Gravimetric vs Volumetric Analysis in Chloride Determination Lab Report

Complete the III. RESULTS and IV. DISCUSSION sections from Experiment 4. Then use this information to write a Full Formal Report for Experiment 4.Remember, you already calculated preliminary results for Experiment 4.You learned about standard deviation, 95% confidence intervals, outliers, Ftests, and t-tests in Virtual Module A – use these statistical tools as appropriate.Refer to “Writing Laboratory Reports” for guidance on a Full Formal Report. You need to include the following sections in your report:o Titleo Abstract Introduction Experimentalo Results Discussiono Referenceso Supplemental InformationThe actual Wt% will be send tomorrow, you can start by the parts that don’t need %wt.

EXPERIMENT 4: DETERMINATION OF CHLORIDE — GRAVIMETRIC AND VOLUMETRIC METHOD COMPARISON word image 1956

In this experiment, you will be given a sample of unknown chloride concentration and use BOTH gravimetric and volumetric methods to determine the amount of chloride.

For the gravimetric analysis, you will dissolve the unknown sample, add AgN03 to the solution which will re-precipitate the chloride using into an insoluble form (AgCl), and then weigh your new chloride compound. From this weight, you will be able to calculate the amount of chloride in your initial unknown, and comment on your precision and accuracy during this experiment.

word image 1957 During volumetric analysis, we will also use AgN03, but as a titrant! You will add AgN03 to a solution of unknown chloride concentration, and as precipitate (AgCl) forms in solution, the solid begins to crystalize. Before the equivalence point is reached, some excess ions in the analyte solution (in our case, Cl-) will become adsorbed onto the crystal surfaces, resulting in a net negative surface charge. However, when excess titrant ions (in our case, Ag+) ions are in solution, the positive ions will adsorb onto the crystal surfaces, resulting in a net positive charge. This change from net negative to net positive charge indicates the equivalence point. To ensure adequate surface area of precipitate crystals for adsorption, particle sizes must remain small and suspended in solution.

This experiment uses a certain kind of precipitation titration called a Fajans titration. Fajans titrations use adsorption indicators – anionic dyes that will bind to the net positive precipitate crystals at the titration end point. The adsorption of the anionic dye onto the positively charged precipitate changes the color of the dye. Dichlorofluorescein, the indicator used in this lab, changes from a pale yellow when negatively charged to pink when adsorbed onto the positive precipitate.

Chemicals/Reagents and additional materials required:

word image 1958 1 : 1 v:v ammonia (NH3):water(H20) solution (—100mL per student)

  • 6M nitic acid (HN03) (25mL per student)
  • silver nitrate (AgN03) solution (AL per student)

word image 1959 Dichlorofluorescein indicator solution (1 dropper bottle per student) word image 1960 Dextrin (A g per student)

word image 1961 Sodium chloride (NaCl, 58.44 g/mol) (—2 g per student, to be dried)

word image 1962 Unknown sample containing chloride ion (Cl-) (Ag per student, to be dried)

word image 1963 word image 1964 word image 1965 l. EXPERIMENTAL o t 473 9

  1. preparation of filter crucibles and unknown chloride sample:

I. Clean four medium porosity filter crucibles according to the steps below:

    1. IN A FUME HOOD, fill crucibles with 1:1 aqueous ammonia word image 1966 sure you place crucibles in a beaker or on a watch glass to prevent spillage ofNH3 solution***); allow it to stand a few minutes, then draw through.
    2. Rinse by filling several times with deionized water and draw through.
    3. Place identifying marks on the crucibles with a pencil and dry at 1500C to constant weight.

2. Dry Ag of unknown sample containing chloride at 110 oc for 1 hr.

word image 1967

Cool

word image 1968 sample in a desiccator.

  1. Gravimetric

Lab Week

  1. Weigh four (—0.4g) unknown samples into 400 ml beakers to the nearest

0.1mg. To each, add 200 ml deionized water and 5ml 6M HN03. Dissolve. ****Thefollowingprocedure must be performed in semidarkness* ***

  1. word image 1969 Slowly add the appropriate amount of 5% AgN03 (aq) to the solution wi good stirring
    1. word image 1970 word image 1971 word image 1972 word image 1973 word image 1974 word image 1975 word image 1976 word image 1977 word image 1978 word image 1979 word image 1980 word image 1981 word image 1982 word image 1983 word image 1984 word image 1985 word image 1986 word image 1987 word image 1988 word image 1989 word image 1990 word image 1991 word image 1992 word image 1993 You want to add enough AgN03 to completely precipitate your dissolved Cl- as AgC1.
    2. After a few minutes, check for complete precipitation by adding a few more drops of the AgN03 (aq).
    3. If more AgCl forms, add another 5 ml of the AgN03 (aq), wait a few minutes, and test again.
    4. Repeat if necessary until an excess of AgN03 (aq) is present.
    5. Store the covered beakers in the darkness overnight or longer.

