Leck Mitchell, Ph.D., PE 

656 Independence Valley Dr.; Grand Junction, CO 81507 

Ph: 970-244-8661; Fax: 970-244-8856; 

Leckmitchell@Bresnan.net 

MITCHELL ATP SUBMISSION FOR CHANGES IN 

METHOD 180.1 USING LASER 

DETERMINATION OF TURBIDITY BY NEPHELOMETRY 

Mitchell Method M5271 

Alternate Test Procedures 

Leck Mitchell, Ph.D., PE 

Patricia Lacy Mitchell, Ph.D. 

Revision 1.0

July 31, 2008 

 

 

 
2

ATP SUBMISSION FOR CHANGES IN METHOD 180.1 USING LASER 

DETERMINATION OF TURBIDITY BY NEPHELOMETRY 

Mitchell Method M5271 

Alternate Test Procedures 

1.0 SCOPE AND APPLICATION:

 1.1 This method covers the determination of turbidity in finished, drinking, filtered 

effluent; distribution; ultra-pure micro-processes, and colorless water with a turbidity of 

less than 40 NTU. 

 1.2 This method introduces a solid state monochromatic laser light source which falls 

 slightly outside the accepted range for EPA 180.1 but meets all other quality control 

 and performance characteristics of 180.1. 

 1.3 The applicable range is 0 to 40 NTU. 

 1.4 This method can be used for compliance monitoring and reporting under 

 the Safe Drinking Water Act (SDWA). 

2.0 SUMMARY OF METHOD:

 2.1 The method is based upon a comparison of the intensity of light scattered by 

 the sample under defined conditions, with the intensity of light scattered by a 

 standard reference suspension. The higher the intensity of scattered light, the 

 the higher the turbidity. Readings in NTUs are made using a laser nephelometer 

 designed according to specifications given in section 6.1 and 6.2. Primary 

 standard suspensions are used to calibrate the instrument. A secondary 

 standard is monitored periodically for deterioration using one of the primary standards. 

 

 2.1.1 Formazin polymer is used as a primary turbidity suspension for 

 Water. 

 2.1.2 AMCO Clear is available from GFS Chemical and is approved by USEPA as a 

 polymer primary standard. It is stable over longer periods of time than 

 Formazin. 

 

2.1.3 Stabilized Formazin, identified as StablCal, is an approved derivative of the 

 Formazin standard and is stable in storage for two years. 

3.0 DEFFINITIONS

 3.1 CALIBRATION BLANK (CB) – A volume of reagent water fortified with 

 the same matrix as the calibration standards, but without the analytes, 
3

 internal standards, or surrogate analytes. 

 

 3.2 INSTRUMENT PERFORMANCE CHECK SOLUTION (IPC) – A solution 

 of one or more method analytes, surrogates, internal standards, or other test 

 substances used to evaluate the performance of the instrument system with 

 respect to a defined set of criteria. 

 

 3.3 LABORATORY REAGENT BLANK (LRB) – An aliquot of reagent water 

 or other blank matrices that are treated exactly as a sample including 

 exposure to all glassware, equipment, solvents, reagents, internal standards, 

 and surrogates that are used with other samples. The LRB is used to 

 determine if method analytes or other interferences are present in the 

 laboratory environment, the reagents, or the apparatus. 

 3.4 LINEAR CALIBRATION RANGE (LCR) – The concentration range over 

 which the instrument response is linear. 

 3.5 MATERIAL SAFETY DATA SHEET (MSDS) – Written information 

 provided by vendors concerning a chemical/s toxicity, health hazards, 

 physical properties, fire, and reactivity data including storage, spill, and 

 handling precautions. 

 3.6 PRIMARY CALIBRATION STANDARD (PCAL) – A suspension prepared 

 from the primary dilution stock standard suspension. The PCAL suspensions 

 are used to calibrate the instrument response with respect to analyte 

 concentration. 

 3.7 QUALITY CONTROL SAMPLE (QCS) - A solution of the method analyte of 

 known concentrations that is used to fortify an aliquot of LBR matrix. The 

 QCS is obtained from a source eternal to the laboratory, and is used to check 

 laboratory performance. 

 3.8 SECONDARY CALIBRATION STANDARDS (SCAL) – commercially 

 prepared, stabilized sealed liquid or gel turbidity standards, or other apparatus 

 or mechanism calibrated against properly prepared and diluted Formazin or 

 styrene divinylbenzene polymers. 

 3.9 STOCK STANDARD SUSPENSION (SSS) – A concentrated suspension 

 containing the analytic solution prepared in the laboratory using assayed 

 reference materials or purchased from a reputable commercial source. Stock 

 standard suspension is used to prepare calibration suspensions or other needed 

 suspensions. 

