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Field Test Methods
Specific Conductance
AOAC Official Method 973.40 and Table
973.40B
A. Principle
- Conductivity of sample is compared with
that of standard KCl solution.
- Method is applicable to drinking,
surface, and saline waters, and domestic and industrial wastes.
B. Apparatus and Reagent
- (a) Conductivity meter
self-contained, Wheatstone bridge type,
capable of being read to ±1%.
- (b) Specific conductance cell.
- Choose cell according to expected
specific conductance so that measured cell resistance is 500–10,000
ohms. Cell constant should be ca 0.1 for solutions of low conductivity
(<100 mmhos), 1 for moderate, and 10 for highly conducting, such as
brines.
- Check complete assembly with KCl
solutions of known conductance shown in Table 973.40B.
- Clean new cells with chromic acid
cleaning solution and platinize new electrodes before use.
- Reclean and platinize electrodes
whenever readings become erratic or if inspection shows any Pt black has
flaked off. To platinize, connect both electrodes together to negative
terminal of 1.5 V dry cell and immerse in solution of 1 g chloroplatinic
acid and 12 mg Pb(CH3COO)2 in 100 mL H2O. Connect positive terminal to
piece of Pt wire and dip into solution. Control current so that only
small amount gas is evolved. Discontinue electrolysis when both
electrodes are coated. Solution may be saved for subsequent use. Rinse
electrodes thoroughly and keep immersed in H2O when not in use.
- (c) Potassium chloride standard
solution. 0.01M.
Dissolve 745.6 mg KCl in freshly boiled double distilled H2O and dilute to 1 L
at 25°. Solution has specific conductance of 1413 mmhos at 25°. It is
satisfactory for most waters when using cell with constant of 1–2. With other
cells, use solution in Table 973.40B and corresponding specific conductance in
calculation. Store in glass stoppered Pyrex bottle.
C. Determination
- Temperature must be constant throughout
determination since specific conductance varies ca 2%/degree. Use 25° if
possible; otherwise use near room temperature but between 20–30°.
- Place 4 tubes standard KCl solution and
2 tubes of each sample in H2 O bath and let stand 30 min.
- Rinse cell in 3 tubes of KCl solution
and measure resistance of solution 4, RKCl.
- Rinse cell thoroughly with tube 1 of
sample and measure resistance of tube 2, RS .
- Do not repeat measurement of KCl
solution unless temperature drift of more than few tenths degree occurs.
- If samples differ in conductivity by
factor of >5, minimize carryover by rinsing in 2 tubes of sample and
measuring third.
D. Calculation
- Calculate cell constant, C, in mhos/cm
=RKCl X 0.001413 at 25°.
- Specific conductance of sample at 25°
= C/Rs in mhos/cm.
- Multiply by 1,000,000 to obtain
umhos/cm.
- If temperature is not exactly 25°,
measure RKCl and RS at same temperature
- and calculate specific conductance =
1413 X RKCl/RS in mmhos/cm.
E. References
- Methods for Chemical Analysis of Water
and Wastes, 1983 (available from National Technical Information Service,
5285 Port Royal Road, Springfield, VA 22161, Stock No. NTIS PB84-128677);
- FWPCA Method Study 1; Mineral and
Physical Analyses, June 1969 (available from National Technical Information
Service, 5285 Port Royal Road, Springfield, VA 22161, PB-230827);
- JAOAC 56, 295(1973).
Table 973.40B
Conductances of KCl Solutions at 25°
|
Concentration, M
|
Conductance
umhos/cm
|
| |
Equivalent |
Specific |
| 0 |
149.85 |
|
| 0.0001 |
149.43 |
14.94 |
| 0.0005 |
147.81 |
73.90 |
| 0.001 |
146.95 |
147.0 |
| 0.005 |
143.55 |
717.8 |
| 0.01 |
141.27 |
1,413 |
| 0.02 |
138.34 |
2,767 |
| 0.05 |
133.37 |
6,668 |
| 0.1 |
128.96 |
12,900 |
| 0.2 |
124.08 |
24,820 |
| 0.5 |
117.27 |
58,640 |
| 1.0 |
111.87 |
111,900 |
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