Standardization of hydrochloric acid PDF

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This practical gives information about how to standardize a solution of HCL using borax and sodium carbonate , also, testing the sutability of the compunds as primary standards...

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KYAMBOGO

UNIVERSITY

FACULTY OF SCIENCE Ordinary diploma in Science Technology Chemistry CHEMI ST RYPRACT I CALREPORT1/ YEARI SEMEST ERI I

Saturday, February 10, 2018

MAKASI GEORGE 17/U/16852/STC/PD Sign ……………………………..

STANDARDIZATION OF HYDROCHLORIC ACID Aim The purpose of this experiment is to standardize a solution of hydrochloric acid using two primary standards; sodium carbonate and borax. And also to introduce one of the methods used to prepare standard solutions. Introduction This experiment involves the standardization of a solution of hydrochloric acid using two primary

standards,

sodium

carbonate

and

borax

(sodium

tetraborate

decahydrate,

Na2B4O7.10H2O). Standardization of an unknown solution involves reacting the solution with another solution whose concentration is already known very accurately. For this experiment, a known quantity of hydrochloric acid called the aliquot is measured and neutralized with say; sodium carbonate whose concentration is known very accurately. Adding an indicator such as methyl orange indicator provides a visual indication (a color change) when the equivalence point is reached i.e when just enough of the standard solution has been added to the unknown solution to neutralize it exactly. By determining how much of the salt solution is required to neutralize the hydrochloric acid, we can calculate a very accurate value for the concentration of the hydrochloric acid. The results obtained will be used to determine the suitability of the two salts (sodium carbonate and sodium tetraborate decahydrate) as primary standards. Theoretical Background An acid –base titration depends on the reaction between an acid and a base to produce a salt. The reaction is normally a quantitative one and is carried out in aqueous solution. An acid can be defined as a substance which, when in aqueous solution, has a tendency to donate a hydrogen ion (proton) to a water molecule. HA(aq) + H2O(l) → A+(aq) + H3O+(aq) Where HA is the acid Page 2 of 12

HA and A- from a conjugate pair in which A - is the base. Similarly H2O and H3O+ are also a conjugate pair in which H3O+ is the acid. A base in this case, is defined as a substance which, when in aqueous solution, has a tendency to accept a proton from water. B(aq) + H2O(l) → BH+(aq) + OH-(aq) Therefore water acts both as an acid and as a base. Salts derived from a combination of strong acids-weak acid also function as bases due to hydrolysis, which results in the release of OH ions. Examples are sodium carbonate and borax. The reaction between an acid and a base is a neutralization reaction. Which takes place between H3O+ and OH- ions H3O+ (aq)

+ OH-(aq) → 2H2O(l)

The endpoint which is the point at which all the acid or base has been completely neutralized by an equivalent amount of base or acid can be marked by a suitable visual indicator such as methyl orange or phenolphthalein.The choice of the indicator is dictated by the type of acid and base used in the titration. A primary standard solution is defined as a solution of known concentration (C) which does not change with time and the concentration is defined by the equation C = n/v Where n = amount (in moles) of solute contained a volume V (expressed in dm3) of solution. In this experiment We draw from the above principles, the experiment uses two primary standards i.e. Sodium carbonate and sodium tetraborate decahydrate in an acid-base titration with methyl orange as indicator. For instance, Borax (Na2B4O7 .10H2O) is used as a primary standard since it does not decompose under normal storage, it is readily obtained in a very pure form (99.999% purity), it reacts with a known stoichiometry and can be weighed and used directly. Page 3 of 12

In water, the salt dissociates: Na2B4O7.10H2O → 2Na+ + B4O72- + 10H2O

and the anion is hydrolyzed: B4O72- + 7H2O → 4H3BO3 + 2OH-

The liberated hydroxide ions above, can be titrated with a strong acid and at the point of their neutralization, the solution in the flask will contain the very weak acid H 3BO3, which will dissociate according to:

H3BO3 + H2O → H2BO3- + H3O+. Ka = 5.8 x 10-10.

