DETERMINATION OF TURBIDITY BY 360o NEPHELOMETRY

Hach Method 10258

Revision 1.0

January, 2016

Hach Company

5600 Lindbergh Drive

Loveland, Colorado 80538
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Hach Method 10258

DETERMINATION OF TURBIDITY BY 360o NEPHELOMETRY

Revision 1.0

January 2016

1.0 SCOPE AND APPLICATION

1.1 This method covers the determination of turbidity from colorless, membrane filtered drinking 

water, conventional filtered drinking water, ground water, surface water, that has a turbidity of 

less than or equal to 40,000 mNTU.

1.2 The applicable turbidity measuring limit for this method is 0.5 mNTU. Note that regulatory 

minimum reporting limit for turbidity may be higher.

1.3 This method may be used for compliance monitoring under the Safe Drinking Water Act 

(SDWA) 40 CFR 141 and the USEPA Long-term Treatment Rule, LT1/LT2.

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 the scattered light, the higher the turbidity. Measurements in mNTU are 

made in a 360o nephelometer designed according to specifications outlined in Section 6 of this 

method. A primary standard suspension of Formazin or stabilized Formazin is used to verify the 

analytical system’s on-going accuracy and stability of measurement (Section 9).

3.0 DEFINITIONS

3.1 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.

3.2 LINEAR CALIBRATION RANGE (LCR) - The turbidity range over which the instrument 

response is linear.

3.3 PRIMARY CALIBRATION STANDARD (PCAL) - A Formazin suspension prepared from the 

primary dilution stock standard suspension or a stabilized Formazin standard such as StablCal. 

The PCAL suspensions are used to calibrate the instrument response with respect to turbidity.

3.4 QUALITY CONTROL SAMPLE (QCS) – A Formazin suspension prepared from a source that is 

different from the primary standards. This may be from an alternate preparation of the stock 

standard suspension (SSS) or an alternate lot of a commercial preparation such as StablCal

(PCAL). 
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3.5 SECONDARY CALIBRATION STANDARD (SCAL) – A manufacturer supplied standard for 

checking instrument performance between required calibration checks with a primary standard.

An example would be optical glass rod that is calibrated by the manufacturer or user to the 

turbidity of a primary standard. 

3.6 STOCK STANDARD SUSPENSION (SSS) - Concentrated Formazin solution (40,000 mNTU)

that is used to prepare diluted primary calibration and calibration verification standards.

4.0 INTERFERENCES

4.1 Condensation around sample vials and light path materials may cause inaccurate readings.

4.2 The presence of true color (i.e., humic substances) will cause turbidity readings to be low, 

although this effect is generally not significant in finished treated drinking water.

4.3 Light-absorbing materials such as activated carbon in significant concentrations may interfere 

with turbidity readings.

5.0 SAFETY

5.1 The toxicity or carcinogenicity of each reagent used in executing 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 field and laboratory facility 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 MSDS documents should be made available to all personnel involved in the chemical analysis. 

A formal safety plan should also be made available.

5.3 Hydrazine sulfate 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

6.1 Turbidimeter

6.1.1 The turbidimeter shall be a 360o nephelometer.

6.1.2 The scattered light produced from the sample shall be collected at an angle of 90° to the 

incident light and 360o around the sample vial. 

6.1.3 The energy and stability of the light source and detector shall be sufficient to meet the 

performance criteria of the measuring limit in Section 1.2, sensitivity/resolution of data 

reporting in Section 13.1, and percent relative standard deviation of spike recovery in 

Section 14.1 of this method.
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6.1.4 The spectral bandwidth (full width at half maximum) of the incident radiation from a 

non-incandescent light source shall be contained in the range of 630 to 690 nm.

6.1.5 The divergence from parallelism of the incident radiation shall be less than ± 1.5° in the 

measurement volume.

6.1.6 The distance traversed by incident light shall be ≤ 10 cm in the measurement sample.

6.1.7 A Light Collector shall be centered at 90° to the incident light path not to exceed 30°

from its center. The scattered light shall be collected around 360° to the axis of the 

incident light. 

6.1.8 A means of directing the light collected at the Light Collector to a photodetector 

responsive to the full spectrum of the incident light. 

6.1.9 A readout device indicating the scattered light collected at 90° to the incident light. 

6.1.10 The volume of the sample vial to be ≤ 20 mL.

6.2 Sample Vial and Optical Glass Rod - The sample vial and optical glass rod to be used with the 

turbidimeter must be of clear, colorless optical glass. 

