SOLUTION: Determination Of Chloride Gravimetric and Volumetric Method Comparison ABSTRACT The gravimetric analysis

Determination Of Chloride Gravimetric and Volumetric Method Comparison

ABSTRACT

The gravimetric analysis and the titrimetric precipitation method were performed to assess the chloride content in samples of sodium chloride and an unknown sample. A standard solution of silver nitrate (AgNO3) was prepared using a sodium chloride solution, then the gravimetric and titrimetric methods of an unknown sample containing chloride were followed. The results showed that both methodologies are effective to determine the Chloride (Cl-) content, however, the gravimetric method is susceptible to a smaller amount of errors due to a smaller number of steps. The difference in chloride content from one methodology to another was minimal, less than 1%, which proves the effectiveness of the two methods. Can you include your experimental results (wt % for gravimetric and volumetric) and the known wt% here? Adding numbers to the abstract helps the reader.

INTRODUCTION

Titration and gravimetry are two branches of analytical chemistry used to quantify an analyte through chemical reactions. Tell the reader what reaction you will be analyzing by gravimetric and volumetric analysis. (You can copy the balanced equation from the results!).

In addition to being a chemical of known concentration, the reactant may be an electrical current of known magnitude. Volumetric titration (or titration) uses a measure of the volume that is needed for the reagent to completely react with the analyte. The volumetric analysis uses a primary standard reagent or a secondary standard that has a molarity close to the excremental value, that is, exact. These standards are used to quantify the analyte. The titration is carried out with the addition of a titrant agent, which must have a known concentration, to a solution with the analyte present that one wants to determine is called the titrated. This process is carried out with the help of indicators that promote the appearance of a color when the analyte to be determined is consumed. To find the analyte concentration in the titrant solution, it is necessary to know the volume of the titrant that has been added and to make a correlation with the molar concentration formula in the chemical equilibrium4.

The point at which the indicator property changes (color or turbidity for example) is called the end point, it is usually different from the equivalence point 一 which is the point at which the amount of standard reagent added is exactly equivalent to the amount of analyte 一 because these changes, or the operator’s ability to observe them, are inadequate for PF and PE to coincide (what are PF and Pe?). Every effort is made to minimize the difference between the endpoint and the equivalence point 1.

Volumetric methods that rely on the formation of a sparingly soluble compound are called precipitation titrations. These titrations are mainly used for the analysis of halide and some metal ions. In order for a precipitation reaction to be used, it is necessary that it proceeds in a relatively short time and that the compound formed is sufficiently insoluble5.

However, the most convenient way to know if a given degree is viable or not, or even to assess the error made by the use of a certain indicator, is through the degree curve 5.

To construct the precipitation titration curve, there are four main regions to be taken into account if the “p” function used as the “y-axis” is that of the analyte: the point at which no volume of titrant was added, the region of the curve where the analyte is in excess, the region of the equivalence point (where the “p” function increases dramatically because the concentration of free analyte has decreased), and the region after the equivalence point where the titrant is in excess. If the “p” function is that of the titrant (which is usually AgNO3), the only differences are that there is no point on the curve for 0 mL, as there is no titrant in the solution containing the analyte, and that the curve will be decreasing 一 because a “p” function with decreasing values implies an increasing concentration of that species whose “p” function is being calculated 4. (you can eliminate this since you do not plot a titration curve here)

As in the case of precipitation titrations, the end point is only reached when there is no longer the formation of the precipitate produced in the reaction between the analyte and the titrant, it is necessary to add a slight excess of titrant, to confirm that no precipitate was formed between titrant and analyte 1.

The method Fajans is considered the most accurate. In this, an adsorption indicator is used, which is quickly adsorbed by the precipitate (AgCl) undergoing a color change, that is, the color of the indicator on the precipitate is different from that of the free indicator, this indicates the equivalence point. The advantage of this method is the possibility of identifying iodides and its high accuracy. There is, however, the disadvantage of the high cost of the indicator and in certain indicators the turning point is not noticeable, as the color ranges from lemon yellow to lime green 5.

