Excelet

Comparison of Total Dissolved Solids (TDS) Methods

TDS can be measured by two different methods:

1. Stream water is filtered to remove suspended (insoluble) material and

then the sample is heated to evaporate the water. The residue obtained

is massed to obtain the milligrams of dissolved solids per volume of water

2. TDS is determined from the conductivity of the water sample, since the

ionic composition for most streams is similar. The conductivity reading is

converted to milligrams of total dissolved solids/liter. TDS = 0.67 * conductivity

Now let's investigate the TDS determination by conductivity and then

compare the two methods and address some possible errors.

Click on the tabs to navigate

evaporated. The residue is dried usually at 103-105oC.

Sinex 2007

Total Dissolved Solids (TDS) by Conductivity What is the cause of the conductivity of the water?

TDS = k * conductivity k = 0.67 67

(0.54 - 0.96)

Cond. TDS

0 0 tracer k composition

200 134 0 670 0.50 NaCl

500 335 1000 670 0.51 KCl

1000 670 1000 0 0.67 natural waters

1500 1005 0.72 442

2000 1340

mg/L

9

Cond. = 1000

The values of "k" for NaCl, KCl, and 442 were determined from data

available:

online at:

mS/cm

http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.php

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Hem, J.D. (1985) Study and Interpretation of the Chemical

Characteristics of Natural Waters, USGS Water-Supply Paper 2254

http://pubs.usgs.gov/wsp/wsp2254/

0 500 1000 1500 2000 25000

500

1000

1500

2000

2500

670

f(x) = 0.67 x

Conductivity, mS/cm

TDS, mg/L

Comparison of Total Dissolved Solids (TDS) Methods530 mg/L

add a constant amount urea = 0 mg/L (used as a fertilizer)

(0 - 300)

(NaCl) (urea) y = x line

0 0 0 0 0 tracer

200 0 200 200 200 0 530

400 0 400 400 400 530 530 530 mg/L change the slider

600 0 600 600 600 530 0 530 to see how the two

800 0 800 800 800 methods compare

1000 0 1000 1000 1000 0 530 (with both yellow

mg/L 530 530 %error cells set at zero)

0.0

How do the methods compare?

add 0 mg/L (commonly used for highway deicing)

(0 - 300)

How does the addition of urea and/or calcium chloride influence the results of the two methods?

TDSevap =

TDSevap

TDScond

CaCl2 =

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0 200 400 600 800 1000 12000

200

400

600

800

1000

1200

TDS by conductivity

TD

S b

y e

vapora

tion

Address the following questions:

1. Explain how and why the addition of calcium chloride influences the results

of both methods including any error.

2. Explain how and why the addition of urea influences the results of both

methods including any error.

3. Suppose a water sample was collected and not filtered. Both methods were

then used to measure the TDS. Explain the results from both methods including

any error.

4. For the TDS = k (conductivity) relationship the value of "k" must remain

constant for this method to work. Why? What could cause "k" to vary?

5. Domino Sugar is a large manufacturer in Baltimore Harbor. If a barge

loaded with finished processed sugar was to sink and the sugar dissolved in the

water of the harbor, how would the TDS results by both methods be influenced?

6. A barge loaded with vinegar (0.8 M acetic acid) develops a leak off shore of

Ocean City. The seawater, which is 0.5 M NaCl, has a pH of 8.3 that is well buffered.

How would the acetic acid influence the results of both methods including any error.

Sinex 2007