# Leck Mitchell , Ph.D., PE

11 Rhine Cour t .; 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 DET ERMINATION OF TURBIDITY BY LED OR LASER NEPHELOMETRY

Mitchell Method M5331

Revision 1.2

1.0 SCOPE AND APPLICATION :

1.1 This method covers the determ ination of turbidity in drinking

, ground, surface and any

col orless 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 turbid ity. 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 f ormazin 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 use d 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 pr operties, 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 suspensi on. 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 mater ials 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, tha t 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 establ ished. 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 regard ing 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 hydr azine 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 shoul d 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 o f 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 l ight source: Monochromatic source operated at a nominal

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

received reference for the scatte red 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 outp ut of the light source.

A Mitchell/Mersch light trap shall be incorporated for on

line 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 sa mple to the pho to 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 t urbidity ≤ 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 fla sk. 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

tu rbidity standards should be prepared daily.

### Mitchell Method 5331 , Revision 1.2 6 7.5 Styrene d ivinylbenzene 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 ins trument.

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 t o 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 Qua lity 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 oper ating 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 sc ale 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/Mers c h self

- check system for a secondary verification of

instrument calibration.

11.0 PR OCEDURE:

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.

1 2.0 DATA ANALYSIS AND CALCULATIONS :

12.1 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:

The following data was collected i n 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 level s ) :

Theoretical

(NTU) Average (NTU) Standard

Deviation % RSD Average %

Recovery

0.10 0.100 0.000 6 0.6 100

0.30 0.294 0.00 7 0.2 98

0.50 0.506 0. 007 0.1 101

5.0 5.0 1 0.00 8 0.1 1 00

10 9.97 0.00 5 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 hie rarchy 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

g eneration. When wastes cann ot 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

s helf life and disposal cost of unused material. Actual reagent preparation v olumes

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 spir it of

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

hazardous waste regulations, particularly the hazardous waste identification rules

and land d isposal 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 , 18 th 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.