What is the Methods used in Sampling and Measuring Water Quality parameters?

Specific parameters are usually analyzed in water quality studies. These specific parameters require different conditions for their determination. In this unit, we discuss the methods used in the collection of water samples and the precautions needed.

Methods used in Sampling and Measuring Water Quality parameters

Sampling for water quality parameters constitutes an important step before the analysis of water. This activity must be carried out carefully to avoid the introduction of errors to such analyses.

Methods used in Sampling and Measuring some Water Quality Parameters

• Measurement of electrical conductivity

When using the hand-held meter, keep it in gentle motion through the water column while taking a reading. Allow several minutes for the meter to stabilize.

Take measurements about 10 cm below the water surface and about 10 cm above the sediment surface where water is deep.

• Dissolved oxygen

For in situ DO measurement with a hand-held meter, use a similar method of sample collection as for electrical conductivity and avoid excessive turbulence to minimize air bubbles in the water.

Allow several minutes for the meter to stabilize before taking measurements about 10 cm below the water surface and above the sediment surface, or a mid- water column reading in shallow water.

Record DO measurements in mg/L (dissolved oxygen concentration) or % (saturation). This in-situ DO measurement is done in the field. Winkler’s method can also be used in the laboratory after fixing water samples in the field immediately after collection

• PH

For in situ field measurement of pH using hand-held meters and laboratory analyses, the same precautions are necessary for electrical conductivity and DO.

Directly collect water into a sample bottle or transfer it into a sample bottle from the collection vessel. Fill completely to exclude air bubbles.

Ensure the sample bottle is pre-rinsed thrice with sample water (3 × 20mL) before final collection and do not freeze but refrigerate at 1-4°C.

Analyze directly or as soon as possible after sample collection and preferably in the field, but within 6 hours if the sample is refrigerated at 1-4°C.

Measure pH electrochemically using a combination electrode (glass plus reference electrode) and calibrated against two or three buffer solutions (APHA, 1998).

• Salinity

Take a 200 ml unfiltered water sample directly in a pre-cleaned plastic bottle with a Teflon liner or transfer it into a sample bottle from a collection vessel.

Pre-rinse water sample bottles thrice with sample water (3 × 20ml) before final collection. Refrigerate at 1-4°C but do not freeze. Avoid excessive turbulence during collection, fill container completely to the top to exclude air and cap tightly.

Analyze directly or as soon as possible after sample collection, but within 24 hours. The unit of measurement for salinity is parts per thousand (ppt or ‰). It is preferable to perform this test in the field. Avoid plastic sample bottles made from low-density polyethylene (LDPE) as they leak easily.

High-density polyethylene (HDPE), polypropylene, polycarbonate, or a fluoropolymer (e.g. Teflon) are suitable.

• Temperature

Measure temperature using a thermometer with a range of 0-50°C or a suitable electronic thermometer. Place the probe in the water to be measured. Allow stabilizing before taking readings. Record temperature measurements in Degrees Celsius (°C).

• Turbidity

Measurements using probes should be at least 1m below the water surface and deeper in clear waters to ensure that there is no influence from ambient light. For laboratory analysis, collect 250 ml unfiltered water samples using pre-cleaned plastic containers or glass bottles.

Pre-rinse sample bottles thrice with sample water (3 × 20mL) before final collection. Do not increase the turbidity of the water while collecting a sample by disturbing the bottom of the aquatic plants.

Store the container in the dark, and refrigerate at 1-4°C but do not freeze to avoid changes in turbidity. Avoid turbulence to minimize air bubbles in water and fill to just below the shoulder of the bottle. Analyze directly or as soon as possible after sample collection preferably in the field, but within 24 hours.

The Secchi disc is used to measure the depth of light penetration or turbidity of water for comparative purposes. The disc is a circular white plate made of non-corrosive rigid material, usually with a diameter of 30 ± 1 cm.

To reduce the effects of currents on the angle of view, a mass of 3.0 ± 0.5 kg is suspended below the centre of the disc on a rigid rod 15 cm long.

The disc is painted with quadrants in black and white waterproof paints and attached to a non-stretch rope, marked at appropriate intervals of depth with waterproof markings (10 cm in turbid water and 50 cm intervals in clearer water). Where the disc cannot be seen is where effective light penetration is extinguished.

The Secchi disk depth is a measure of the limit of vertical visibility in the upper water column and a direct function of water clarity.

High Secchi depth reading simply high water clarity and light penetration while low Secchi depth readings indicate reduced water clarity, limited light penetration, and primary production due to suspended particles, and algal blooms.

Highly colored waters (e.g. with tannins) have low Secchi transparency, which may not indicate poor water quality. The Secchi disk is prone to error if strong flows and clouds casting shade are present.

