Leck Mitchell, Ph.D., PE 

11 Rhine Court.; Grand Junction, CO 81507

Ph: 970-244-8661; ; 

Leckmitchell@Bresnan.net

DETERMINATION OF TURBIDITY BY LED OR LASER

NEPHELOMETRY

Mitchell Method M5331

Leck Mitchell, Ph.D., PE

Patricia Lacy Mitchell, Ph.D.

Revision 1.2

February 2016
Mitchell Method 5331, Revision 1.2 2

DETERMINATION OF TURBIDITY BY LED OR LASER NEPHELOMETRY

Mitchell Method M5331

Revision 1.2

1.0 SCOPE AND APPLICATION:

1.1 This method covers the determination of turbidity in drinking, ground, surface and any

 colorless water with a turbidity of less than 40 NTU.

 1.2 The applicable range is 0 to 40 NTU. 

 1.3 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 LED or 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 Any approved primary standard may be used in conjunction with this method. 

These standards include formazin polymer, stabilized formazin polymer and 

styrene divinylbenzene polymer suspensions.

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, 

 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 
Mitchell Method 5331, Revision 1.2 3

 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 

 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.
Mitchell Method 5331, Revision 1.2 4

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 LED or laser light source: Monochromatic source operated at a nominal 

 wavelength of 525 ± 15nm. The light source shall be used as a directly 

 received reference for the scattered nephelometric signal. 

 6.2.2 There shall be no divergence from parallelism as the incident 

 radiation and any convergence shall not exceed ±1 1⁄2 degrees in the

measurement area.

 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 90 degrees. 

 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 for online use.

 

 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. 
Mitchell Method 5331, Revision 1.2 5

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

 line use and Mitchell/Mersch anti-fog windows for on-line use. 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 for on-line use.

6.2.7 Turbidimeter example meeting these specifications is the Mitchell M5331.

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

6.5 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 necessary. Water should have a turbidity ≤ 0.03 

NTU. This value should be considered when preparing standards.

7.2 Stock standard suspension (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, concentrated stock 

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

 turbidity standards should be prepared daily.
Mitchell Method 5331, Revision 1.2 6

7.5 Styrene divinylbenzene polymer in commercially prepared, certified, concentrated 

stock or ready to use dilutions. Manufacturer’s instructions should be followed for 

choosing the appropriate standard for the instrument.

7.6 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 AND INSTRUMENT SETUP

8.1 Online instrumentation does not require sample cooling, preservation or 

 storage.

8.2 Install and set up the instrument according to the manufacturers instructions.

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.
Mitchell Method 5331, Revision 1.2 7

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 a secondary verification of 

 instrument calibration. 

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.

 

12.0 DATA ANALYSIS AND CALCULATIONS:

12.1Report 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:

 The following data was collected in using styrene divinylbenzene polymer standards (n=10 

for 0.10, 0.30, 0.50 and n=5 for 5.0, 10, 20 and 40 NTU levels): 

Theoretical 

(NTU)

Average (NTU) Standard 

Deviation

% RSD Average % 

Recovery

0.10 0.100 0.0006 0.6 100

0.30 0.294 0.007 0.2 98

0.50 0.506 0.007 0.1 101

5.0 5.01 0.008 0.1 100

10 9.97 0.005 0.1 100

20 20.0 0.09 0.4 100

40 40.0 0.2 0.4 100
Mitchell Method 5331, Revision 1.2 8

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 

 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.