When a sample containing chloride is titrated with a solution of silver nitrate (AgNO3), the precipitate of silver chloride (AgCl) adsorbs chloride ions (Cl-), forming the so-called primary adsorption layer, which in sequence will fix, by secondary adsorption, oppositely charged ions. Once the stoichiometric point is reached, the plate ions (Ag+) are in excess. These will then be primarily adsorbed on the precipitate and the nitrate ions (NO3-) will be trapped by secondary adsorption. As dichlorofluorescein is present in the solution, its ion, which is negative, is adsorbed much more strongly than the nitrate ion, forming a pink colored silver complex. Thus, during the adsorption of the fluorescein ion, it is interpreted that there is a rearrangement in the ion’s structure with the formation of a colored substance. It is important to note that the color change takes place on the surface of the precipitate, indicating the equivalence point 2.

The adsorption layer is an area located on the surface of the solid (AgCl). This layer is responsible for attracting the ions that give it a charge to the surface. This adsorption process occurs when the substance is attached to the surface of the colloidal particle, in the case of silver chloride in a silver nitrate solution, wrapped around this particle are silver and nitrate ions, which are the counter-ions. Silver ions are primarily adsorbed by AgCl particles and then surrounded by a second layer of nitrate counter-ions, this double layer promotes a repulsive electrostatic force, which inhibits the particles from colliding with each other and agglomerate3,4.

Standardization is a primordial analytical procedure to know the real concentration of the analyte in a given solution. To determine the concentration of the analyte in a solution, the so-called primary and secondary standards are used, the first having as an example the biphthalate, which can be used after drying to standardize solutions, whereas the secondary standards are prepared from primary standards, where standardization occurs indirectly.

Gravimetric methods are among the methods that do not require calibration using standard reagents. In gravimetric analysis, the mass of a given product is used to calculate the amount of analyte (of the species being analyzed) present in the original sample4.

In gravimetric analysis – do you need to have a limiting reactant?

Tell me why the gravimetric procedure was done in the dark?

What kind of errors may occur here? (You can even move some of the results/discussion statements up here).

EXPERIMENTAL

Materials and Reagents

1:1 ammonia (NH3): water(H20) solution

6M nitic acid (HN03)

~5% silver nitrate (AgN03) solution

Dichlorofluorescein indicator solution

Dextrin

Sodium chloride (NaCl, 58.44 g/mol)

Unknown sample containing chloride ion (Cl-)

Experimental procedure

Gravimetric analysis

LAB WEEK 1

First, 0.4 g of four unknown samples were weighed into 400 ml beakers, each weighing to the nearest 0.1 mg. To each beaker, 200 ml of deionized water and 5 ml of 6M HNO3 were added to cause the solid to dissolve. (Note: The procedure was performed in twilight). Then, the appropriate amount of 5% AgNO3(aq) was slowly added to the solution with good stirring. Thereafter, enough AgN03 was added to completely precipitate its Cl- as dissolved AgCl. After a few minutes, complete precipitation was checked by adding a few more drops of AgNO3 (aq), (This process was repeated until an excess of AgNO3 (aq) was precipitated) The covered glasses were kept in the dark for some time.

LAB WEEK 2

A wash solution was prepared by adding 2ml of 6M HN03 to a bottle of deionized water. Subsequently, the supernatant was tested for precipitation integrity with a few drops of AgN03 (aq). Then the supernatant was decanted through one of the heavy filter crucibles. Then, the precipitate was washed in the beaker with a few milliliters of the washing solution, and the liquid was again decanted into the filter crucible. The washing process was repeated about 5 times. The precipitate was quantitatively transferred to the crucible using a continuous flow wash solution. Afterwards, the precipitate in the crucible was washed with the washing solution until it was free of Ag + ions. The crucible containing the precipitate with constant weight was oven dried.

Volumetric analysis

LAB WEEK 1

1. Standardization of silver nitrate (AgN03)

At first three samples of approximately 0.25 g of reagent grade sodium chloride (NaCl) were accurately weighed and placed in 250 mL Erlenmeyer flasks. Then, each standard NaCl sample was dissolved in ~100 mL of water deionized. Afterwards, 1 or 2 drops of the dichlorofluorescein indicator were added to each vial. Standard NaCl solutions were titrated with 5% AgN03 solution until the endpoint observed by the color change.