Conditions for measuring Secchi disc depth are:

a) Clear sky with the sun directly overhead or the sun is at your back to minimize reflection from the sun on the water and the sampler must wear sunglasses.

b) Take measurements on the protected side of the boat, with minimal waves or ripples.

c) The same person should record Secchi disc depth all through a sampling day to ensure consistency.

d) If the conditions vary from this ideal situation, record any differences in infield notes or the field observations form.

e) Tie the end of the rope onto a float (e.g. a bucket) to prevent accidental loss of the disc.

f) Lower the disc into the water in a position away from the shadow and record the depth at which the black/white interface on the disc just disappears from sight.

Raise the disc until it just becomes visible and records this depth to the nearest 10 cm, then lower it just to the point where the disc disappears again. The depths at disappearance and reappearance are averaged and referred to as the Secchi disc depth.

Read Also: Water Chemistry Elements and their Characteristics

• Sampling procedures for total suspended solids

Take 1L unfiltered sample in a pre-cleaned bottle or plastic container directly or from a collection vessel. Pre-rinse sample bottle thrice with sample water (3 × 20 ml) before final collection.

Avoid disturbing the water, aquatic weeds, or increasing air bubbles in the sample during collection. Fill the bottle to the shoulder and refrigerate at 1-4°C but do not freeze. Analyze sample directly or as soon as possible after collection, but within 24 hours. Do not hold samples longer than 7 days. The unit of measurement is mg total suspended solids/L.

• Sampling procedures for volatile suspended solids

Use the same procedures as for total suspended solids to collect samples for volatile suspended solids. The unit of measurement is mg/L (mg volatile suspended solids/L). Volatile solids can be determined by ignition at 550°C (2540-E, APHA, 1998). VSS is determined as the weight of the lost material on ignition at 550°Ccompared to constant weight at 105°C.

• Total nitrogen (TN)

Total nitrogen includes all forms of nitrogen- nitrate, nitrite, ammonia, and organic nitrogen. The concentration of nitrogen can be used to assess nutrient status in waterways.

Enrichment by nitrogenous compounds can cause nuisance or toxic algal blooms. Sources of nitrogen enrichment include fertilizers, animal wastes, sewage, nitrogen-fixing plants, and lightning.

Collect 200ml unfiltered water in a pre-cleaned plastic container or bottle directly or similarly transfer from the collection vessel to total and volatile suspended solids. Fill to just below the shoulder of the bottle. Store at 1-4°C or freeze and store in the dark.

Analyze within 24 hours if the sample is kept refrigerated at 1-4°C or within 30 days if kept frozen below -20°. Units of measurement mg/L (mg nitrogen/L).

The method of analysis is the Persulphate digestion method 4500-N C. (APHA, 1998), and the automated cadmium reduction method 4500-NO3- F (APHA, 1998).

• Total oxidized nitrogen

Total oxidized nitrogen is the sum of the nitrate (NO3-) and nitrite (NO2-) expressed as concentrations in mg/L nitrogen. The nitrite and nitrate species can be determined separately.

Total oxidized nitrogen (NOx-N) = [Nitrate (NO3-) +Nitrite (NO2-)] Nitrite is an intermediate form of nitrogen and is rapidly oxidized to nitrate. Nitrate is an essential plant nutrient and its levels in natural waterways are typically low (less than 1 mg/L).

Excessive amounts of nitrate can cause water quality problems and accelerate eutrophication, altering the densities and types of aquatic plants in affected waterways. Some bacteria convert nitrate into gaseous nitrogen through denitrification. Nitrate is determined by the automated cadmium reduction method 4500-NO3- F (APHA, 1998).

Take a 125 ml filtered water sample in a new pre-cleaned plastic container or bottle. Collect the sample in a clean sample container before filtration. Place the filtered sample in a different sample bottle, after rinsing thrice with filtered sample water (3 × 20 ml) before final collection.

Fill to just below the shoulder of the bottle. Filter the sample through 0.45µm pore diameter cellulose acetate (membrane) filter C. Analyse within 24 hours if the sample is kept refrigerated at 1-4°C or within 30 days if kept frozen below -20°C. The unit of measurement is mg/L (mg oxidized nitrogen as nitrogen/L).

• Sampling procedures for nitrogen as ammonia/ammonium

Ammonia nitrogen and ammonium nitrogen species are determined using the same analytical method. Ammonia and ammonium exist in equilibrium in an aqueous solution. In alkaline solutions, ammonia’s dominant species is ammonia (NH3), while ammonium (NH4+) predominates at lower pH. Sources of ammonia include fertilizers and the decomposition of organic matter.

Collect 125 ml filtered water sample in a pre-cleaned plastic container or bottle. You can collect the sample in a clean container before filtration. Filter the sample through 0.45 µm pore diameter cellulose acetate (membrane) filter C.

