Nutrient Measurement Using Ion Selective Electrodes

Kelly Sweazea, Technical Sales Manager Thermo Scientific Electrochemistry Products

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Need for Nutrient Measurement in Water

• In the news… ...“The area of hypoxia is formed every year when phytoplankton, stimulated by nutrients such as nitrogen and phosphorous from agricultural runoff…removes almost all of the oxygen from the bottom waters of the Gulf, making it unfit for life.”...

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The Rise of Slime: Casco Bay, Maine “Nitrogen pollution has wreaked havoc

in coastal waters around the world, causing fish kills, outbreaks of shellfish poisoning, marine mammal deaths and dead zones.”

“Parts of Casco Bay are now showing signs of this problem - green algae smothering mud flats, prolonged red tides and murky waters choking out eelgrass beds. Deteriorating coastal water quality threatens our marine resources, our quality of life, and our economy. ”

Need for Nutrient Measurement in Water

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• In the news… 13 Mar 2012, MSNBC: Farming communities facing crisis over nitrate pollution, study says

... “Nearly 10 percent of the 2.6 million people living in the Tulare Lake Basin and Salinas Valley might be drinking nitrate-contaminated water, researchers found. And if nothing is done to stem the problem, the report warns, nearly 80 percent of residents could be at risk of health and financial problems by 2050.

Need for Nutrient Measurement in Water

High nitrate levels in drinking water have been linked to thyroid cancer, skin rashes, hair loss, birth defects and “blue baby syndrome,” a potentially fatal blood disorder in infants.”...

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What are the Nutrients?

•  Nitrogen, Phosphorous, and Carbon Compounds •  Phosphorous is often the limiting (key) nutrient in freshwaters •  Nitrogen is typically the limiting nutrient in marine waters • Organic carbon is a food source •  Nutrients come in dissolved and particulate forms

•  Forms of Nitrogen •  Inorganic nitrogen – nitrate, nitrite, ammonia • Organic nitrogen – proteins, amino acids, peptides, etc.

• e.g., glutamic acid, urea, nicotinic acid, glycine, etc.

•  Forms of Phosphorous •  Inorganic phosphorous – ortho phosphates and poly phosphates • Organic phosphorous – ATP, AMP

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• Agricultural Activities (irrigation and drainage, fertilizer application, runoff and erosion)

• Overuse of Fertilizers Used for Landscaping Applications

• Urban and Industrial Runoff

•  Livestock Waste

• Detergents

•  Industrial Wastes •  Paper mills, etc.

• Mining and Forest Harvesting Activities

Non-point Sources of Nutrient Contamination

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• Hazardous spills

• Underground Storage Tanks

• Storage Piles of Chemicals

• Mine-Waste Ponds

• Deep-well Waste Disposal

•  Industrial or Municipal Waste Outfalls

• Runoff

•  Leachate from Municipal and Hazardous Waste Dumpsites and Septic Tanks

Point Sources of Nutrient Contamination

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Importance of Nutrient Level Monitoring and Control

• Pollution Reduction • Desire to have clean water

• Protection of Aquatic Life • Excess nitrogen encourages algal

blooms which result in water oxygen depletion and aquatic life destruction

• Ammonia is toxic to aquatic organisms at very low levels

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Methods for Measuring Nutrients

•  Ion Selective Electrodes • Colorimetry (color reactions)

•  Oldest Technology (100+ years) •  Limited Range •  Interferences

• Color, Turbidity, Other Species

•  Ion Chromatography •  Expensive •  Requires Extensive Training to Operate

• Spectrophotometry •  Titration •  Capillary Ion Electrophoresis / Ultra Violet detection (CIE/UV)

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Comparing Methods for Testing Wastewaters

General Equipment Cost Lower to Higher

Methodology General Cost Titration - manual (visual indicator) $100 - $200 (buret)

Colorimetry - manual $1,000 (multi wavelength)

ISE Meter and Probe (NH3) $1,800

Spectrophotometric - manual $3,800

Titration - automatic pH (potentiometric indicator) $7,500

Colorimetry / Spectrophotometric (automatic)

