1. If the indicators identified the unknown(s) as being acidic, the ph meter needs to be calibrated for the acidic range:

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1 PURPOSE: To identify three unknown acid, base or salt solutions from the ph values of their solutions, obtained by the use of acid-base indicators and/or a ph meter. PRINCIPLES: Distinguishing between an acidic or basic solutions is often easily achieved by the use of common indicators, such as litmus paper and phenolphthalein, if the ph of the solution range is between specific ph values. The main use of these indicators is to test whether a solution is acidic or basic. However these indicators do not distinguish between strong and weak acids or bases. Furthermore, often they are not useful in distinguishing between acidic or basic solutions of salt solutions. For an accurate identification of unknown solutions based on ph values, a ph measurement with a correctly calibrated ph meter is required. The rationale is as follows: 1. If the indicators identified the unknown(s) as being acidic, the ph meter needs to be calibrated for the acidic range: First: calibration with a ph= 7.01 buffer Next: calibration with a ph = 4.01 buffer 2. If the indicators identified the unknown(s) as being basic, the ph meter needs to be calibrated for the basic range: First: calibration with a ph= 7.01 buffer Next: calibration with a ph = buffer 3. If the indicators did not identify the unknown(s) as either being acidic, or basic the type of ph calibration needs to be determined by the use of a Universal Indicator. A Universal Indicator is a ph indicator composed of a solution of several compounds that exhibits several smooth color changes over a range of ph values (from 1-14). PROCEDURE: Your unknowns will be issued to you only after you have turned in to your laboratory instructor the Report Form for Experiment 7, PART I You may work with your lab partner on the tests performed on the known solutions, but you are required to do the testing and the identification of the three unknown solutions individually. I. CHECK OUT YOUR THREE UNKNOWN SOLUTIONS Record the numbers of your unknowns on the check-out sheet and in your laboratory notebook Your unknowns are randomly assigned from the following choices: 1. Hydrochloric acid HCl(aq) 2. Acetic acid HC2H3O2(aq) 3. Ammonium chloride NH4Cl(aq) 4. Sodium acetate NaC2H3O2(aq) 5. Aqueous ammonia NH3(aq) 6. Sodium hydroxide NaOH(aq) 2015 www/proffenyes.com 1

2 II. INDICATOR TESTING 1. Prepare your Chemplate for Indicator Testing of nine solutions (six known solutions and three unknown solutions). The known solutions are available in dispenser bottles. The three unknown solutions are provided in vials. To dispense the three unknown solutions from the vials onto the depressions of the Chemplate you may use disposable plastic droppers (if available), or a hollow glass tube. Be aware that contamination of the solutions in the depressions of the Chemplate or the contents of the vials will compromise your observations and the correct identification of your unknowns. 2. Perform indicator testing. Blue and Red Litmus Test. Lay nine pieces of both red and blue pieces of litmus paper, on a piece of clean dry paper towel. Using disposable plastic droppers (if available), or a hollow glass tube, or a glass rod, transfer one or several drops of each solution on both kinds of litmus paper. You may not use the same disposable plastic dropper when switching from one solution to another. If you use a hollow glass tube or a glass rod for transfer, make sure you wash them very well before switching from one solution to another. Phenolphthalein Test. Use the same depressions of the spot plate to perform the phenolphthalein test. 3. Enter your observations in DATA TABLE 1 For some tested solutions (knowns and unknowns) the indicator tests will not determine if the solution is acidic or basic. This DOES NOT IMPLY that the solution is neutral (e.g. ph = 7), but rather that the ph of these solutions lies in a ph range in which the indicators do not respond with a color change. If this is the case, enter? for Acid/Base character of solution in DATA TABLE 1 for both known and unknown solutions. Although you have previously calculated and determined experimentally the ph values of all six known solutions and in fact you do know their acidic/basic character, it will help you in the identification of your unknowns if you base all your answers in Data Table 1 strictly on the indicator tests. 4. Wrapping up of the experiment. Discard the solutions from the Chemplate. Wash the Chemplate and put it away in your locker. Make sure you keep your three unknown solutions in the vials. You will need these solutions for I of the experiment www/proffenyes.com 2