Place the filtered sample in a different sample bottle, after pre- rinsing thrice with filtered water sample (3x20ml) before final collection. Fill to just below the shoulder of the bottle and refrigerate at 1-4°C or freeze and store in the dark.

Analyze within 24 hours if the sample is filtered and kept refrigerated at1-4°C or within 30 days if filtered and kept frozen below -20°C.

Unit of measurement mg/L (mg N/L). The analysis method is the automated phenate method 4500-NH3 G. (APHA, 1998).

• Total organic nitrogen (TOrgN)

Calculate total organic nitrogen from the concentrations of total nitrogen, nitrite, nitrate, and ammonium nitrogen, by subtracting the concentrations of inorganic fractions of nitrogen, namely nitrite and nitrate (NOx) and ammonium nitrogen (NH3-N/NH4-N) from the total nitrogen (TN) concentration:

TOrgN = TN – (NOx + NH3-N/NH4-N).

• Total Kjeldahl nitrogen (TKN)

Kjeldahl nitrogen describes all dissolved nitrogen in the tri-negative oxidation state (e.g. ammonium, ammonia, urea, amines, amides, etc) and comprises all the dissolved nitrogen except some inorganic species (nitrite and nitrate) and organic compounds (azo-compounds, nitriles, oximes, etc).

The Kjeldahl method hydrolyses all the amino nitrogen to ammonium, which is then measured by the ammonium/ammonia method.

Assuming that the concentrations of many of the other nitrogen species are very low, the TKN concentration is therefore approximately equal to the TN concentration less the nitrite and nitrate concentrations.

Alternatively, the TKN concentration is approximately equal to the sum of the total organic nitrogen and ammonia/ammonium as nitrogen concentrations. Calculate TKN (total) directly by subtracting nitrate and nitrite from total nitrogen (TN) on an unfiltered sample.

The Kjeldahl determination is rarely used because it is not as precise as the persulphate digestion method used to calculate TN. It also uses mercuric sulfate-sulphuric acid digest, leaving mercury as an undesirable waste product.

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• Dissolved organic nitrogen (DOrgN)

Calculate dissolved organic nitrogen (DOrgN) by analyzing TN in a filtered sample and subtracting the NH3-N/NH4-N and NOx-N (i.e. the dissolved inorganic fractions of nitrogen) from the result.

Take 125 ml filtered sample in a pre-cleaned plastic container or bottle like that used for total oxidized nitrogen. Refrigerate at 1-4°C or freeze and store in the dark. Analyze within24 hours if the sample is filtered and kept refrigerated at1-4°C or within 30 days if filtered and kept frozen below -20°C.

Unit of measurement is mg/L (mg DOrgN as nitrogen/L). The analysis method of Total nitrogen is by persulphate digestion method 4500-N C. (APHA, 1998) and the automated cadmium reduction method 4500-NO3- F (APHA, 1998).

• Total phosphorus (TP)

Phosphorus occurs in natural waters and wastewaters as phosphates. These are classified as orthophosphates (PO43-), condensed phosphates (pyro-, meta-, and other polyphosphates), and organically bound phosphates.

They occur in solution, in particles or detritus, and in bodies of aquatic organisms (APHA, 1998). Sources of phosphorus enrichment include detergents, fertilizers, animal feces/ manure, sewage, and industrial wastes.

High levels of phosphorus and/or other key nutrients may lead to related problems such as nuisance or toxic algal blooms, although some waterways are naturally eutrophic (nutrient enriched).

To sample for total phosphorus, collect 200 ml of unfiltered water in a pre-cleaned Teflon -capped bottle or plastic directly or from a collection vessel. Fill to just the shoulder of the bottle. Ensure the sample bottle is pre-rinsed thrice with sample water (3 × 20mL) before final collection. Refrigerate at 1-4°C or freeze, and store in the dark.

Analyze immediately or within 24 hours for samples refrigerated at 1-4°C and stored in the dark. Analyze within 30 days if kept frozen below -20°C. The unit of measurement is mg/L (mg phosphorus/L). The analysis method is persulphate digestion method 4500-P B.5. (APHA, 1998), and the automated ascorbic acid reduction method 4500-P F. (APHA, 1998).

• Total organic carbon (TOC)

The total organic carbon (TOC) concentration represents all the carbon covalently bonded in organic molecules and so not filtered.

Total organic carbon does not take into account the oxidation state of the organic matter and does not measure other organically bound elements, such as nitrogen, hydrogen, and inorganics that can contribute to the oxygen demand measured by biological oxygen demand (BOD).

Drinking water TOC concentrations range from less than 100 µg/L to more than 25 mg/L. Wastewaters may contain very high levels of organic carbon(>100mg/L).

