Each indicator changes colors over specific

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1 in determining the LH1 around the color change point. noted at a specific concentration of H or OH ions. Each individual indicator is most useful For each of the indicators listed in the laboratory materials list) a distinct color is color 1) color 2) (associated) (dissociated, Hln(aq) <- > H(aq) + ln-(aq) mixture of the associated and dissociated indicator. ph ranges. This change in color reflects a change in the concentration of the equilibrium Each indicator changes colors over specific is the result. hydroxide ions is the focus of this exercise. The mathematical tool known as the ph scale studied. The relationship of the color of an indicator to be concentration of hydrogen and For this experiment) the color variation of several individual indicators are to be litmus from lichens are well known sources of natural indicators. or base were vegetable dyes or natural minerals. Compounds found in red cabbage and or base solutions. Similar compounds exist in nature. In fact the first indicators of acid dyes which interact with light in different ways when their structures are altered with acid with H(aq) or OH(aq). The chromophores used in this experiment are common organic This experiment use organic compounds, called indicators to demonstrate the color changes which occur when a chromophoric (color producing) compound interacts In this experiment, you will observe the variation in color with ph of several specific applications according to the ph range in which they show a color change. compounds that change color with changing ph. Some indicators are mixtures of compounds, each of which changes color at a different ph. Indicators are chosen for determination, indicators are commonly used. ph indicators are organic electronic instrument known as a ph meter. For simpler though less accurate ph highly basic ph 14. The most accurate method of ph determination is by use of an indicators. The normal ph range is from the highly acidic ph 1 to the neutral ph 7 to the acidic, basic, or neutral. The ph scale represents the hydronium ion concentration of a solution and is used to indicate how acidic or basic a solution is. Many kinds of solutions must be tested to determine whether they are USING INDICATORS TO DETERMINE ph 1

2 In each of the rows of diluted acid and base, a single well or division between wells yields a color change. This color change can be explained in two distinct ways. 1) Color change A change in color signals a change in the structure of the indicator molecule. 2) K1 of the indicator The indicator itself is a chemical compound which interacts with light based on the concentration of reactant and product in a chemical equilibrium system. Using phenophthalein as an example, the reaction below illustrates this point: Hln(aq) < > H(aq) + ln(aq) (associated, colorless) (dissociated, pink) Phenophthalein is colorless in a solution which is high (relatively) in H(aq) ions. In wells I through 8, almost all the phenophthalein is in the associated state The associated molecule Hin interacts minimally with white light. The resulting solution of Hin, in relatively high concentrations of H, give a solution which is clear and colorless If, however, the concentration of H(aq) ions decreases, by reaction with OH (aq), for example, the concentration of Hin will decrease (according to Le Chatelier s Principle) until the only species left in the solution is the lrr(aq) ion. This molecule interacts with white light to give an aqueous solution which is bright pink The point where the equilibrium mixture of dissociated and associated molecules are present is given by the well in which a slight pink color predominates OBJECTIVES Prepare solutions of varying ph. Observe the color changes of ph indi cators in these solutions. Determine the ph ranges of each indicator. MATERIALS NflV apron goggles 96-well microplate nñcrotip pipets (3) 100-mL beaker white paper distilled water

3 PROCEDURE. 1. Place a microplate on apiece of white piper with the numbered columns at the top áñd the lettered rows to the left. Label the rows of your Microplate as follows. Row A: Universal indicator Row B: Bromothymol blue Row C: Phenolphthalein RowD: methyl orange A A B QQQQQQQQQQQ B QQQQQQQQQQQQ F F QQQQQQQQQQQQ H QQQQQQQQQQQQ H Fill a 100-mL beaker half-full with distilled water. 3. Place 9 drops of distilled water in each of wells 2 through 11 in rows A through D. 4. Add 10 drops of O.1MHC1 to each of wells Al, Bi, Cl, and Dl. 5. Using a clean microtip pipet, add 10 drops of 0.O1M NaOH to each of the wells A12, B12, C12, and D12.