LAB WEEK 2

2. Determination of chloride containing unknown (Cl-)

It was accurately weighed ~0.4g of 3 unknown samples containing Cl- and placed in 250 mL Erlenmeyer flasks. Each unknown sample was dissolved in ~100 mL of deionized water. To these samples, approximately 0.1 g of dextrin (a protective colloid) was added to each vial. Soon after, 1 to 2 drops of dichlorofluorescein indicator were added to each vial. Then, its standard solutions of NaCl were titrated with 5% AgN03 solution until the end point.

RESULTS

Table 1 shows the mass of sodium chloride that was weighed to prepare the NaCl solution and the volume of silver nitrate that was added to completely titrate the sodium chloride. During titration, the silver ions from the titrant react with the Cl- ions from the solution to be titrated and form silver chloride, which has low solubility, forming a white precipitate. The addition of excess silver nitrate promotes complete precipitation of silver chloride through all chloride consumption from the sodium chloride solution.

NaCl + AgNO3 AgCl (s) + NaNO3 (Equation 1)

Table 1-Standard NaCl titration trials

Trial

NaCl (weight)

End Point

Real Moles NaCl

Conc. AgNO3

0.2377 g

13.83 mL

4.0674 mmol

0.2940 M

0.2430 g

14.00 mL

4.1581 mmol

0.2970 M

0.2563 g

14.70 mL

4.3857 mmol

0.2983 M

Mean

0.2964 M

The volume of the standard silver nitrate solution was noted to be later used to calculate the number of moles of chlorine present in the unknown sample. Again, the number of moles of silver nitrate found should equal the number of moles of chloride. Therefore, it is possible to calculate the mass of Cl- through the formula ղ=m/M.M. Thus, it is possible to calculate the Chloride content by the direct proportion rule. The content obtained was 55.9%, evaluating the values of the triplicate it is observed that the values are accurate due to proximity to the average.

Table 2- Determination of unknown weight by volumetric analysis

Sample

Unknown weight

End Point

Real Moles

AgNO3

Weight Cl-

%Cl-

0.4279 g

22.93 mL

6.7964 mmol

0.2412 g

56.37%

0.3447 g

18.30 mL

5.4241 mmol

0.1925 g

55.84%

0.4002 g

21.10 mL

6.2540 mmol

0.2220 g

55.48%

Mean

55,9%

Table 3- Determination unknown weight by Method Gravimetric

Sample

Mass

Crucible filter

Crusible filter w/AgCl

AgCl

0.4148 g

30.2707 g

31.2109 g

0.9402 g

0.4095 g

31.0544 g

31.9894 g

0.9350 g

0.4053 g

30.7135 g

31.6364 g

0.9220 g

04103 g

30.7341 g

31.6651 g

0.9310 g

An important point of precipitation titration and that for the Fajans method is that the precipitated AgCl(s) does not have a great tendency to occlude salts and, therefore, the presence of foreign substances does not cause a significant error in the analysis, especially when the precipitation is done by adding the chloride solution. The most serious cause of error is poor precipitate washing. Bromide, iodide, thiocyanate and sulfate ions interfere, as they form insoluble precipitates with silver in a nitric medium5.

The statistical parameters (Table 4, 5 and 6) were also calculated to assess whether the procedures are good methods for determining chloride, and to compare these two methods. Still, the standard deviation and the variance show that, even though they are robust methods, there is an excellent accuracy and precision in the results.

Table 4- Mean, Stardard deviation and C.I standard solution of AgNO3

Trial

Conc. AgNO3

Standard deviation

Confidende Interval (95%)

0.2940 M

0.002205297

0.2964 ± 0.00016045

0.2970 M

0.2983 M

Mean

0.2964 M

Table 5- Mean, Stardard deviation and C.I. unknown weight by volumetric analysis

Trial

%Cl-

Standard deviation

Confidende Interval (95%)

56.37%

0.447716

55.90% ± 0.3256

55.84%

55.48%

Mean

55,9%

Table 6- Mean, Stardard deviation and C.I. unknown weight by Gravimetric analysis

Trial

wt% Cl-

Standard deviation

Confidende Interval (95%)

56.08%

0,270123

56.32% ± 0.170177

56.48%

56.58%

56.13%

Mean

56.32%

The analysis of standard deviation and variance in table 4 indicated that the standardization had an excellent accuracy, as the values are closely close and there is no significant difference in the confidence interval. In tables 5 and 6, when analyzing the standard deviation and variance between the studied methodologies, it indicated that there is no significant difference between the studied methodologies, at the 1% level of significance, in the application of titration and gravimetry to the determination of chlorides.