Lab Week 2

  1. Prepare a wash solution by adding 2ml 6M HN03 to a wash bottle of deionized

water.

  1. Once again, test the supernatant for completeness of precipitation with a few drops of AgN03 (aq).
  2. Decant the supernatant through one of the weighed filter crucibles. Then, wash the precipitate in the beaker with a few milliliters of the wash solution, again decanting the liquid through the filter crucible. Repeat the washing process —5 times.
  3. Quantitatively transfer the precipitate to the crucible using a stream of wash solution (the rubber policeman may be used to loosen any AgCl particles which adhere to the beaker).
  4. Wash the precipitate in the crucible with the wash solution until it is free of Ag+ ions.
  5. Dry the crucible containing the precipitate at 1500C to constant weight.

word image 1994 C. Volumetric Analysis

Lab week 1

l. Standardization of silver nitrate (AgN03)

  1. Accurately weigh 3 samples (by difference) Of—O.2SUfreagent grade sodium chloride (NaCI) and place into 250 mL Erlenmeyer flasks
  2. Dissolve each standard NaCl sample in —100 mL of deionized water.
  3. Add —0.1 g of dextrin (a protective colloid) to each flask
  4. Add 1-2 drops (no more!) dichlorofluorescein indicator to each flask
  5. Titrate your NaCl standard solutions with 5% AgN03 solution to the endpoint (note the solution used— use the same one forpart 2!).

Lab Week 2 0.4

2. Determination of unknown containing chloride (Cl-)

word image 1995 word image 1996 a. Accurately weigh 3 samples (by difference) of—0.4 g of the unknown containing Cl- and place into 250 mL Erlenmeyer flasks b. Dissolve each unknown sample in —100 mL of deionized water.

  1. word image 1997 Add g of dextrin (a protective colloid) to each flask word image 1998
  2. Add 1-2 drops (no more!) dichlorofluorescein indicator to each flask

word image 1999 e. Titrate your NaCl standard solutions with 5% AgN03 solution to the endpoint

Il. PRELIMINARY RESULTS

  1. From your results in I.C.I. (Standardization ofsilver nitrate), calculate the real [AgN03J using each ofyour standard NaCl titration trials. Then, calculate a mean [AgN031.
  2. From your results in I.C.2. (Determination ofan unknown containing chloride), calculate the weight percent of Cl- in your unknown sample using each ofyour unknown sample titration trials. Then, calculate a mean wt% cr•
  3. From your results in I.B. (Gravimetric Analysis), calculate the weight percent of Cl- in your unknown sample using each ofyour unknown sample precipitation trials. Then, calculate a mean wt% CI-•

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111. RESULTS (These will build up on the Preliminary Results in part Il above)

  1. From your results in l.c. 1. (Standardization ofsilver nitrate), calculate the real [AgN03] using each ofyour standard NaCl titration trials. Then, calculate a mean, standard deviation, and 95% confidence interval for the IAgN031
  2. From your results in I.C.2. (Determination ofan unknown containing chloride), calculate the weight percent of Cl- in your unknown sample using each of your unknown sample titration trials. Then, calculate a mean, standard deviation, and 95% confidence interval for the wt% Cl-•
  3. From your results in I.B. (Gravimetric Analysis), calculate the weight percent of Cl- in your unknown sample using each of your unknown sample precipitation trials. Then, calculate a mean, standard deviation, and 95% confidence interval for the wt% Cl-•
  4. word image 2001 word image 2002 word image 2003 word image 2004 Email your instructor with the label of your chloride unknown — she will provide you with the actual wt% Cl- in your “unknown” sample.

IV. DISCUSSION

  1. word image 2005 Are your results from the volumetric analysis of Cl- accurate? Are they precise? How do you know? What could be done differently to improve results?
  2. Are your results from the gravimetric analysis of Cl- accurate? Are they pregisg? How do you know? What could be done differently to improve results?
  3. Compare and contrast the gravimetric and volumetric methods of analysis. Be

word image 2006

sure to include information about ease of analysis, experimental error, accuracy, precision, pros/cons etc. of each method. word image 2007

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