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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 = 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
  • 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-
  • 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 
  • 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?
  • 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?
  • 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
  • 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