 was added to the precipitate in the beaker, the washing was repeated but this time the precipitate and washings were transferred onto the filter. The washing was continued until the filtrate is essentially free of silver ion. To check whether the washing is completed the small amounts of washing were collected in a test tube and a few drops of HCl were added in a test tube but the remaining silver was not detected thus the washing was completed.

The washings from the filter flask was then emptied and placed in the vacuum filtration system and washed by the 5mL of acetone, this was repeated three times. The acetone was placed in a special waste bucket and the filter paper was removed containing the precipitate from the funnel and placed in the 50mL beaker as that beaker was labeled before. The beaker was then placed in the oven at a temperature of 110°C for about 30 minutes. The beaker was then cooled for about 5 minutes and the final weight was obtained by placing the beaker on an analytical balance. The results were recorded and the station and the equipments were cleaned and dried.

Results and Observations

Table 1: Experimental Data and Calculations

Calculations

Amount of AGNO3 Required (Calculated Amount +5 ML)

(mass of sample)*(0.55)÷(35.5)÷(0.1)*(1000)+(5)

(0.1149) * (0.55) ÷ (35.5) ÷ (0.1) * (1000) +(5)

= 22.80140845 mL = 23 mL PARTNER

= 25 mL

Calculation of % Chloride in the Sample

〖moles Cl〗^- in sample=moles AgCl

=mass AgCl÷〖MW〗_AgCl =(28.2743-29.9751)÷(107.85+35.45) = -0.011868806 mol

mass 〖Cl〗^-=# moles 〖Cl〗^-*〖MW〗_Cl

=(-0.011868806)*(35.45)

= -0.420749172 g

% Cl in sample=(mass 〖Cl〗^-÷mass sample)*100

=((-0.420749172)÷(0.1149))*100

= -366.1872689 %

= -366.2 %

Calcuation of % Uncertainity

((0.0002) ÷(0.1149g))*(100)+((0.0001)÷(-1.7008))*(100)

=0.168184817

=0.1682

AVERAGE =(0.168184817+0.197368421) ÷2

=0.182776619

=0.1828 PARTNER

=0.197368421

=0.1974

Calculation of Accuracy (Relative Error in %)

(Using Average Values, Mine and Partner)

(Accepted-Experimental ÷Accepted)*(100%)

=((58.81)-(-157.0533071) ÷(58.81))*(100)

=367.052044 %

=367.1 %

Calculation of Precision (Relative Spread in Ppt)

(Difference between extreme results÷Avg Value)*(1000 ppt)

(((52.0806547 %)-(-366.1872689 %))÷(-157.0533071 %))*(1000ppt)

= -2663.222643 ppt= -2663 ppt

Discussion

My results were much lower than expected (% chloride = -366.2%), even the mass of the beaker is higher than compared to mass of the beaker with precipitate (29.9751 > 28.2743), which is not possible. There could be numerous reasons that could affect my results, some are listed as follows. There could be a strong possibility that I didn’t correctly measured the weight of the sample, like I didn’t correctly zeroed the analytical balance before taking the measurement, and the same could apply while I was taking the final weight. There is also a strong possibility that I could have lost some of the precipitate in the washings since there is a great amount of chance losing the precipitate.

I could also have exposed the precipitate to light for a long amount of time in air, as the precipitate decomposes in light and I could have lost some in decomposition. There is also a strong possibility that the tests for completeness of precipitation and washing could have failed, since I could have easily missed to see the formation of the precipitate in the solution, and also I could have failed in detecting the silver in the solution. There could be numerous possibility in errors that could have affected my results to be lower than expected. The result is not possible to have without making errors in the experiment, the true % chloride = 55.20% as mentioned in the above section there could have numerous errors that could have negatively impacted my results to be much lower than it should be.

While my partner’s results were also lower but in the accepted range, (% chloride = 52.08 %) and his mass of the beaker is lower than compared to mass of the beaker with precipitate (26.4947 < 26.7587), as expected. But still, the calculated % chloride is not equal to the true % chloride, so there could be numerous errors in the experiment. My partner could have exposed the precipitate in air for a relatively long period of time, since the precipitate decomposes in air, and in light, it could have lowered his result. The washing process could have contributed in reducing the amount of precipitate, thus lowered his result. He could have lost some of the precipitate in the completion tests, thus lowering the amount of precipitate. He may have added the acid too fast thus not allowing enough time for chloride ions to form.

Conclusion

The sample number was 354. The average %Cl was -157.1 %. The real %Cl was 55.20%. The average uncertainty of the experiment was 0.1828. The precision of the experiment was -2663 ppt. The accuracy of the experiment was 367.1 %.