Turbidity
Standards
This information supplied by:
D & A Instrument Company / Copyright 2005, all rights
reserved.
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In the
USA, formazin is the primary standard for the calibration
of turbidimeters and it is the one we use to certify
our instruments. The median particle size of formazin is 1.5
µm; the standard deviation of size is 0.6 µm (see
size distribution graph), and as shown by the SEM images below,
formazin particles have many different shapes. The preparation,
storage, and handling of formazin will affect its accuracy
and stability. Recommended formazin storage times are listed
in the accompanying table. Working standards are prepared
by volumetric dilution of 4000-NTU stock formazin with distilled
water. So for example, a 2000 NTU calibration standard is
made by mixing equal volumes of stock formazin and distilled
water.
| Turbidity
(NTU) |
Maximum
Storage Time |
1 -10 |
1 day |
2 - 20 |
1 day |
10 - 40 |
1 day |
20 - 400 |
1 month |
> 400 |
1 year |
Besides being the primary standard,
formazin has two other advantages. It is available from several
chemical and scientific suppliers (www.vwrsp.com,
www.ColePalmer.com,
www.riccachemical.com,
and www.labchem.net)
and it is the least-expensive, commercially available standard.
Formazin also has a couple of disadvantages, which include:
1) it has a MSDS health-hazard rating of 2, 2) turbidity can
vary by ± 2% from the lot to lot; 3) the size, shape,
and aggregation of formazin particles change with temperature,
time, and concentration; 4) it settles in storage and must
be mixed immediately prior to use, and 5) dilute formazin
standards have a storage life as short as one hour.
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AMCO Clear, supplied by GFS Chemicals (www.gfschemicals.com),
is the other approved calibration standard. It is made from
styrene divinylbenzene (SDVB) microspheres. SDVB spheres have
a median size and standard deviation of 0.28µm (~1/5
that of formazin particles) and 0.10 µm respectively
and a refractive index of 1.56. As shown on the SEM image,
they are dimensionally uniform. SDVB standards are formulated
especially for OBS meters and cannot be used with different
meters. Superior physical consistency of AMCO Clear results
in a more precise calibration standard, giving standard errors
less than 1% compared to 2.1% for formazin and better linearity,
0.15 NTU compared to 0.32 for formazin. |
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(Photo courtesy of GFS Chemicals)
The key benefits of SDVB
standards are: 1) < 1% lot-to-lot variation in turbidity;
2) consistent optical properties from 10 to 30o C; 3) guaranteed
one-year stability; 4) mixing and dilution are not required;
and 5) they are not toxic. Two drawbacks are that SDVB standards
can only be used with the instruments for which they are
made and they are more expensive than formazin. For example,
one liter of 4000-NTU standard costs about twice as much
as an equivalent amount of 4000-NTU formazin. Our instruction
manuals explain how to use turbidity standards and the instructions
provided by the suppliers tell how they should be handled.
We must emphasize that unlike SSC, which has physical units,
turbidity values (NTUs, FTUs, etc.) do not. Therefore, if
you measure water turbidity to be 100 NTU, you cannot directly
infer any physical quantities from it. Turbidity values
do not represent particular SSC values, indicate light levels
at the bottom of a stream, or quantify biological process’.
Moreover, it is often assumed that turbidity standards behave
optically like sediment. This is possible when the size,
NIR reflectivity, refractive index, and shape of the sediment
and the turbidity standard are similar, this is an extremely
rare occurrence. For example, even the median diameters
of the two approved calibration standards differ by a factor
of more that five and the shape of SDVB and formazin particles
also differ; see NTU-SSC relationships.
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Reference:
John Downing. 2005. Turbidity Monitoring.
Chapter 24 in: Environmental Instrumentation and
Analysis Handbook. John Wiley & Sons, Pages:
511-546. 2005.
Sadar, M. 1998. Turbidity Standards.
Hach Company Technical Information Series – Booklet
No. 12. 18 pages.
Papacosta, K. and Martin Katz. 1990. The Rationale
for the Establishment of a Certified Reference Standard for
Nephelometric Instruments. In: Proceedings,
American Waterworks Assoc. Water Quality Technical Conference.
Paper Number ST6-4, pp. 1299-1333.
Zaneveld, J.R.V., R.W. Spinrad, and R. Bartz. 1979. Optical
Properties of Turbidity Standards. SPIE Volume
208 Ocean Optics VI. Bellingham, Washington. pp. 159-158.
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