6.2.1 Vials and rods are to be kept scrupulously clean, both inside and out, and discarded when 

they become scratched or etched. 

6.2.2 The vials and rods 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. 

7.0 REAGENTS AND STANDARDS

7.1 Reagent water for sample dilutions and facility prepared Formazin calibration standards, 

turbidity-free to less than 30 mNTU.

7.2 StablCal Primary Calibration Standard Suspensions or equivalent.

7.3 Hach Optical Glass Rod or equivalent.

7.4 User Prepared Stock Standard Suspension (Formazin only).

7.4.1 Dissolve 1.00 g hydrazine sulfate, (NH2)2.H2SO4, (CASRN 10034-93-2) in reagent water 

and dilute to 100 mL in a volumetric flask. CAUTION -- carcinogen.
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7.4.2 Stock Standard Suspension - Dissolve 10.00 g hexamethylenetetramine (CASRN 100-97-

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

7.4.3 Intermediate Suspension - In a 100 mL volumetric flask, add 5.0 mL of each stock 

solution in 10.5.1 and 10.5.2. Allow to stand 24 hours at 25 ± 3°C, then dilute to the mark 

with reagent water.

7.4.3.1 Primary Calibration Formazin Standards - Mix and dilute 10.0 mL of the 

Intermediate Suspension (10.5.3) to 100 mL with reagent water. The turbidity of 

this suspension is defined as 40,000 mNTU. For other values, mix and dilute 

portions of this suspension as required.

7.4.3.2 A new intermediate suspension should be prepared each month. Primary 

calibration standards should be prepared daily by dilution of the stock standard 

suspension.

8.0 GRAB SAMPLES FOR DILUTION AND ANALYSIS

8.1 Collect each sample in a soft or hard plastic or glass container. Immediately refrigerate or ice the 

sample to 4oC and analyze within same day of collection.

8.2 Prolonged time to analysis of grab samples may yield different turbidity results than in-line 

samples, as turbidity in water is unstable.

9.0 QUALITY CONTROL

9.1 Each facility using this method for regulated environmental monitoring is required to operate a 

formal quality assurance/control program. The minimum initial requirements of this program 

consist of the demonstration of the facility’s capability with this method by analyzing an LCR 

and QCS. 

9.1.1 Linear Calibration Range (LCR) 

9.1.1.1 Verify the analytical system’s linearity (light source, sample vial, and detector) 

using either StablCal or Formazin Calibration Standard as described in Section 7. 

The calibration verification linearity test solution should be in the range of 

10,000 mNTU to 800 mNTU. Lower level calibration standard solutions may be 

used if the accuracy can be established within 10% of its stated value.

9.1.2 Quality Control Sample (QCS)

9.1.2.1 When beginning the use of this method, on a quarterly basis or as required 

complying with data-quality needs, verify the calibration standards and 

acceptable instrument performance with the preparation and analysis of a QCS as 
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defined in Section 3.4. 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.

10.0 CALIBRATION AND CALIBRATION VERIFICATION STANDARDIZATION

10.1 Calibration

10.1.1 Using StablCal or Formazin (PCAL), calibrate the instrument following the 

manufacturer’s operating instructions.

10.2 Calibration Verification

10.2.1 The calibration verification standard (PCAL or SCAL) should be in the range of 10,000 

mNTU to 800 mNTU. Lower level calibration standards may be used if their accuracy 

can be independently established within 10% of its stated value.

10.2.2 The measured value of the PCAL or SCAL standard shall be within 10% of its stated 

value. If the measured value greater than 10%, repeat the test with a new standard and a 

clean test vial.

10.2.3 If upon following step in 10.2.2 the calibration verification continues to fail, perform a 

new calibration as described in Section 10.1.1 and verify calibration as in Section 10.2.2.

11.0 TURBIDITY MEASUREMENT PROCEDURE

11.1 Follow the manufacturer’s instructions for proper set up and operation of the instrument.

11.1.1 Turbidities exceeding 40,000 mNTU: Dilute the sample with one of more volumes of 

distilled water or membrane filtered water until the turbidity falls below 40,000 mNTU. 

The turbidity of the original sample is then computed from the turbidity of the diluted 

sample and the dilution factor. For example, if 5 volumes of 0.2 μm filtered distilled 

water were added to 1 volume of sample, and the diluted sample showed a turbidity of 

30,000 mNTU, then the turbidity of the original is calculated to be 180,000 mNTU. 

Samples requiring dilution shall be analyzed using static flow conditions.