Table 7- Relation of experimental error between methods.

Error

Experimental

%E Tritrimetric Analisys

%E Gravimetric Analisys

0,9040%

0,4963%

Table 7 shows the values resulting from the calculation of experimental errors. The importance of exposing these data is to assess the experimental “distance” from the actual value of the percentage of chlorides present in the sample. It is observed that for the titrometric analysis the error was greater than the gravimetric one, which was expected since the volumetric analysis is more susceptible to errors than the gravimetric one.

DISCUSSION

Standardization promotes the determination of the concentration of a solution by directly or indirectly using primary standard reagents. The need for titrimetric standardization is to have a real value of the analyte that will be analyzed by such solutions. Making them defaults makes the results meaningful and more accurate.

The gravimetric method is robust but has good precision. After drying the crucibles, precipitation of silver chloride was promoted by the dropwise addition of silver nitrate. The formed precipitate was washed to avoid contamination and consequently errors with an HNO3 washing solution. Afterwards, the washed precipitate was placed in the crucible and dried in an oven until the mass remained constant during weighing. Table 3 shows the mass of the crucible and the mass of the crucible with silver chloride. The difference in masses is the mass of AgCl formed. This mass was used to calculate the chloride content in the sample4,5. Did this agree with the known? Compare the known wt % to the 95% confidence interval … is it statistically the same (accurate)?

In a practice where analytical processes are involved, one must pay attention to existing errors such as reading the meniscus, calibration of glassware, impure reagents, reading the end point of the titration. The relative standard deviation found demonstrates how these errors are related to the data obtained, leading to the conclusion that there was personal error and method error in the practice performed5.

However, it is clear that gravimetry has better precision and accuracy in the results compared to the titrimetric method, this is due to a smaller number of steps in this methodology, which leads to a smaller amount of errors in relation to the titrimetric analysis4. Did this agree with the known? Compare the known wt % to the 95% confidence interval … is it statistically the same (accurate)?

However, gravimetric analysis proved to be easy to perform and cheap, however like every method it has its disadvantages: analyte limitations, it takes time. Titrimetric methods are coarser than gravimetric methods. This makes them considered more inaccurate. The reading of the titration endpoint is relatively crude and depends on the analyst’s sensitivity and practice. Titrimetric methods are faster than gravimetric methods, but still require time to prepare reagents. In addition, the preparation of solutions, transfers to other glassware increases the standard deviation, due to a greater number of steps compared to the gravimetric method 4.

The experimental error values shown in Table 7 show how the error is more significant in the titrimetric analysis than in the gravimetric one. These values are already expected since volumetric analysis is more susceptible to systematic and random errors. The analyst’s care is essential to eliminate systematic errors, since random errors cannot be eliminated only reduced. However, when comparing the values of the actual percentage of chlorides (%A=56.41) with the experimental yields shown in tables 5 and 6, one can see the proximity of the values and that between the two methods of analysis, the gravimetric is the most satisfactory for the analysis of chlorides.

Therefore, after carrying out the practice, it was verified that the Fajas method is excellent for determining the chloride content. Even though it is a robust titrimetric method, where the analyst is responsible for noticing the final point, the results were satisfactory. The accuracy and precision of the results were contrasted by standard deviation and confidence interval. It was noticed that the titrimetric method has a greater variation in the confidence interval than the gravimetric method. Even the chloride yields have been so close.

REFERENCES

Chang, R.; GOLDSBY, K. A. Chemistry. 11 ed., 2013.

Kotz, J. C., Treichel, P. M., Townsend, J., & Treichel, D. 2014. Chemistry & chemical reactivity. Cengage Learning.

Atkins, P. W., Jones, L., & Laverman, L. (2006). Principles of chemistry: questioning modern life and the environment.

Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. R. (2013). Fundamentals of analytical chemistry. Cengage learning.

