The Science

Jeff Jeffers

TOC Method

• Acidify and Sparge with Zero-Grade Oxygen or Air - Removes TIC

Interference while Providing Oxygen to support Oxidation

• Oxidize Organics to CO2 by UV Radiation

• Concentrate CO2 - Removes Ionic Interference

• Measure Conductivity of CO2 Concentrate

• Software Model Converts Conductivity/Temperature Signals to TOC

Based on Chemical Equilibrium Among CO2 Species and

Concentration Values…with Data Smoothing (as needed)

Counter-Flow

Sparger

UV

Reactor

Acid

CO2

Concentrator Condenser

Conductivity

Temperature Sensor

GLS

Back-Pressure

Regulator

Heater

Sample

Oxygen (Air)

Sparge Gas

Conceptual Schematic TOC Instrument with Conductivity Detection of Concentrated CO2

Mist Trap

Gas & Excess Liquid Drain

N2, O2 Vent

Liquid Drain

Condensate

Flow Control

Liquid

Steam with

Volatile Gases

CO2, N2, O2

S

Pump

UV Oxidation

• Generate Free-Radical Oxidizer

2 H2O + O2 4 [OH]*

• Hydrocarbon Oxidation (Sucrose Example)

C12H22O11 + 48 [OH]* 12 CO2 + 35 H2O

• At least 4 mgO2/L is Required to Oxidize 1 mgC/L for many Organic Compounds

185nm, 254nm

Why Conductivity?

• Advantages • Disadvantage

– High Sensitivity -- Not Ion-Selective

– Fast Online Response -- Temp. Dependent

– Inexpensive Equipment

– Low O & M Cost

– Reliable

– Easy to use

Conductivity Measurement

• Cell Geometry

k = (L/A) * 1/R

• Individual Ions

ki = ⋀i * Ci

• Multiple Ions at Temperature t

kt = ∑ {⋀i * [1 + ßi * (t – 25)] * Ci }

Pure Water Example

Kw

H2O [H+] + [OH-]

[H+] * [OH-] = Kw [H+] = [OH-] = √Kw

{ H+ } + { OH- }

Kt = { ⋀H+ * [1 + ßH+ * (t - 25)] * [H+] } + { ⋀OH-* [1 + ßOH-* (t - 25)] * [OH-] }

Pure Water Example

• Shows conductivity dependence on temperature

• Shows that low cell constant is required for reliable resistance measurement

Variation In Water Quality

• Pure Water ………………...0.055 µS/cm

• Power Plant Boiler Water….1.0 µS/cm

• Good City Water… 50 - 500 µS/cm

• Sea Water…………….50,000 µS/cm

Equilibrium – CO2 in H2O

Gas Liquid

Phase Phase

CO2 CO2 + H2O H2CO3 H+ + HCO3- 2H+ + CO3

=

H K1 K2 K3

H2O H+ + OH-

Kw

Equilibrium Constants H, K1, K2, K3 and Kw are Temperature Dependant

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 100

Co

nd

uct

ivit

y, µ

S/c

m

Temperature, ºC

CO2 Solubility in Water

1,000 ppmCO2 Atmosphere

CO2 Atmosphere

Pure Water

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 500 1000 1500 2000 2500 3000

TIC

, p

pm

C a

nd

Con

du

ctiv

ity, µ

S/c

m

Atmosphere, ppmCO2

Water in Equilibrium with Atmosphere

25ºC

µS/cm

ppmC

Contributors to Conductivity Response

0.0

0.2

0.4

0.6

0.8

1.0

10 100 1,000 10,000

Con

du

ctiv

ity, µ

S/c

m

CO2 in Water, pptC (nanogramsC/l)

