Stuyvesant High School Department of Biology & Geo-Science LABORATORY EXERCISE #5 ACIDS, BASES and the pH SCALE INTRODUCTION Cells are chemical factories in which metabolic processes occur All chemical reactions in cells must occur at a favorable pH pH is a measure of the concentration of the hydrogen ions in a solution If the concentration of hydrogen ions is too high or too low, the metabolic reactions will not occur properly Today you will learn how to measure the pH of a number of solutions using special indicator papers These papers, such as litmus paper and pHydrion paper, are impregnated with chemicals that turn specific colors depending on the pH of the solution You will also discover how cells control hydrogen ion concentration STUDENT OBJECTIVES Learn to determine the pH value of a solution using different indicator paper Determine and compare the pH value of several solutions Understand the effect of a buffer solution on the pH of a solution PRE-LABORATORY QUESTION What you understand by the term pH and how would you expect a low hydrogen ion concentration to affect the function of a cell? You can use your textbook and other resource material to answer these questions MATERIALS Paper Towel, China marker, forceps, red litmus paper, blue litmus paper, pH papers, measuring cylinder, test tubes, test tube rack, drinking straw, small beaker, cabbage juice, buffer solution, bromthymol blue solution, test solutions PROCEDURE You will be working with caustic chemicals so recall the safety precautions that you learned during your first laboratory exercise I Determining pH Working in pairs, take a paper towel and divide it up into large squares Label the top of each square with numbers 1- Square numbers and should be labeled as on the data collection sheet Each square is large enough to place a small piece of each of the indicator paper and so labeled (see diagram on the board) The solutions to be determined are as following: 1) 0.1N NaOH 2) 0.1N HCL 3) Lemon Juice 4) Antacid 5) Water 6) Vinegar 7) Baking soda 8) Pepsi/Coke Using forceps, place a very small piece of red litmus and blue litmus and pHydrion paper onto the #1 square Place a SMALL drop of the #1 solution on each piece of indicator paper Observe and record any change in the color of the indicator papers Compare the color of the pHydrion paper to the chart on the vial and record the pH indicated Repeat steps and until you have tested all different solutions, from #1 to #8 II Titration of Cabbage Juice, a natural pH indicator Pour about ml of cabbage juice indicator into three small test tubes Label these tubes A, B, and C Use pHydrion paper to determine the pH of the indicator in tube C and record this and the color of the indicator on the data sheet Save tube C as a reference Add ONE DROP AT A TIME of 0.1N NaOH (0.1N is a measure of the concentration of the solution) to tube A until there is a color change Gently mix after each drop Remember to keep track of the NUMBER of drops needed to produce the color change Record this and the new color on the data sheet Now add ONE DROP AT A TIME of 0.1N HCl to tube A until the indicator color returns to the original color (Refer to tube C) Record the number of drops needed to this on the data sheet Regents Living Environment Laboratory Manual Stuyvesant High School Department of Biology & Geo-Science Add ONE DROP AT A TIME of 0.1N HCl (0.1N is a measure of the concentration of the solution) to tube B until there is a color change Gently mix after each drop Remember to keep track of the NUMBER of drops needed to produce the color change Record this and the new color on the data sheet Now add ONE DROP AT A TIME of 0.1N NaOH to tube B until the indicator color returns to the original color (Refer to tube C) Record the number of drops needed to this on the data sheet III Controlling the pH of a solution Pour about ml of water into three small test tubes Label these A, B, C Use pHydrion paper to determine the pH of the solution in tube C and record this on the data sheet Save tube C as a reference Add drops of 0.1N HCl to tube A Use pHydrion paper to determine the pH and record your results Add drops of 0.1N NaOH to tube B Use pHydrion paper to determine the pH and record your results Add about ml of Buffer Solution to three small test tubes Label these D, E, F Use pHydrion paper to determine the pH of the solution in tube F and record this on the data sheet Save tube F as a reference Add drops of 0.1N HCl to tube D Use pHydrion paper to determine the pH and record your results Add drops of 0.1N NaOH to tube E Use pHydrion paper to determine the pH and record your results Optional Exercise (if time) IV Using an indicator in a biological experiment Place about ml of bromthymol blue indicator into three small test tubes Label these tubes A, B, C Add HCl to tube A ONE DROP AT A TIME until there is a color change and measure the pH Record the change in color and save tube A as a reference Add NaOH to tube B ONE DROP AT A TIME until there is a color change and measure the pH Record the change in color and save tube B as a reference Gently exhale through a straw into the liquid in test tube C being careful not to blow the liquid out of the tube Compare the color of this tube to your two reference tubes and measure the pH Record the color and your conclusions on the data sheet Regents Living Environment Laboratory Manual ... until there is a color change and measure the pH Record the change in color and save tube A as a reference Add NaOH to tube B ONE DROP AT A TIME until there is a color change and measure the pH. .. determine the pH and record your results Add about ml of Buffer Solution to three small test tubes Label these D, E, F Use pHydrion paper to determine the pH of the solution in tube F and record... on the data sheet III Controlling the pH of a solution Pour about ml of water into three small test tubes Label these A, B, C Use pHydrion paper to determine the pH of the solution in tube C and