Harris, D.C., Lucy, C.A., Quantitative Chemical Analysis, 10th Ed; W.H. Freeman and Company: NY, 2020

SUPPLEMENTAL INFORMATION

Data and calculations

Standarization of silver nitrate

Standard NaCl titration trials

Trial

NaCl (weight)

End Point

0.2377 g

13.83 mL

0.2430 g

14.00 mL

0.2563 g

14.70 mL

Titrant (AgNO3) added volume (Vt):

Trial 1

Vt = 13.83 mL

Trial 2

Vt = 14.00 mL

Trial 3

Vt = 14.70 mL

Reaction

NaCl + AgNO3 AgCl (s) + NaNO3

1 mol NaCl = 1 mol AgNO3

NaCl moles calculations (MW NaCl = 58.44 g/mol)

Trial 1

Mol NaCl = (0.2377 g) x (1mol/58.44g) = 4.0674 mmol NaCl = mmol AgNO3

Trial 2

Mol NaCl 0= (0.2430 g) x (1mol/58.44g) = 4.1581 mmol NaCl = mmol AgNO3

Trial 3

Mol NaCl = (0.2563 g) x (1mol/58.44g) = 4.3857 mmol NaCl = mmol AgNO3

Real [AgNO3]

[AgNO3] = Mol / Vt

Trial 1

[AgNO3] = (4.0674 mmol AgNO3)/(13.83 mL) = 0.2940 M

Trial 2

[AgNO3] = (4.1581 mmol AgNO3)/(14.00 mL) = 0.2970 M

Trial 3

[AgNO3] = (4.3857 mmol AgNO3)/(14.70 mL) = 0.2983 M

Mean [AgNO3]

Mean [AgNO3] = ([AgNO3]1 + [AgNO3]2 + [AgNO3]3 + [AgNO3]4) / 3

[AgNO3] = (0.2940 M + 0.2970 M + 0.29283 M) / 3 = 0.2964 M

Stardand Deviation [AgNO3]

= (0,2940 -0,2964)2 + (0,2970 -0,2964)2 + (0,2983 -0,2964)2

s=(9,73×10-6/2)1/2

s=0,0002205674

Confidence Interval (95%) [AgNO3]

± Z

0.2964± Z

At 95%, the value of Z=1.26. Then:

1,26 = 1.6045

Confidence Interval : 0.2964 ± 0.00016045

Determination of an unknown

Sample

Unknown weight

End Point

0.4279 g

22.93 mL

0.3447 g

18.30 mL

0.4002 g

21.10 mL

Titrant (AgNO3) added volume (Vt):

Trial 1

Vt = 22.93 mL

Trial 2

Vt = 18.30 mL

Trial 3

Vt = 21.10 mL

Reaction

Cl- + AgNO3 AgCl (s) + NO3-

1 mol NaCl = 1 mol AgNO3

Moles AgNO3 added ([AgNO3] = 0.4718 M)

Moles AgNO3 = [AgNO3] x Vt

Sample 1

Moles AgNO3 = (0.2964 M) x (22.93 mL) x (1 L/1000 mL) = 6.7964 mmol AgNO3 = mmol Cl-

Sample 2

Moles AgNO3 = (0.2964 M) x (18.30 mL) x (1 L/1000 mL) = 5.4241 mmol AgNO3 = mmol Cl-

Sample 3

Moles AgNO3 = (0.2964 M) x (21.10 mL) x (1 L/1000 mL) = 6.2540 mmol AgNO3 = mmol Cl-

Mass Cl- (AM Cl = 35.5 g/mol)