Conductivity of Carbon Dioxide in Water

x20 @ 40°C

x20 @ 25°C

x1 @ 25°C

Concentrated

Test Data

0

100

200

300

400

500

600

16:30 17:30 18:30 19:30 20:30 21:30 22:30 23:30 0:30 1:30 2:30 3:30 4:30 5:30

TOC

, pp

bC

8/26/13

Low-Level Sample Cycling Shows Good

Repeatability

0

10

20

30

40

50

60

70

80

21:30 22:30 23:30 0:30 1:30 2:30 3:30 4:30 5:30

TOC

, pp

bC

Linearity Test

10

100

1000

10000

10 100 1000 10000

TO

C R

epo

rted

, p

pb

C

TOC Added, ppbC

100 % Recovery Line

Sucrose

Benzoquinone

Isopropanol

Urea

Potential MDL

• Cell Constant 0.00884 cm-1

• 0.536 µS/cm FS

• 30ºC

• 2 % FS Detectable Change

• MDL

– No Concentration – 1.0 ppbC

– X5 Concentration – 0.2 ppbC

– X10 Concentration – 0.1 ppbC

Range Selection Criteria

Calibration / Verification

Water Blank – Determines background TOC subtracted from calibration standard signals.

Calibration – Determines calibration factor applied to compensate for non-ideal TOC recovery.

Calibration Verification – Verifies instrument recovery of Sucrose complies with USP.

System Suitability - Verifies instrument recovery of Benzoquinone complies with USP.

Procedures

• Calibration

– Run Blank (DI used in Cal Standard prep)

– Run Cal Standard

– Record Values of Each After 30 min Stabilization Time

• Calibration Verification (w/Sucrose) & System Suitability (w/Benzoquinone)

– Run Blank (DI Used in Standard prep)

– Run Cal Standard (Sucrose/Benzoquinone)

– Record Values of Each After 30 min Stabilization Time

• Online Sample

– Run Blank (Online Sample with Reactor off)

– Record Value After 30 min Stabilization Time

– Turn Reactor On…Wait 30 min for Stabilization Before Reading Sample

0

100

200

300

400

500

600

0 10 20 30 40 50 60

TO

C,

pp

bC

Minutes

Characteristic Instrument Response to Step Change

Test Reports

• Calibration Verification (Sucrose)

Verification Result, % = { TOT - ( DI + IT ) } / ADD *100

• System Suitability (Benzoquinone)

Response Efficiency, % = { TOT - ( DI + IT ) } / ADD *100

Where,

TOT = Instrument Response to Cal Sample with TOC Added, ppbC

ADD = TOC Added to Cal Sample (same for both tests), ppbC

( DI + IT ) = Cal Sample before TOC Added = Blank, ppbC

DI = TOC Added by DI, ppbC

IT = TOC Added by Instrument, ppbC

Weight/Volume Stock TOC Test Solutions

• Sucrose……….....50,000 ppbC…..118.8 mg/L

• Benzoquinone…..50,000 ppbC….....75.0 mg/L

• KHP…………….50,000 ppbC…...106.3 mg/L

• Urea………….....50,000 ppbC…...255.0 mg/L

• Isopropanol……500,000 ppbC……1.062 cc/L

• Ethylene Glycol.500,000 ppbC…....1.160 cc/L

• Nicotinic Acid….50,000 ppbC…….85.4 mg/L

• Methanol………500,000 ppbC……1.686 cc/L

Pump Setup

Diaphragm pumps P1, P2, P3, and P4 are each calibrated so that measured flow rates equal the

programmed flow rates shown on the ABOUT FLOW RATES screen.

The instrument controls flow by calculating the pump control signal frequency for the desired

flow rate as determined by the following relationship:

F = q / V

Where,

F = Pump control signal frequency (one impulse per chamber volume), impulses/min

q = Flow rate, ml/min

V = Pump chamber volume, ml/stroke

Pump Setup

Pump Setup

Pump Setup

Symbols

A = Electrode Surface Area, cm2

L = Distance Between Electrodes, cm

k = Conductivity, µS/cm

ki = Conductivity of an Ion @ 25ºC, µS/cm

kt = Conductivity of All Ions in Solution @ 25ºC, µS/cm

⋀i = Equivalent Conductance of an Ion at Infinite Dilution @ 25ºC, µS/cm per µmol/L

Ci = Concentration of an Ion, µmol/L

ßI = Temperature Coefficient of an Ion

∑ = Sum of all Ionic Species in the Solution

F = Pump control signal frequency (one impulse per chamber volume), impulses/min

q = Flow rate, ml/min

V = Pump chamber volume, ml/stroke