State Certification Number: 1396
Phone (661) 765-2364 ♦ Fax (661) 765-6920 ♦ midwaylab@verizon.net ♦ www.midwaylaboratory.com
Water Analysis Quality Control Criteria
July 29, 2011
Introduction:
The following information is in part to clarify, define and explain Quality Assurance – Quality Control (QA
– QC) calculations, guidelines and related information that is presented on the lower area of our water
analysis reports. This information is provided to insure that the data presented on our water analysis
reports is within the precision and accuracy of the instrumentation and ability of the Chemist – Technician
performing the analysis and thereby validates the correctness of the data presented.
The first and primary guideline for the accuracy and correctness of data generated for a water analysis
report is the quality of the sample that is representative of the water system to be evaluated.
“The quality of the analysis is only as good as the quality of the sample”.
Topics related to water analysis quality control criteria …
1.
2.
3.
4.
5.
6.
7.
8.
Measured EC – Calculated EC.
Calculated TDS to EC Ratio.
Measured TDS to EC Ratio.
Anion – Cation Balance.
Calculated and Measured Sodium.
Anomalies associated with Conductivity and pH.
Alkalinity Measurements and the Presence of Organic Acids.
Alkalinity Measurements and the Presence of Borates.
1. Measured EC – Calculated EC:
-
If the calculated conductivity is higher than the measured value, reanalyze the
higher ion sum.
If the calculated conductivity is less than the measured value, reanalyze the
lower ion sum.
The acceptable ratio is:
0.9 < Calculated EC ÷ Measured EC < 1.1
2. Calculated TDS to EC Ratio:
-
If the ratio of calculated TDS is falls below 0.55, the lower ion sum is suspect;
reanalyze it.
If the ratio of calculated TDS is above 0.7, the higher ion sum is suspect;
reanalyze it.
If reanalysis causes no change in the lower ion sum, an unmeasured
constituent (i.e. ammonia, nitrite, nitrate, phosphate etc.) may be present at
significant concentrations. Poorly dissociated calcium and sulfate ions if
present will cause ratios 0.8 times or greater the EC.
The acceptable criteria are as follows:
Calculated TDS ÷ Conductivity = 0.55 – 7.0
Water Analysis Quality Control Criteria
Page 1 of 4
State Certification Number: 1396
Phone (661) 765-2364 ♦ Fax (661) 765-6920 ♦ midwaylab@verizon.net ♦ www.midwaylaboratory.com
3. Measured TDS to EC Ratio:
The acceptable criteria for this ratio are from 0.55 to 0.70. If the ratio of the TDS to EC is outside
these limits, measured TDS and (or) the measured conductivity is suspect.
-
Reanalyze a fresh aliquot of the sample.
Calcium and sulfate ions when present in appreciable amounts present the
inability to properly dissociate. Dilute the sample and compare with a
conductivity standard at a lower concentration other than the recommended
1412 µmho for most fresh and waste waters.
4. Anion – Cation Balance:
The Milliequivalent summation of cations to anions must balance in that all potable, waste waters,
production brines possessing relatively neutral pH values are electrically neutral. The test is based on
the percentage difference defined as follows:
% Difference = 100 X [ ( ∑Cations - ∑ Anions) ÷ ( ∑Cations - ∑ Anions) ]
The acceptance criteria are as follows:
Anion Sum
Acceptable %
(meq/L)
Difference
______________________________________________
0 -3.0
3.0 – 10.0
10.0 – 800
± 0.2 meq/L
±2%
±2–5%
If the acceptance criteria are not in acceptable differences, perform the following:
-
Review all raw data and calculations for possible errors.
Re-evaluate the sample, preferably a fresh sample, for conductivity, alkalinity
and pH
Check major analytes (i.e. chlorides, sodium or largest ion present in report.
Evaluate ions that posses multiple equivalents (i.e. boron, sulfate) that add to
equivalent weights.
5. Calculated and Measured Sodium:
Sodium ion is calculated by summating the equivalent weights of all anions present in the sample
minus the equivalent weight of the Cations (less Sodium) and multiplying by the molecular weight of
Sodium.
The acceptable criteria are calculated as follows:
0.9 < Calculated Sodium
-
÷ Measured Sodium < 1.1
If this value falls outside the 0.9 – 1.1 acceptable limits, re-evaluate the sample
for additional ions (i.e. ammonia, nitrite, nitrate, phosphate etc.).
Refer to acceptance limits as itemized described in topic four of the Water QC
Acceptance Criteria.
Water Analysis Quality Control Criteria
Page 2 of 4
State Certification Number: 1396
Phone (661) 765-2364 ♦ Fax (661) 765-6920 ♦ midwaylab@verizon.net ♦ www.midwaylaboratory.com
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Sodium as well as chloride ion is the commonest analytes acceptable to
contamination in the laboratory from an array of sources. Fingers that have
touched food (i.e. sun flower seeds, potato chips etc.) are of many sources of
contamination and by following simple hygiene (cleaning surfaces and hands)
minimize this potential prior to performing sodium or chloride measurements.
Sweat and tears have the potential of adding several milligrams of sodium.