 

4.0 INTERFERENCES:

 4.1 The presence of floating debris and coarse sediments that settle out rapidly will 
4

 give low readings. Finely divided air bubbles can cause high readings. 

 4.2 The presence of true color, that is the color of water that is due to dissolved 

 substances that absorb light, will cause turbidities to be low, although this 

 effect is generally not significant with drinking water. 

 4.3 Light-absorbing materials such as activated carbon in significant 

 concentrations can cause low readings. 

5.0 SAFETY:

5.1 The toxicity or carcinogenicity of each reagent used in this method has not been 

 fully established. Each chemical should be regarded as a potential health 

 hazard and exposure should be as low as reasonably achievable. 

5.2 Each laboratory is responsible for maintaining a current awareness file of 

 OSHA regulations regarding the safe handling of the chemicals specified in this 

 method. A reference file of Material Safety Data Sheets (MSDS) should be 

 made available to all personnel involved in the chemical analysis. The 

 preparation of a formal safety plan is also advisable. 

 

 5.3 Hydrazine sulfate (7.2.1) is highly toxic and may be fatal if inhaled, swallowed or 

 absorbed through the skin. Formazin can contain residual hydrazine sulfate. Proper 

 protection should be employed. 

 

6.0 EQUIPMENT AND SUPPLIES:

 6.1 The turbidimeter shall consist of a nephelometer, with a light source for 

 illuminating the sample, and one or more photo-electric detectors with a 

 readout device to indicate the intensity of light scattered at right angles to the 

 path of the incident light. The turbidimeter should be designed so that little 

 stray light reaches the detector in the absence of turbidity and should be free 

 from significant drift after a short warm-up period. 

 6.2 Differences in physical design of turbidimeters will cause differences in 

 measured values for turbidity, even though the same suspension is used for 

 calibration. To minimize such differences, the following design criteria should 

 be observed: 

 6.2.1 Laser light source: Monochromatic source operated at a nominal 

 wavelength of 650 ±30nm. The light source shall be used as a directly 

 received reference for the scattered nephelometric signal. Alternative light 

 sources may be incorporated as long as they fall within the proscribed 

 wavelengths and meet all quality control performance characteristics. In 

 addition, the Mitchell designs allow a variation in light source without 

 variation in turbidity response. 

 6.2.2 There shall be no divergence from parallelism as the incident 

 radiation and any convergence shall not exceed ±1 1⁄2 degrees. 
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 6.2.3 Distance traversed by incident light and scattered light not to exceed 10 cm. 

 6.2.4 Detector/Light Receiver: Centered at 90o ± 1 1⁄2 degrees to the incident 

 light path and the light cone not to exceed ± 30 degrees from 90degrees. 

 The receiver, if used, may be coupled to a photo detector using an optical 

 path. 

 6.2.4.1 The photo detector must have a spectral response that 

 encompasses complete spectral output of the light source. 

 A Mitchell/Mersch light trap shall be incorporated. 

 

 6.2.5 An optical device may be used to carry light from the light source 

 to the sample or to carry scattered light from the sample to the 

 photo detector. 

 6.2.6 Instrument shall have incorporated a Mitchell/Mersch bubble trap for 

 on line use and Mitchell/Mersch anti-fog windows. Sensor shall be 

 horizontal with vertical windows. Both windows shall be immersed 

 in the measurement fluid for on-line use. Instrument shall be 

 capable of being pressurized to 30 psig. 

 6.2.7 Turbidimeter example which meets these specifications is the 

 Mitchell (M5271). 

 6.3 The sensitivity of the instrument should permit detection of a turbidity 

 difference of .001 NTU or less in waters having turbidities less than 1 

 NTU units. The instrument should measure from 0 to 40 NTU turbidity 

 units. 

 6.4 The sample tubes for laboratory, bench top or portable units to be used with 

 the available instrument must be of clear, colorless glass or plastic. They 

 should be kept scrupulously clean, both inside and out, and discarded when 

 they become scratched or etched. A light coating of silicon oil may be used 

 to mask minor imperfections in glass tubes. They must not be handled at all 

 where the light strikes them, but should be provided with sufficient extra 

 length, or with a protective case, so that they may be handled. Tubes should 

 be checked, indexed and read at the orientation that produces the lowest 

 background blank value. 

 6.5 Balance – Analytical, capable of accurately weighing to the nearest 0.0001g. 

 6.6 Glassware – Class A volumetric flasks and pipettes as required. 

 

7.0 REAGENTS AND STANDARDS

7.1 Reagent water, turbidity-free: Pass deionized distilled water through a 0.45 

 micron pore-size membrane filter, if such filtered water shows a lower turbidity 

 than unfiltered distilled water. 
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7.2 Stock standard suspension (stabilized Formazin) 

 7.2.1 Dissolve 1.00g hydrazine sulfate (CASRN 10034-93-2) in reagent 

 water and dilute to 100 mL in a volumetric flask. 