Thus at the neutralization point (or equivalence point: the point when all the liberated OH- have been converted to H2O) the solution will be slightly acidic (pH ~ 4.8; the actual pH depending on volumes and amounts used). The indicator used to identify the strong base-strong acid equivalence point must therefore change color over the pH range 4.8 ± 1 and as such methylorange indicator is used. Methyl indicator is an acid –base indicator and its structure is as shown below.

Now using both sodium carbonate and borax as primary standards; Reaction equation between hydrochloric acid and sodium carbonate Na2B4O7 ⋅ 10 H2O (aq) + 2 HCl(aq) → 2 NaCl(aq) + 4 H3BO3(aq) + 5H2O(l) Page 4 of 12

Reaction equation between hydrochloric acid and borax Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + CO2(g) Since water is sometimes contaminated with acidic or basic impurities, the indicator blank on the water is obtained. It is then subtracted on the obtained titres in order to obtain corrected values of the volumes obtained. Obtaining the actual volume of (sodium carbonate or borax) required to completely neutralize the acid, averaging it and employing the values in a set of calculations and analysis, can enable us obtain with precision the concentration of Hydrochloric acid and standardize it. Requirements Conical flasks Measuring cylinders 10ml and 100ml Burette 10cm3 pipette Retort stand Distilled water 0.1 M hydrochloric acid 250cm3 volumetric flask Methyl orange indicator Pasteur pipette

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Experimental setup

0.1M hydrochloric acid in a burette

Retort stand/ burette stan

250ml conical flask

Sodium carbonate/ borax with methyl orange indicator

Procedure/Method About 1.3249g anhydrous sodium carbonate was weighed into a clean 250cm3 volumetric flask and 100cm3 of distilled water added, the flask was stoppered and shaken until all the solid had dissolved. Distilled water was then added to the mark. The flask was labelled clearly. 4.7673g of pure borax was weighed into another clean 250cm3 volumetric flask and 100cm3 of distilled water added, the flask was stoppered and shaken until all the solid had dissolved. Distilled water was then added to the mark. The flask was labelled clearly. Page 6 of 12

Titration with sodium carbonate 10cm3 of the stock carbonate solution was pipetted into a clean 250cm3 conical flask, and 3 drops of the methyl –orange indicator added. The acid solution was transferred into the burette and the meniscus was adjusted to a suitable starting point. A piece of white paper was then placed under the flask. While swirling, the acid solution was run into the flask until the indicator showed a distinct orange color. The procedures were then repeated and values read recorded in a suitable table. Titration with Borax 10cm3 of the sodium tetraborate decahydrate solution was pipetted into a clean 250cm 3 conical flask, and 3 drops of the methyl –orange indicator added. The acid solution was transferred into the burette and the meniscus was adjusted to a suitable starting point. A piece of white paper was then placed under the flask. While swirling, the acid solution was run into the flask until the indicator showed a distinct orange color. The procedures were then repeated and values read recorded in a suitable table. Indicator blank The indicator blank is found by measuring out accurately a volume of which equals the total volume of the solution in the flask at the end point. To this volume the same number of drops of the indicator as used in the titration was added. The water was then titrated against the same acid. The acid solution was run into the flask until the indicator showed a color change, the volume read and recorded in a suitable table. Results a. Hydrochloric acid and Sodium Carbonate Molarity of sodium carbonate solution = 0.05M Capacity of the pipette used = 10.0cm3 Number of drops of methyl-orange = 3 drops