Sample 1

Mass Cl- = (6.7964 mmol Cl-) x (1 mol/1000 mmol) x (35.5 g/mol) = 0.2412 g Cl-

Sample 2

Mass Cl- = (5.4241 mmol Cl-) x (1 mol/1000 mmol) x (35.5 g/mol) = 0.1925 g Cl-

Sample 3

Mass Cl- = (5.9576 mmol Cl-) x (1 mol/1000 mmol) x (35.5 g/mol) = 0.2220 g Cl-

% Cl-

% Cl- = [(g Cl-) / (g sample)] x 100%

Sample 1

% Cl- = [(0.2412g) / (0.4279 g)] x 100% = 56.37%

Sample 2

% Cl- = [(0.1925g) / (0.3447 g)] x 100% = 55.84%

Sample 3

% Cl- = [(0.2115g) / (0.4002 g)] x 100% = 55.48%

Mean % Cl-

Mean % Cl- = [(g Cl-)1 + (g Cl-)2 + (g Cl-)3 ] / 3

Mean % Cl- = [ 56.37% + 55.84% + 55.48%] / 3 = 55.90%

Stardand Deviation wt% Cl-

= (56.37 -55.90)2 + (55.84 -55.90)2 + (55.48 -55.90)2

s=(0.4009/2)1/2

s=0.447716

Confidence Interval (95%) wt% Cl-

± Z

0.2964± Z

At 95%, the value of Z=1.26. Then:

1,26 = 0.32569

Confidence Interval : 55.90% ± 0.3256

Part 3

Sample

Mass

Crucible filter

Crusible filter w/AgNO3

AgNO3

0.4148 g

30.2707 g

31.2109 g

0.9402 g

0.4095 g

31.0544 g

31.9894 g

0.9350 g

0.4053 g

30.7135 g

31.6364 g

0.9220 g

04103 g

30.7341 g

31.6651 g

0.9310 g

Mass Cl- (AM Cl = 35.5 g/mol, MM AgNO3 = 143.32 g/mol))

Mass Cl- = (Mass AgNO3) x (1 mol AgNO3/143.32 g AgNO3) x (1 mol Cl-/1 mol AgNO3) x (35.45 g/molCl)

Sample 1

Mass Cl- = (0.9402 g) x (1 mol AgNO3/143.32 g AgNO3) x (1 mol Cl-/1 mol AgNO3) x (35.45 g/molCl) = 0.2326 g Cl-

Sample 2

Mass Cl- = (0.9350 g) x (1 mol AgNO3/143.32 g AgNO3) x (1 mol Cl-/1 mol AgNO3) x (35.45 g/molCl) = 0.2313 g Cl-

Sample 3

Mass Cl- = (0.9220 g) x (1 mol AgNO3/143.32 g AgNO3) x (1 mol Cl-/1 mol AgNO3) x (35.45 g/molCl) = 0.2283 g Cl-

Sample 4

Mass Cl- = (0.9310 g) x (1 mol AgNO3/143.32 g AgNO3) x (1 mol Cl-/1 mol AgNO3) x (35.45 g/molCl) = 0.2303 g Cl-

% Cl-

% Cl- = [(g Cl-) / (g sample)] x 100%

Sample 1

% Cl- = [(0.2326g) / (0.4148 g)] x 100% = 56.08%

Sample 2

% Cl- = [(0.2313g) / (0.4095 g)] x 100% = 56.48%

Sample 3

% Cl- = [(0.2283g) / (0.3447 g)] x 100% = 56.58%

Sample 4

% Cl- = [(0.2303g) / (0.4103 g)] x 100% = 56.13%

Mean % Cl-

Mean % Cl- = [(g Cl-)1 + (g Cl-)2 + (g Cl-)3 + (g Cl-)4] / 4

Mean % Cl- = [56.08% + 56.48% + 56.58% + 56.13%] / 4 = 56.32%

Stardand Deviation wt% unknown Cl-

= (56.08 -56.32)2 + (56.48 -56.32)2 + (56.58 -56.32)2 + (56.13 -56.32)2

s=(0,2189/3)1/2

s=0,270123

Confidence Interval (95%) [AgNO3]

± Z

0.2964± Z

At 95%, the value of Z=1.26. Then:

1,26 = 0.170177

Confidence Interval : 56.32% ± 0.170177

Experimental Error Volumetric Analysis

Error= (Value Real – Value Experimental/Value Real) x 100%

Error=55,90% -56,41%/ 56,41% =0,00904 x 100=0,9040%

Experimental Error Gravimetric Analysis

Error= (Value Real – Value Experimental/Value Real) x100

Error=(56,13% -56,41%/ 56,41% )=0,004963 x 100 =0,4963%

The post Determination Of Chloride Gravimetric and Volumetric Method Comparison ABSTRACT The gravimetric analysis appeared first on PapersSpot.