$35,000 - $50,000 (depending on accessories)

Ion Chromatography $60,000

CIE/UV $60,000

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Comparing Methods for Testing Wastewaters

•  Colorimetry & Spectrophotometry

(manual, kit)

•  ISE

•  Titration - manual (visual or potentiometric indicator)

•  Colorimetry / Spectrophotometry (automatic)

•  Ion Chromatography (IC)

•  Capillary Ion Electrophoresis / Ultra Violet detection (CIE/UV)

Methods Shown with Increasing:

•  Equipment Complexity

•  Training Required

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Comparing Methods for Testing Wastewaters

•  Colorimetry & Spectrophotometry

(manual, kit)

•  ISE

•  Titration - manual (potentiometric indicator)

•  Capillary Ion Electrophoresis / Ultra Violet detection (CIE/UV)

•  Colorimetry/Spectrophotometry (automatic)

•  Ion Chromatography (IC)

Methods Shown with Increasing:

•  Daily Equipment Preparation Time

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Commonly Tested Forms of Nutrients

• Forms of Nitrogen • Nitrate • Nitrite • Ammonia • Organic Nitrogen (TKN minus ammonia) •  Total Kjeldahl Nitrogen (organic N and ammonia) •  Total Nitrogen

• Forms of Phosphorous • Orthophosphate (reactive phosphorous) • Acid-hydrolyzable Phosphorous (inorganic forms of

phosphorous) •  Total Phosphorous

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EPA Approved Test Procedures - P

Orthophosphate Methods •  Colorimetry (manual or auto) •  CIE/UV •  Ion chromatography

Total Phosphorous Methods Digestion (various types) followed by: •  Semi-auto block digestor (TKP) •  ICP/AES (after digestion) •  Gas-Liquid Chromatography •  Colorimetry (manual or automatic)

Methodology Parameters Approved Test Procedureso-PO4 TP EPA SM ASTM Other

Persulfate Digestion x --- 4500-P B.5 ---

Ion Chromatography x300.0, Rev 2.1; 300.1, Rev 1.0 4110 B or C D4327

CIE/UV x --- 4140 D6508 D6508, Rev.2

Colorimetric (ascorbic acid) x x365.1, Rev 2.0;

365.3 4500-P E, FD515-88(A)

Semi-automated block digestor (TKP) x 365.4D515-88(B)

ICP/AES x 200.7, Rev 4.4 3120Gas-Liquid Chromatography x Addison (1970)

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EPA Approved Test Procedures – NH3 and TKN

Sample treatment followed with testing by:

•  colorimetry (manual or auto) •  ISE

Methodology Parameters Approved Test ProceduresNH3 TKN EPA SM ASTM Other

Manual Kjeldahl Digestion x ---4500 Norg B or C; 4500 NH3 D3590 (A)

Ammonia distillation or gas diffusion x 350.1, Rev 2.0 4500-NH3 B ---Colorimetric (Nesslerization) x x --- --- D1426 (A)Titration x x --- 4500-NH3 C ---

ISE x x ---4500-NH3 D or

E D1426 (B)Colorimetric (manual phenate, salicylate*, etc) x --- 4500-NH3 F ---

Colorimetric (automated or semi-automated phenate, salicylate*, etc) x x 350.1 Rev 2.0

4500-NH3 G or H ---

Automated Electrode x --- ---B&L Auto Analyzer

Ion Chromatography x --- D6919Titration x x --- 4500-NH3 C ---Automated Kjeldahl Digestions w/ analysis by titration, Nessler, or FIA. x 351.1; 351.2 4500-Norg D D3590 (B)

•  titration •  ion chromatograph

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Methodology Parameters Approved Test ProceduresNO2 NO3 TN EPA SM ASTM Other

