pH & Buffers

pH pH is a measurement of the level of acid or base in a solution pH level affects molecular structure and function. For instance, proteins maintain their structure and activity only within certain pH ranges. Specifically, pH is a measurement of the number of hydrogen ions (H+) in solution (-log) of H+ concentration H2O H+ + OH- (1x10-7 mol/L) Water is neutral because it has an equal number of H+ to OH- No matter the pH value, the concentration of H+ and OH- adds up to 10-14 .

Measuring pH of a Solution The pH of a solution is determined by measuring the number of H+ ions. This is done in the lab one of 2 ways: pH Paper pH Meter

pH Paper The easiest way. A chemical indicator in the paper changes color depending on the number of H+ ions When determining the pH of only 1 sample it is usually easier to use pH paper.

pH Meter The pH meter is calibrated using buffered solution of known pH. An electrode then determines the pH by measuring the number of H+ ions and comparing to the pH calibration buffers. A pH meter is convenient to use when determining a large number of samples or when a pH is to be altered.

Calibrating and using a pH Meter Although every brand of pH meter is different, the basic method of calibrating and using a pH meter is the same. Use buffer standards to calibrate the meter. Turn on the pH meter. Rinse off the electrode with distilled water. If the meter has a temperature setting knob, set it to room temperature. Place the electrode in the pH 7 buffer standard. While swirling the solution, adjust the calibration until the display reads “7” Rinse the electrode with distilled water, being very careful of the very delicate tip. Place the electrode into the solution to be tested. Swirl the solution until the pH display stops changing. Read the pH value. If the solution is a strong acid, use a pH 4 buffer or a pH 10 buffer for a strong base to slope the meter after it has been calibrated to pH 7.

Buffers A buffered solution resists changes in pH A buffer is a solution made with some kind of salt or organic molecule added to deionized water, and is prepared at a certain pH. The buffering salt or molecule ionizes and interacts with H+ ions or OH- ions that may be released or added to a solution To adjust the pH of a buffer down or up, HCl or NaOH can be added until the desired pH is reached

pH of Common Household Items

HACH 440D pH Meter

Screen Display

Dual Screen Display

Checking Standards

Viewing & Printing Stored Data

How to Connect to a Printer

Troubleshooting

Troubleshooting (continued)

Molar Concentration Moles/Liter= Molarity The unit “1 mole” is the number of molecules of a substance that gives a mass in grams, equal to its molecular weight (formula weight) Formula weight can be determined by using the Periodic Table of Elements or by looking at the label of a chemical reagent. A moles worth of molecules is defined by mass but also by evidence that a mole contains 6x1023 of atoms or molecules.