11.2 Calibration Verification and Calibration Frequency

11.2.1 Calibration verification test frequency is dependent on the conditions of the sample vial 

(biofilm, scratches in the wall surface) and the less likely change in light source and 

detector stability. 

12.0 CALCULATIONS 
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12.1 No analyst calculations are required for undiluted sample analysis as the processor has a built-in 

sample calculator.

12.2 Matrix Sample Dilution Calculation – An additional calculation is required for diluted samples.

12.2.1 Dilution Formula: 

T = Ax where: T = Final calculated mNTU turbidity result

A = mNTU result from diluted sample 

x = (B + C) / C = Dilution factor

B = volume of dilution water, in mL

C = sample volume taken for dilution, in mL

12.3 Example of Dilution Calculation:

Measurement from diluted sample (A) – 35 mNTU

Sample volume to be diluted (C) – 10 mL

Volume of dilution water (B) – 990 mL

Dilution factor (x) = (990 + 10) / 10 = 100

Final Turbidity (T) = 35 mNTU x 100 = 3,500 mNTU

13.0 DATA REPORTING

13.1 The recommended rounding of data for regulatory reporting purposes is presented in the 

following table: 

Turbidity Range (mNTU) Record to the Nearest 

Value (mNTU)

Water Treatment 

Application

0.0 – 100 0. 1 Membrane Filtration

100 – 1000 1 Membrane Filtration

100 – 1000 3 Conventional Filtration

1000 – 10000 10 Conventional Filtration

10000 – 40000 20 Conventional Filtration
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14.0 METHOD PERFORMANCE

14.1 In a single laboratory using RO membrane filtered and conventional filtered water, the following 

average percent recovery of turbidity spike and relative standard deviation was determined with 

Hach Method 10258.

Single Facility Matrix Spike Recovery

Water Type

Number of 

Measurements at 

Each Spike Level

Spike Value 

(mNTU)

Average % 

Recovery of 

Spike

% RSD of 

Recovery

Membrane 

Filtration 10

1.5 102 3.1

111 103 1.2

555 102 0.4

Conventional 

Filtration 10

1.4 102 12

111 94.0 0.3

431 97.5 0.1

14.2 In the multiple-facility validation study using membrane filtered and conventional filtered 

finished water, the following average percent recovery and relative standard deviation of turbidity 

spike was determined with Hach Method 10258.

Multiple-facility Matrix Spike Recovery

15.0 POLLUTION CONTROL

15.1 Pollution prevention encompasses any technique that reduces or eliminates the quantity or 

toxicity of waste at the point of generation. Numerous opportunities for pollution prevention exist 

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 

Water Type Spike Value (mNTU)

Average % 

Recovery of 

Spike

% RSD of 

Recovery

Membrane 

Filtration

1.5 93.3 11

100 106 1.9

500 104 0.7

Conventional 

Filtration

1.5 – 2.5 92.9 5.3

100 104 1.5

500 106 0.6
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address their waste generation. When wastes cannot be feasibly reduced at the source, the Agency 

recommends recycling as the next best option.

15.2 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.

16.0 WASTE MANAGEMENT

16.1 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 the 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 and hazardous waste 

regulations, particularly the hazardous waste identification rules and land disposal restrictions. 

For further information on waste management consult the "Waste Management Manual for 

Laboratory Personnel," available from the American Chemical Society at the address listed in 

Section 15.3.

17.0 BIBLIOGRAPHY

1. Annual Book of ASTM Standards, Volume 11.01 Water (1), Standard D1889-88A, p. 359, 

(1993). 

2. Standard Methods for the Examination of Water and Wastewater, 18th Edition, pp. 2-9, 

Method 2130B, (1992). 

3. Standard Methods for the Certification of Laboratories Analyzing Drinking Water: Criteria 

and Procedures, Quality Assurance, EPA/570/9-90/008, April, 1990. 

4. Hach Method 10133, Determination of Turbidity by Laser Nephelometry, Hach Company, 

5600 Lindbergh Drive, Loveland, Colorado 80538.

5. Guidance Manual for Compliance with the Interim Enhanced Surface Water Treatment Rule: 

Turbidity Provisions, United States Environmental Protection agency, Office of Water 

(4607), EPA 815-R-99-010, April 1999.

6. Final Long Term 1 Enhanced Surface Water Treatment Rule, United States Environmental 

Protection Agency, EPA 815-F-02-001, January 2002.

7. Long Term 2 Enhanced Surface Water Treatment Rule, United States Environmental 

Protection Agency, January 5, 2006.