ISE x --- 4500-NO3 D ---Colorimetric (Brucine) x 352.1 --- ---

Colorimetric (Cadmium Reduction) x 353.2 Rev 2.0 4500-NO3 E or FD3867 (A

or B)Colorimetric (Automated hydrazine) x --- 4500-NO3 H ---Colorimetric (Reduction) x Easy NO3Colorimetric (Diazotization) x --- 4500-NO2 B --- Hach 8507

Colorimetric (bypass Cd Reduction) x 353.2 Rev 2.0 4500-NO3 E or FD3867 (A

or B)

Ion Chromatography x x300.0, Rev 2.1; 300.1, Rev 1.0 4110 B or C D4327

CIE/UV x x --- 4140 D6508 D6508, Rev.2Persulfate Digestion x 4500-P B.5Colorimetric (chromotropic acid) x No approved procedures for Total N at this time (April 2012)Colorimatric (dimethyl phenol) x No approved procedures for Total N at this time (April 2012)

EPA Test Procedures – NO2, NO3, TN

Approved test procedures for nitrite and nitrate:

•  ISE •  Colorimetry

•  IC •  CIE/UV

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•  Ion Selective Electrodes are devices which detect specific ion species in solutions

•  ISEs consist of a sensing membrane in a rugged, inert body

What are Ion Selective Electrodes (ISEs)?

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• Responsive over a wide concentration range

• Not affected by sample color or turbidity

• Rugged and durable

• Rapid response time

• Real time measurements

• Low purchase and operation cost

• Easy to use

Ion Selective Electrode (ISE) Advantages

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Why Use Ion Selective Electrodes?

• Specific ion detection in solutions

•  ISE meters report concentrations • No manual calibration curves are required

•  ISE meters generate sophisticated curves which are held in the meter’s memory

• Run standards

• Run unknowns

• Read results

•  ISE use is endorsed through EPA approved methods

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Nutrient Regulations

• USEPA & NPDES •  Govern permits for limits of nutrients •  Mandate that standard methods must be used for reporting

• USEPA Approved Methods using an electrode include: •  Acidity •  Alkalinity •  Ammonia •  Chloride •  Chlorine, Total •  Cyanide

•  Fluoride •  Total Kjeldahl Nitrogen (TKN) •  Nitrate •  Dissolved Oxygen / BOD •  pH •  Sulfide

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Types Of Sensing Electrodes

• Glass Membrane: pH, sodium

• Liquid Membrane: chloride, nitrate, etc.

• Gas Sensing: ammonia, CO2, etc.

• Solid State: chlorine, copper, fluoride, etc.

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Plastic Membrane Electrodes

• Ammonium • Calcium • Chloride half-cell • Fluoroborate half-cell • Nitrate • Perchlorate half-cell • pH • Potassium • Surfactant half-cell • Water Hardness half-cell

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• Sensing membrane is an ion carrier dissolved in a soft plastic

• Short term storage of plastic membrane electrodes:

in dilute standard

•  Long-term storage: store module dry in vial

Plastic Membrane Electrodes

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• Example: Nitrate • EPA approved for drinking water or

wastewater

• 0.1 ppm detection limit

• Available in combination or half-cell versions

Plastic Membrane Electrodes

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Nitrate ISE Assembly

•  Replace module every 2-6 months

•  Do not over-tighten module

•  Replace fill solution weekly

•  Do not immerse electrode beyond o-ring on half-cell module

•  Use full strength ISA or Optimum Results as fill solution

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•  Check slope with ISA (ionic strength adjustor) not ISS (interference suppressor solution) •  Depressed slope normal when using ISS

•  Use ISS with interfering anions

•  Soak module In DI water for cleaning

•  Store module in 10-100ppm standard

•  Make sure junction flow is adequate

•  Calibrate with standards that bracket sample concentration

Nitrate ISE Hints

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• Ammonia • Carbon Dioxide • Nitrogen Oxide • Oxygen

Gas Sensing Electrodes

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• Gas sensing electrodes work by measuring a pH change caused by diffusion of gas through a hydrophobic but porous membrane