Note: Ten milligram addition of a sodium contaminant to a 1000 to one dilution will
yield an increase of 10,000 mg/L sodium to results.
6. Anomalies associated with Conductivity and pH:
The presence of pH values in aqueous samples exceeding the pH of 10.0 may result in erroneous
conductivity (EC) results due to hydroxide ion interference.
-
-
Verify conductivity measurements by the addition of Gallic Acid (3, 4, 5Trihydroxybenzoic Acid) to the phenolphthalein (pH 8.3) indication point to the
sample and re-evaluate sample conductivity.
Steam quality measurements are subject to this anomaly. Add equal amounts
of Gallic Acid to both boiler feed waters and condensate effluents prior to
conductivity measurements.
7. Alkalinity Measurements and the Presence of Organic Acids:
The presence of organic acid salts (i.e. sodium acetate, CH3COONa + pH-H +1 ↔ acetic acid,
CH3COO1- ↔ H +1 ) adds to carbonate alkalinity measurements.
a.
- Verify the presence of organic acid salts and quantitatively determine
concentrations by ion chromatography.
- Utilize the Milliequivalent weights of the Anion – Cation balance in order to
determine if this is the source of error.
- Determine the residual organic acids after alkalinity determinations with an
additional pH adjustment to pH 3.5 and titrate with standardized sodium
hydroxide to pH 11.5. Graph pH vs. volume of standardized NaOH and
calculate “Total Organic Acids” at the graphically observed equilibrium point.
- Mathematically correct the alkalinity to “True” carbonate alkalinity.
8. Alkalinity Measurements and the Presence of Borates:
Alkalinity measurements by titration for the bicarbonate – carbonate – hydroxide buffering system in
production brines is critical for determining scaling tendencies for calcium carbonate deposition
(Langelier - Ryznar, Stiff – Davis), water treatment calculations, buffering potentials and related
topics. The analytical procedure for determine alkalinity is by titration and will include any analyte that
resists change in pH.
The correct determination of carbonate alkalinity is directly related to the expense involved in the
proper treatment of Oil-Field-Waters and the multi-million dollar expenditure for scale deposition,
corrosion control and sub surface water disposal the petroleum community realizes annually.
-
The presence of boron in Oil-Field-Waters is usually in the form of boric acid, borates and
organo-borates. When Boron is present as undissociated boric acid and (or) as a soluble
borate, B4O7-2, it is intrinsic to the buffering mechanisms, being second only to the
carbonate system. Boron with its three equivalent points has the potential of adding
dramatically to alkalinity results.
Water Analysis Quality Control Criteria
Page 3 of 4
State Certification Number: 1396
Phone (661) 765-2364 ♦ Fax (661) 765-6920 ♦ midwaylab@verizon.net ♦ www.midwaylaboratory.com
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Boron may be useful in identifying the sources of intrusive brines and together with
bromine and iodine is always associated with waters accompanying petroleum and as
with chlorine, is of marine origin.
Note: Titrations for carbonate alkalinity, organic acids and related buffering agents may be compromised by the
presence of sulfites, SO3-2, borates, B4O7-2 and other related analytes.
I hope this information will be of service to you in understanding and evaluating water reports generated
by Midway Laboratory. If you have any questions, please give us a call or send an e-mail.
Alan J. Harris
Director - President
References:
ASTM; “Water and Environmental Technology”; 2007 American Society for Testing and Materials, Vol.
11.01, 02, Water.
2. Todd, D.; Mays, L.; “Ground Water Hydrology”; 2005 3rd Ed. John Willey & Sons Publishers.
3. Skoog, D.; West, D. et al; “Fundamentals of Analytical Chemistry”; 8th Ed. 2004 Brooks/Cole Publishers.
4. APHA, AWWA, WEF; “Standard Methods for the Examination of Water and Waste Water”; 20th Ed. 1998
American Public Health Association and Assoc.
5. Stumm, W.; Morgan, J.; “Aquatic Chemistry”; 1996 3rd Ed. John Willey & Sons Publishers.
6. Economides, M.; Hill, D.; Ehlig-Economides, C.; “Petroleum Production Systems”; 1994 Prentice Hall
Petroleum Engineering Series, Prentice Hall Publishers.
7. Patton, C.; “Applied Water Technology”; 1986 1st Ed. Campbell Petroleum Series, Publishers.
8. API; “Analysis of Oil-Field-Waters”; 1981 American Petroleum Institute, Washington D.C., API RP 45.
9. EPA; “Handbook for Analytical Quality Control in Water and Wastewater Laboratories”; 1979 Unites
States Environmental Protection Agency publication EPA-600/4-79019.
10. Case, L. C.; “Water Problems in Oil Production,” 2nd Ed. 1970, The Petroleum Publishing Co., Tulsa
Oklahoma.
11. Levorsen, A.; “Geology of Petroleum”; 1954 W.H. Freeman & Co. Publishers.
1.
Alan J. Harris
Director – President
Rev. 2-11-05
Rev. 12-4-08
Rev. 8-10-09
Rev. 4-29-10
Rev. 7-29-11
Water Analysis Quality Control Criteria
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