 7.2.2 Dissolve 10.00g hexamethylenetetramine (CASRN 100-97-0) in 

 reagent water and dilute to 100 mL in a volumetric flask. In a 100 

 mL volumetric flask, mix 5.0 mL of each solution (7.2.1 + 7.2.2). 

 Allow to stand 24 hours at 25 ± 3 degree C, then dilute to the 

 mark with reagent water. 

7.3 Primary calibration standards: Mix and dilute 10.00 mL of stock standard 

 suspension (7.2) to 100 mL with reagent water. The turbidity of this 

 suspension is defined as 40 NTU. For other values, mix and dilute portions 

 of this suspension as required. 

 7.3.1 A new stock standard suspension (7.2) should be prepared each 

 month. Primary calibration standards (7.3) should be prepared daily 

 by dilution of the stock standard suspension. 

7.4 Formazin in commercially prepared, certified, primary, concentrated stock 

 standard suspension (SSS) may be diluted and used as required. Dilute 

 turbidity standards should be prepared daily. 

7.5 AMCO Clear- Styrene Divinylbenzene polymer primary certified, turbidity 

 standards are available for specific instruments and require no preparation or 

 dilution prior to use. 

7.6 Stabilized Formazin, turbidity standards, certified, primary, polymer 

 suspensions have long-term storage stability. 

7.7 Secondary standards may be acceptable as a calibration check, but must 

 be monitored on a routine basis for deterioration and replaced as required. 

8.0 SAMPLE COLLECTION, PRESERVATION AND STORAGE

8.1 Samples for bench top and portable laboratory units should be collected in 

 plastic or glass bottles. All bottles must be thoroughly cleaned and rinsed 

 with turbidity free water. Volume collected should be sufficient to insure a 

 representative sample, allow for replicate analysis (if required), and minimize 

 waste disposal. 

8.2 No chemical preservation is required for samples. Cool sample to 4 degrees C. 

8.3 Samples for bench top and portable laboratory units should be analyzed as 

 soon as possible after collection. If storage is required, samples maintained at 

 4 degrees C may be held for up to 48 hours. 

8.4 Online instrumentation does not require sample cooling, preservation or 

 storage. 
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9.0 QUALITY CONTROL

9.1 Each laboratory using this method is required to operate a formal quality 

 control (QC) program. The minimum requirements of this program consist of 

 an initial demonstration of laboratory capability and analysis of laboratory 

 reagent blanks and other solutions as a continuing check on performance. 

 The laboratory is required to maintain performance records that define the 

 quality of data generated. 

9.2 INITIAL DEMONSTRATION OF PERFORMANCE 

 9.2.1 The initial demonstration of performance is used to characterize 

 instrument performance (determined of LCRs and analysis of QCS). 

 9.2.2 Linear Calibration Range (LCR) – The LCR must be determined 

 initially and verified whenever a significant change in instrument 

 response is observed or expected. The initial demonstration of linearity 

 must use sufficient standards to insure that the resulting curve is linear. 

 The verification of linearity must use a minimum of a blank and three 

 standards. If any verification data exceeds the initial values by ± 10%, 

 linearity must be reestablished. If any portion of the range is shown to 

 be nonlinear, sufficient standards must be used to clearly define the 

 nonlinear portion. 

 9.2.3 Quality Control Sample (QCS) – When beginning the use of this 

 method, on a quarterly basis or as required to meet data-quality needs, 

 verify the calibration standards and acceptable instrument 

 performance with the preparation and analysis of a QCS. If the 

 determined concentrations are not within ± 10 % of the stated values, 

 performance of the determinative step of the method is unacceptable. 

 The source of the problem must be identified and corrected before 

 continuing with on-going analyses. 

9.3 ASSESSING LABORATORY PERFORMANCE 

 9.3.1 Laboratory Reagent Blank (LRB) – The laboratory must analyze at 

 least one LRB with each batch of samples. Data produced are used to 

 assess contamination from the laboratory environment. 

 9.3.2 Instrument Performance Check Solution (IPC) – For all 

 determinations, the laboratory must analyze the IPC (A midrange 

 check standard) and a calibration blank immediately following daily 

 calibration, after every tenth sample (or more frequently, if required) 

 and at the end of the sample run. Analysis of the IPC solution and 

 calibration blank immediately following calibration must verify that the 

 instrument is within ± 10 % of calibration. Subsequent analyses of the 

 IPC solution must verify the calibration is within ± 10 %. If the 

 calibration cannot be verified within the specified limits, reanalyze the 

 IPC solution. If the second analysis of the IPC solution confirms 
8

 calibration to be outside the limits, sample analysis must be 

 discontinued, the cause determined and/or in the case of drift the 

 instrument recalibrated. All samples, following the last acceptable IPC 

 solution, must be reanalyzed. The analysis data of the calibration blank 

 and IPC solution must be kept on file with the sample analyses data. 