• Short term storage of gas sensing electrodes: in dilute standard

•  Long-term storage: store dry

Gas Sensing Electrodes

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• Example: Ammonia • EPA approved for

wastewater

• 0.01 ppm detection limit

• Combination electrode

• Replaceable membranes

Gas Sensing Electrodes

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pH Internal Fill Solution Ag/AgCl Reference

NH3 Permeable Membrane

NH3 IFS, NH4Cl

NH3 diffusion pH Sensing Glass

NH3 + H2O NH4+ + OH-

Ammonia Electrode Structure

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• Theory of Operation •  Membrane permeable only to gas

•  Ammonia diffuses through until partial pressure is equal on both sides

•  Inner stem: pH electrode •  Internal Fill Solution: fixed amount of NH4+ (excess) •  NH3 is directly proportional to OH- •  pH electrode can indirectly detect change in NH3

Ammonia Electrode Structure

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Ammonia ISE Assembly

•  Soak inner body in electrode fill solution for a minimum of 2 hours, or preferably, overnight.

•  Replace the membrane every 2-4 weeks.

•  Use tweezers to handle membrane.

•  Gently place membrane over outer body threads until surface is smooth and without wrinkles… do not overstretch.

•  Add ~2.5 mL internal fill solution (approx. 50 drops).

•  Carefully shake electrode down after assembly.

•  Rinse, dry, then soak in a 10ppm standard at least 15 minutes.

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Ammonia ISE Hints

•  Use electrode at a 20 degree angle: check for bubbles at membrane

•  Replace membrane and / or clean inner body in 0.1M HCl when response is sluggish or slope is low, then recalibrate

•  Sample Stirring •  Set stirrer to sit just below electrode surface

- Reduces carryover and bubbles on

probe surface

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• Direct measurement is generally preferred: •  Measure many samples with similar backgrounds

•  Measure high volume of samples

•  Measure wide range of concentrations

•  Easy

• Read measurement by using an ISE meter or by preparing a calibration curve

• Precision is +/- 2%

Direct Measurement with ISEs

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• Two-point calibration for linear portion of curve

• Low-level measurements require non-linear multi-point calibration or blank correction. The ISE meter may have an “auto-blank” feature in the settings.

Direct Measurement with ISEs

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• Calibrate every 2 hours

• Always calibrate with standards that bracket expected concentration range

• Always use at least two standards that are ten fold apart in concentration

• Slope range for monovalent ions at 20°-25°C: 54-60 mV (includes ammonia and nitrate probes)

• Slope range for divalent ions at 20°-25°C: 26-30 mV

Direct Measurement with ISEs

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Practical Considerations

• Method interferences

• Electrode interferences

• Temperature effects

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• Method interferences Many method interferences are overcome by using an

Ionic Strength Adjuster (ISA) • maintains a constant background when added to samples and standards

• minimizes ionic strength differences • complexes many interferences • adjusts pH to proper range

Practical Considerations

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• Method interferences

ISA example

• Ammonia ISA adjusts pH to proper range for conversion to ammonia

Practical Considerations

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• Electrode interferences • Some ion species cause increased electrode response

• With some ISEs, there is a maximum allowable ratio •  Example: not more than 400x as much chloride for the bromide

electrode

•  For some ISEs, interferences introduce a gradual error •  Example: at 10 ppm nitrate, a level of 760 ppm chloride will cause

10% error

•  For some ISEs, interference suppressors are available •  Example: Sodium ISA removes H+ interferences

Practical Considerations

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• Temperature effects A change in temperature will cause electrode response to shift

and change slope • On average, a 1°C change in temperature gives rise to a 2%

error for monovalent ISEs (this type includes ammonia and nitrate probes)

• On average, a 1oC change in temperature gives rise to a 4% error for divalent ISEs

•  ISE temperature compensation is generally accomplished by keeping samples and standards at the same temperature, between 20°-25°C.

• Some, but not all, meters will allow adjustment of the isopotential point for each different ISE electrode, and then the temperature input can be used to adjust the calibration curve

Practical Considerations

Thank You!