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Run

1

2

3

4

Final burette reading /cm3

4.40

8.80

13.60

18.60

Initial burette reading /cm3

0.00

4.40

9.00

13.60

Volume used/cm3

4.40

4.40

4.60

4.40

Indicator Blank/cm3

0.20

0.20

0.20

0.20

1

2

3

4

Final burette reading /cm3

30.70

41.50

25.80

36.50

Initial burette reading /cm3

20.00

30.70

15.00

25.80

Volume used/cm3

10.70

10.80

10.80

10.70

Indicator Blank/cm3

0.20

0.20

0.20

0.20

Corrected average titre = 4.27cm3 b. Hydrochloric acid and Borax Molarity of borax solution = 0.05M Capacity of the pipette used = 10.0cm3 Number of drops of methyl-orange = 3 drops

Run

Corrected average titre = 10.57cm3 Analysis, Calculations and Evaluation of data a. Using Sodium Carbonate as primary standard; The balanced equation for the reaction is; Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + CO2(g) --------------------------------(i) The average titre can be obtained:

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Total volumes used / number of runs; (4.40 + 4.40 + 4.60) / 3 = 4.46cm 3; Indicator blank (0.2 + 0.2 + 0.2) / 3 = 0.2cm3. Thus the corrected averaged value = volumes used – indicator blank; (4.47 – 0.2) = 4.27cm3 To obtain the concentration of Hydrochloric acid, we need to first obtain the number of moles of sodium carbonate that reacted; Please note that a 0.05M sodium carbonate solution is used in the first experiment, this means every 1000cm3 of water contain 0.05 moles of the salt. As such 1cm 3 of water contains; (0.05 / 1000) x 1moles of salt; hence a volume of 10cm 3 of sodium carbonate will contain (0.05 / 1000) x 10 moles of sodium carbonate; 0.0005 moles of sodium carbonate. Equation (i) shows that 1 mole of sodium carbonate reacts with 2 mole of hydrochloric acid; as such the mole ratio is 1:2, base to acid respectively. To obtain the number of moles of acid in 4.27cm3 simply multiply number of moles of base x 2; (0.0005 x 2) = 0.001 moles of hydrochloric acid reacted. The concentration of acid is obtained as follows. If 4.27cm3 contains 0.001 moles of hydrochloric acid; then in 1000cm3 will shall have (0.001/ 4.27) x 1000 = 0.23M. The concentration of Hydrochloric acid while using sodium carbonate as primary standard is exactly 0.23M. b. Using Borax as primary standard; The balanced equation for the reaction is; Na2B4O7 ⋅ 10 H2O (aq) + 2 HCl(aq) → 2 NaCl(aq) + 4 H3BO3(aq) + 5H2O(l) -------- (ii) The average titre can be obtained: Total volumes used / number of runs; (10.70 + 10.80 + 10.80) / 3 = 10.76cm3; Indicator blank (0.2 + 0.2 + 0.2) / 3 = 0.20cm3. Thus the corrected averaged value = volumes used – indicator blank; (10.76 – 0.20) = 10.57cm3

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To obtain the concentration of Hydrochloric acid, we need to first obtain the number of moles of sodium tetraborate decahydrate that reacted; Please note that a 0.05M sodium tetraborate decahydrate solution is used in the second experiment, this means for every 1000cm3 of water contain 0.05 moles of the salt. As such 1cm3 of water contains; (0.05 / 1000) X 1moles of salt; hence a volume of 10.56cm 3 of sodium tetraborate decahydrate will contain (0.05 / 1000) X 10 moles of sodium carbonate; 0.0005moles of sodium tetraborate decahydrate. Equation (ii) shows that 1 mole of sodium tetraborate decahydrate reacts with 2 moles of hydrochloric acid; as such the mole ratio is 1:2, base to acid respectively. To obtain the number of moles of acid in 10.56cm3 simply multiply number of moles of base x 2; (0.0005 x 2) = 0.001 moles of hydrochloric acid reacted. Obtaining the concentration of acid is obtained as follows. If 10.57cm3 contains 0.001moles of hydrochloric acid; then in 1000cm3 will shall have (0.001 / 10.57) x 1000 = 0.094M. The concentration of Hydrochloric acid while using sodium tetraborate decahydrate as primary standard is exactly 0.09M Discussion of Results It was found that the concentration of the given hydrochloric acid was 0.09M while using sodium tetraborate decahydrate as primary standard and 0.23M while using sodium carbonate as primary standard. This result is acceptable and reliable especially for sodium tetraborate decahydrate because no great error was made in the experiment and since 0.1M hydrochloric acid is provided for this experiment. Sodium carbonate shows a relatively big difference in the concentration of the acid as compared to sodium tetraborate decahydrate. However these values are what is expected for this experiment, since one of the goals of the experiment was to determine the suitability of a salt over another in the standardization of hydrochloric acid i.e. one salt giving a much more precise value to the other. Obtaining a 0.09 M concentration with sodium tetraborate decahydrate and 0.23M while using sodium in this experiment demonstrates the differences in the salts as primary standards. It can be observed that sodium carbonate is a not entirely suitable for standardization of the acid because there is a

