Acids and bases
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Acids and Bases. Biotechnology I. Life Chemistry. Based on water Cells contain 80-90% water Proper pH essential to ALL living systems Plants cannot live in poor pH soil Animals die if blood pH is abnormal Microorganisms need specific pH to grow & multiply

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Acids and bases

Acids and Bases

Biotechnology I


Life chemistry

Life Chemistry

  • Based on water

  • Cells contain 80-90% water

  • Proper pH essential to ALL living systems

    • Plants cannot live in poor pH soil

    • Animals die if blood pH is abnormal

    • Microorganisms need specific pH to grow & multiply

  • Maintaining proper pH is CRITICAL to survival of Cells and Biological systems


Ph environments

pH Environments

  • Biological and Industrial processes require specific pH environments

    • Food processing

    • Water purification

    • Rx production

    • Sewage treatment

  • Requires pH monitoring


Water

Water

  • Water = H2O  H+ + OH-

  • Pure water at 25 C

    • Concentration of H+ = concentration of OH- [1 x 10-7 mole/L]

  • Aqueous = water based

  • H+ is the symbol for hydrogen ion

  • OH- is the symbol for the hydroxide ion


Ph is

pH is

  • A way to express hydrogen ion concentration in a solution

  • Measurement of the acidity/alkalinity of an aqueous solution

  • pH is the –log of the H+ concentration

  • pH is measured on a scale

    • Ranges from 0 to 14

  • Pure water

    • H+ concentration is 1x10-7 mole/L

    • The log of 1x10-7 = -7

    • The – log of –7 = 7

    • The pH of pure water = 7


Acids

Acids

  • Definition: electrolyte that donates hydrogen ions

  • Properties:

    • Acids in water conduct electricity

    • The stronger the acid the stronger the conductivity

    • Acids react w/metals to produce H2 gas

    • Acids are indicators; they cause reversible color changes

      • Phenolphthalein and litmus are two examples of acid-base indicators

    • Acids react w/hydroxide compounds to form water and salt; this type of reaction is called “neutralization”

    • Strong acids completely dissociate in water to release hydrogen ions = H+

      • i.e. hydrochloric acid (HCl): HCL in water H+ + Cl-


Bases

Bases

  • Definition: electrolyte that yields hydroxide ions or accepts hydrogen ions

  • Properties:

    • Bases in water conduct electricity

    • The stronger the base the stronger the conductivity

    • Bases react with acids in neutralization reactions to form water and a salt

    • Bases cause reversible color changes in acid-base indicators (color is pH dependent)

    • Bases in water solution are slippery to the touch

    • Caution: even dilute bases can be caustic!

    • Strong bases completely dissociate in water to release hydroxide ions = OH-

      • NaOH in water  Na+ + OH-

      • TheOH- ions react with H + to form water, thereby  the concentration of hydrogen ions


Buffer

Buffer 

  • Substance(s) that when in aqueous solution resists a change in H+ concentration even if acids or bases are added

  • Some buffers change pH as their temperature and/or concentration changes

  • Tris buffer is widely used in molecular biology; it is very sensitive to temperature and the pH will vary greatly at various temperatures.


Neutralization reaction

Neutralization Reaction

  • One mole of H+ from an acid combines with one mole of OH- from a base to form H2O.

  • In addition, one mole of negative ions from the acid combine with one mole of positive ions from the base to form a salt.

H+Cl- + Na+OH -

H20 + NaCl


Logarithmic scale

Logarithmic Scale

  • pH scale is logarithmic

    • Means each whole number increases by the factor of 10.

      • A solution with pH=6 is 10x more acidic than pure water with pH=7.

    • pH 5.0 has 10 x more H+ then pH of 6.0

    • pH of 7.0 is 100 x less acidic than pH of 5.0

    •  pH of 7.0 has 100 x less what then a solution with a pH of 5.0?


Acids and bases

Quiz

  • What is OH- ?

  • What is the pH of a solution w/ an H+ ion concentration of 10-4 mole/L?

  • What is the concentration of H+ ions in a solution w/ a pH of 9.0?


Answers

Answers

  • Hydroxide ion

  • pH = -log [H+] = -log 10-4 = -(-4) = 4

  • pH = -log[H+]; 9.0 = -log [H+] -9.0 = log [H+]

    antilog (-9.0) = 1 x 10-9 mole/L


Review questions

Review Questions

  • Which pH value describes the most acidic solution?

    • 4 2 14 10

  • What is one of the most common bases used in the lab?

    • Sodium Hydroxide

    • Describe it when it is in solution

    • Given what you know, what would you say about “Clorox” bleach?

      • It is slippery to the touch


Measuring ph

Measuring pH

  • Indicators

    • Phenophthalein, phenol red, bromothymol blue, universal indicator to name a few

  • pH Paper

  • pH Meters


Ph meter

pH Meter

  • Meter / electrode system for measuring pH in laboratory

  • Provides greater accuracy, sensitivity than chemical indicators

  • Can measure pH of a solution to the nearest 0.1 unit

  • Can be used with variety of aqueous solutions

  • Consists of:

    • Voltmeter – measures voltage

    • Two electrodes connected to one another (sensor probe)

      • When immersed in the sample they develop an electrical voltage that is measured by the voltmeter

  • Calibration recommended with each use, when battery replaced and when fluid in sensor is changed


Calibration

Calibration

  • Important in operating the pH meter

    • It tells the meter how to translate the voltage difference between the measuring and reference electrodes into units of pH

    • Temperature sensitive

    • Two buffers of known pH are used to calibrate a pH meter

    • Refer to pH meter manual


Adjusting the ph of a buffer

Adjusting the pH of a buffer

  • Most often you will adjust the pH using NaOH or HCL

  • Adjust the pH at the temperature it will be used at

    • For example, if you are running an enzyme assay at 37C then adjust the pH at 37C

  • When making a buffer, do not bring it to final volume until you have adjusted the pH. Why?


Adjusting the ph of a buffer1

Adjusting the pH of a buffer

  • Place pH probe in solution

  • Check the pH and temperature

  • Add base or acid SLOWLY as required, soln. should be stirring

  • Re check pH to see if it is at specified pH.


Critical tips for using ph meter

Critical Tips for Using pH Meter

  • Depth of immersion – do not immerse to the bottom of a solution if there are particulates settled there

  • Make sure air bubbles are not trapped in the probe

  • Rinse probes w/ distilled water after each series of measurements

  • Be sure stir bars are not hitting the probe


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