4 well 9 of each row contains a solution /GccXcD EZXEZ The rows of solutions niw have H /BEXEXEXEDcXEZXDEXDcDcD /FcEXEXXEXEXEXEXDEZXEZXEZXD the same way that you did well A2. / EcEXECDXE ce CEZXEJ. DcXDE) row A for the wells in rows B, C, and D. well A8, but do not add any solution to row A for the wells in rows B, C, and D. /DEXJcDcZXDcDccxDcDcDc wella7. ph scale from 1 to 12. Well 4 of each row /ccdcdcdezxdcxddcdcxd that is about ph 9, and so on. contains a solution that is about ph 4, 10. Use a clean pipet to remove 1 drop of 12. Repeat the base dilution process used in well All. Mix as in step 6. any row. of step 10 from well A12 back through Do not transfer any solution to well 7 in not transfer any solution to well A?. 8. Repeat the dilution and mixing process 9. Repeat the acid dilution process used in 10. Each row of wells approximates the concentrations that vary by a factor of and place it in well A3. Mix well AS in 7. Remove 1 drop of solution from well A2 11. Repeat the dilution and mixing process the lnpet an4-returning the hq.uii drayruig up the-ecrntents of the well-4nto immediately to ii A2 of steps 6 and 7 through well A6, but do pipet. Mix the contents of well A2 by solution from. well A12 and place it in Al and add it to well A2 with your acid 6. Remove 1 drop of acid solution from well cxcx C I /

5 WhIch would 2 Which would would be a good Indicator be a good Indicator be a poor one? 1Which for an HCI/NaOH tltra.tlon? for general use? /What is the significance of the color changes In each row? tht4g4zs and rinse the inicroplate. Discard the solutions in your microplate according to your teacher s instructions your observations of colors 16. Finally, add 1 drop ofjrni*blue to each well in row C..--4.,jI121 well in row D. 15. Add 1 drop of phenolphthalein to each well in row B. J Je well in row A. 13. Add 1 drop of universal inthcator to eacn 14. Add 1 drop of bromothymol blue to each 17. Observe the wells in each row. Record

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7 wells 2 through 11 in rows A through D. wells A12, B12, C12, and 1)12. wellsal,b1,c1, anddl. drops of 0 O1M NaOH to each of the 3. Place9dropsofdi8tffledwaterinehof Row A: Universal indicator Row B: Bromothymol blue Row C: Phenolphthalein 2. Fill a 100-mL beaker half-full with as follows. PROCEDURE 5. Using a clean microtip pipet, add Add 10 drops of O.1MHC1 to each of distilled water F QQQQQQQQQQQQ F QQQQQQQQQQQ QQQQQQQQQQQQ QQQQQQQQQQQQ H QQQQQQQQQQQQ H B QQQQQQQQQQQQ B A A RowD: methyl orange top and the lettered rows to the left. Label the rows of your Microplate paper with the numbered columns at the 1. Place a microplate on a piece of white

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9 Thymol blue yellow blue Alizarin yellow yellow red Phenolphthal.in colorless red Brczmothvmol blue yellow blue Methyl violet yellow violet Methyl orange red yellow Methyl red red yellow Indicator Ebbective ph range Acid borm Base form PROPERTIES OF SONE INDICATORS HN 4.9 x 10-. HOC) 3.1 x 10 Al(HiO)j 7.2 x 10 i CH,NH2 3.9 x 1O CHaCOOH 1.8 x 1O HF 3.5 x 10-4 NH, 1.8 x 10 Acid Ka Base Kb EOLJILIBRItJI CONSTANTS OF ACIDS AND SES. problems br this chapter. It mar be copied and distributed to students. The 4o)lowing tables contain data necessary to solve some of the cid Base Equilibria Chapter 14 the

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