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discrepancy in the values (0.23M and 0.1M) while sodium tetraborate decahydrate gives a more precise and accurate value. One of the reasons for this could be that primary standards is are expected to be pure reagents which do not gain any weight by absorbing moisture. Hence any error during weight measurement is prevented. Borax is a hydrate and it gives consistent results. The .10H2O hydration number will remain constant and will not fluctuate to 11 or 99 within normal temperature and humidity ranges. This means that per mass the same amount of sodium ions are dispensed every time and gives the same results time after time and experiment after experiment even with different batches of powder. Also the tetraborate conjugate base is very weak, which is ideal for a titration agent as well. This demonstrates that borax is a much more suitable primary standard for the standardization of Hydrochloric acid. Because there is a discrepancy in values particularly with sodium carbonate, there must be some errors. Error Analysis Sodium carbonate is hygroscopic hence it could have absorbed water molecules from the atmosphere during weighing thereby increasing its weight. The greatest error in the experiment was that when determining the end-point of titration, it was easy to get over the end-point with a few drops. Although the color was still orange, the amount of hydrochloric acid used was different. To improve the experiment and reduce this error, it was suggested to repeat the titration for some more times in order to help adjusting the average volume of hydrochloric acid to a more accurate number or to add in hydrochloric acid drop by drop when it was near to the end-point. Thus even a few over added drops can be avoided. Markings on a burette can be easily misread. One way to misread the volume is by looking at the measurement on an angle. From above, it can seem like the volume is lower, while from below, the apparent volume looks higher. This could have generated an error in readings and thus recording of wrong values, this error can be progressively carried on to the analysis and thus yield wrong results.

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Errors in concentrations directly affect the measurement accuracy. Errors include using the wrong concentration to begin with, which can occur from chemical decomposition or evaporation of fluids. The solutions may have been prepared incorrectly or contaminants could have been introduced into the solution. Conclusion The Concentration of hydrochloric acid was found to be 0.23M and 0.09M for sodium carbonate and borax respectively, which do not differ greatly to the expected concentration. The concentration of the analyte is also found to be dependent on the purity of the standard solution used. The results obtained are acceptable and thus the objective of the experiment fulfilled. Recommendations Accurate measurements are essential when it comes to a standardization experiment, as such all equipment such as the electric balance and all volumetric glassware should be calibrated for correct measurements, to assess and reduce on random errors. Ensure the primary standards used are in their most pure states especially with regard to sodium carbonate. Sodium carbonate will rapidly absorb water from the air. If not actively weighing it, this primary standard should be stored in a desiccator. References http://www.foragetesting.org/lab_procedure/sectionB/3/part3.1a.1.htm https://www.pharmaguideline.com/2010/09/preparation-and-standardization-of-1m.html https://www.quora.com/What-is-the-standardization-of-hydrochloric-acid-with-sodiumcarbonate

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