3 III. ph MEASUREMENTS 1. Prepare the buffers and the solutions for ph testing Buffers You will use the three buffer solutions (ph = 4.01, ph = 7.01 and ph = 10.01) stored in your locker in plastic test tubes.. Solutions Prepare shell vials to test the ph of solutions (known and unknown) whose acidic character was not determined by the use of indicators. (Wash, rinse with D.I. water and drain off the water from the vials) Deliver 10 ml of the known solution(s) whose acidic/basic character was not determined by the use of indicators. Label this (these) vial(s). 2. Calibrate your ph meter Recall that to calibrate the ph meter, you will need to use the buffer solution(s) whose ph value(s) is (are) in relatively close range with the assumed ph (acidic or basic) of your solution. In this experiment the ph-meter will be used to measure the ph of two solutions whose ph values are known and one, two or three unknown solutions whose ph values are NOT known. Since a correct identification of your unknown solutions is mostly determined by an accurate ph determination, it is imperative that the ph meter be calibrated for the proper range. You will determine this range by using Universal ph Indicator Paper. Since this is a rather vague estimate of the ph value, a comparison table (DATA TABLE 2) that includes data for both known an unknown solutions is very helpful. Be aware that every one of your unknowns is exactly the same as one of the six known solutions. As such, if there is match between one known and one correctly identified unknown, the data obtained should be similar or very close. Do this with your partner: Lay two strips of Universal ph Indicator Paper on a piece of clean dry paper towel. Using disposable plastic droppers (if available) or a hollow glass tube sweep all areas of the Universal ph Indicator Paper strip, with one of the known solutions whose acidic/basic character was not determined by the use of the indicators. Repeat this for the second known solution whose acidic/basic character was not determined by the use of the indicators. You will notice a color change on the Universal ph Indicator Paper strip. The colors on all areas of the test strip will match one of the color set corresponding to a specific ph on the box of the Universal ph Indicator Paper box. Since this color is not stable and changes in time, it is advisable to test, observe and record the findings, one solution at the time. Record the ph values obtained for these two known solutions and use them to determine the ph calibration range (7/4 or 7/10) www/proffenyes.com 3

4 Do this individually: Depending on the unknowns assigned to you: You may not need to perform this test at all, or You may need to perform this test on one, two or all three of your unknowns. Lay one to three strips of Universal ph Indicator (as needed) to determine the calibration range to be used for ph measurements. Perform the ph tests using Universal ph Indicator paper as described above. 3. Measure the ph of the solutions (known and unknown) whose acidic/basic character was not determined by the use of the indicators. Calibrate first your ph meter for acidic solutions (7.01 followed by 4.01) Measure and record the ph of the known acidic solution(s) and the unknown solution(s) whose ph range has been identified as being acidic, by the Universal ph Indicator Paper (if any) by immersing the ph electrode in the appropriate vial(s). Calibrate next your ph meter for basic solutions (7.01 followed by 10.01) Measure and record the ph of the known basic solution(s) and the unknown solution(s) whose ph range has been identified as being basic, by the Universal ph Indicator Paper (if any) by immersing the ph electrode in the appropriate vial(s). Record the ph values of the solutions tested in DATA TABLE Wrapping up I of the experiment. Keep your buffer solutions in your locker in tightly capped plastic test tubes. The buffer solutions will be used for the next experiment. Your laboratory instructor will instruct you on how to store the ph mete IV. IDENTIFY YOUR UNKNOWNS 2015 www/proffenyes.com 4

5 REPORT FORM NAME: Date: Note: Part of this this experiment is performed individually. HCl(aq) Conc. Color with Blue Litmus paper DATA TABLE 1 Color with Red Litmus paper Color with phenolphthalein solution CLEAR OR PINK Acid/Base Character of Solution. ACIDIC OR BASIC OR? HC2H3O2(aq) NH4Cl(aq) NaC2H3O2(aq) NH3(aq) NaOH(aq) Unknown Unknown Unknown Note: Use? if indicators did not respond with a color change 2015 www/proffenyes.com 5

6 DATA TABLE 1I HCl(aq) Conc. ph Calculated Value ph determined by using Universal Indicator Paper (Write N/A if this test is unnecessary) Calibration of ph meter 7/4 OR 7/10 ph Experimental Value (measured with ph meter) HC2H3O2(aq) NH4Cl(aq) NaC2H3O2(aq) NH3(aq) NaOH(aq) Unknown Unknown Unknown CONCLUSIONS: UNKNOWN is (formula) UNKNOWN is (formula) UNKNOWN is (formula) 2015 www/proffenyes.com 6