 NOTE: Secondary calibration standards (SS) may also be used as the 

 IPC. 

 9.3.3 Where additional reference materials such as Performance Testing 

 samples are available, they should be analyzed to provide additional 

 performance data. The analysis of reference samples is a valuable 

 tool for demonstrating the ability to perform the method acceptably. 

10.0 CALIBRATION AND STANDARDIZATION

 10.1 Turbidity Calibration: The manufacturer’s operating instructions should 

 be followed. Measure standards on the turbidimeter covering the range 

 of interest. If the instrument is already calibrated in standard turbidity 

 units, this procedure will check the accuracy of the calibration scales. At 

 least one standard should be run in each instrument range to be used. Some 

 instruments permit adjustments of sensitivity so that scale values will 

 correspond to turbidities. Solid standards, if used, must be protected from 

 surface scratches which may cause potential calibration changes. If a pre- 

 calibrated scale is not supplied, calibration curves should be prepared for 

 each range of the instrument. Calibration must be performed under 

 identical optical conditions as operational conditions. The unit may 

 optionally incorporate a Mitchell/Mersch self-check system for verification 

 of operation. 

 

11.0 PROCEDURE:

 11.1 For on-line use, all instrument parameters should be set to insure constant sample 

 flow is fed to the instrument. This will insure bubble interferences are minimized. 

 Refer to the manufacturer’s instrument manual for the set up of measurement 

 parameters and flow rates. 

 11.2 Turbidities less than 40 NTU: In instruments other than on-line, allow samples to 

 come to room temperature before analysis. Mix the sample to thoroughly disperse 

 the solids. Wait until air bubbles disappear, and then pour the sample into the 

 turbidimeter tube. Read the turbidity directly from the instrument scale or from the 

 appropriate calibration curve. 

12.0 DATA ANALYSIS AND CALCULATIONS:

 12.1 Multiply sample readings by appropriate dilution to obtain final reading. 
9

 12.2 Report Results as follows: 

NTU Record to Nearest

0.0 – 1.0 .01 

1 – 10 0.1 

10 – 40 1. 

40 – 100 5. 

100 – 400 10. 

400 – 1,000 50. 

≥1,000 100. 

 

13.0 METHOD PERFORMANCE:

 In an intra laboratory test facility the following readings for precision and accuracy were 

 established for M5271: 

 True Value Sample Instrumentation Average Standard Deviation

 ______________________________________________________________________ 

 0 .015 .0012 

 .1 .097 .0004 

 .3 .297 .001 

 .5 .506 .0012 

 5 5.044 .005 

 10 10.022 .009 

 20 20.048 .054 

 40 39.971 .482 

 

14.0 POLLUTION PREVENTION:

 14.1 Pollution prevention encompasses any technique that reduces or eliminates the 

 quantity or toxicity of waste as the point of generation. Numerous opportunities for 

 pollution prevention exists in laboratory operation. The EPA has established a 

 preferred hierarchy of environmental management techniques that places pollution 

 prevention as the management option of first choice. Whenever feasible, laboratory 

 personnel should use pollution-prevention techniques to address their waste 

 generation. When wastes cannot be feasibly reduced at the source, the Agency 

 recommends recycling as the next best option. 

 14.2 The quantity of chemicals purchased should be based on expected usage during its 

 shelf life and disposal cost of unused material. Actual reagent preparation volumes 

 should reflect anticipated usage and reagent stability. 

 

15.0 WASTE MANAGEMENT: 

 15.1 The U.S. Environmental Protection Agency requires that laboratory waste 

 management practices be conducted consistent with all applicable rules and 

 regulations. Excess reagents, samples and method process wastes should be 
10

 characterized and disposed of in an acceptable manner. The Agency urges 

 laboratories to protect air, water, and land by minimizing and controlling all 

 releases from hoods and bench operations; complying with the letter and spirit of 

 any waste discharge permit and regulations; and by complying with all solid 

 hazardous waste regulations, particularly the hazardous waste identification rules 

 and land disposal restrictions. 

16.0 REFERENCES:

 16.1 Annual Book of ASTM Standards, Volume 11.01 Water (1), Standard D1889-88A, 

 p. 359, (1993). 

 16.2 Standard Methods for the Examination of Water and Wastewater, 18th Edition, 

 pp.2-9, Method 2130B (1992). APHA, AWWA, and WEF. Port City Press, 

 Baltimore, MD. 

 16.3 McKee, Gerald F., (1985); Turbidity Standard, EPA Record.