Take 125 ml of unfiltered sample in a new and pre-cleaned amber Teflon-lined capped bottle free from organics. The sample bottle should be pre-rinsed thrice with sample water(3 × 20 mL) before final collection. Fill the bottle completely to exclude air bubbles.
Refrigerate at 1-4°C, do not freeze, and store in the dark. Add 10% sulphuric acid (H2SO4) in the field until the pH is < 2 to acidify the sample.

Carry out the analysis as soon as possible but the maximum holding time for acidified sample is 7 days. Refrigerate at 1-4°C and store in the dark. Do not freeze. The unit of measurement is mg/L (mg carbon/L or µg non-purgeable organic carbon/L).

The analysis method for total organic carbon is by high-temperature combustion and IR detection,
method 5310 (APHA, 1998). Inorganic carbon must be removed before analysis so volatile organic carbon will be lost. Report as non-purgeable organic carbon.

• Dissolved organic carbon (DOC)

Dissolved organic carbon (DOC), represents all the soluble organic carbon (or carbon covalently bonded in organic molecules) that can pass through a 0.45 µmpore diameter filter.

Dissolved or total inorganic carbon (TIC, or DIC) represents the carbonate (CO32), bicarbonate (HCO3-), and dissolved carbon dioxide (CO2) present in a sample.

Interference of DIC in the measurement of DOC is removed by acidifying the sample to a pH of less than 2 to convert all carbonates to CO2. The CO2 gas produced is purged from the sample before DOC analysis.

Removing CO2 purging from the sample also purges volatile organic carbon from the sample. Only non-purgeable organic carbon will be determined in the DOC measurement.

Take 125ml of filtered sample in a pre-cleaned glass (amber or brown) container free from organics. The cap should be lined with Teflon. Filter sample through 0.45 µm pore diameter cellulose acetate (membrane) filter.

Place the filtered sample into the sample bottle after rinsing three times with a filtered water sample (3x20ml) before final collection. Fill the container completely to the top to exclude air bubbles. Refrigerate at 1-4°C, do not freeze, and .tore in the dark.

Carry out the test as soon as possible or analyze it within 7 days if acidified, refrigerated at 1-4°C, and stored in dark. The unit of measurement is mg/L (mg carbon/L or µg non-purgeable organic carbon/L). Analysis method Total organic carbon by high-temperature combustion and IR detection, method 5310 (APHA, 1998). Note that:

a) Samples should be filtered as soon as possible after sample collection, preferably on site. Filter paper should be washed with the sample first before filtration. Do not re-use filter paper.

b) If the sample has high particulate matter content, it may be necessary to pre-filter using a glass fibre filter paper (GFC 1.2 µm).

• Soluble reactive silica (SiO2-Si)

Diatoms use silica to build their cell walls and can become a nuisance if their numbers increase rapidly and cause a bloom. Therefore, it is important to quantify the amount of soluble reactive silica in estuaries and catchments.

Collect 250 ml of filtered sample in a pre-cleaned plastic or glass bottle before filtration. Filter sample through 0.45 µm pore diameter cellulose acetate (membrane) filter C. Place the filtered sample in the bottle, after rinsing three times with filtered sample water(3 × 20 ml) before final collection. Fill to the shoulder of the bottle.

Refrigerate at 1-4°C, do not freeze, and analyze within 24 hours if the sample is only kept refrigerated at 1-4°C or within 1 month for sample filtered and kept refrigerated at 1-4°C. Unit of measurement is mg/L (mg silica as silicon/L). Analysis method Automated method for molybdate-reactive silica method 4500-SiO2 D, E (APHA, 1998).

Note that:

a) Samples should be filtered as soon as possible after collection, preferably on site. Wash filter paper with sample before filtration. Do not re-use filter paper.
b) Avoid plastic sample bottles made from low-density polyethylene (LDPE). Use high-density polyethylene (HDPE), polypropylene, polycarbonate, or fluoropolymer (e.g. Teflon) bottles.
c) Do not pre-filter with glass fibre filter paper (GFC 1.2 µm) as these contain silica and can contaminate the sample.

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• Biochemical oxygen demand (BOD)

Biochemical oxygen demand is a measure of the amount of biologically and/or chemically degradable organic material in water. It indicates the amount of oxygen aerobic aquatic organisms could consume in the process of metabolizing all the organic matter available to them. High BOD causes low levels of dissolved oxygen in affected water.

Obtain a 1L unfiltered sample in a brown or amber plastic container or glass. Use new pre-cleaned bottles only and do not a pre-rinse container with the sample. Collect samples directly into a sample bottle or transfer them into a sample bottle from the collection vessel.

Fill the container to the top to exclude air. Analyze directly or as soon as possible after the sample is collected or within24 hours if the sample is refrigerated at 1-4°C in the dark. Do not freeze. The analysis method is the 5-day BOD test method 5210 B (APHA, 1998). The unit of measurement is mg/L. Dark (or amber) glass bottles are preferable for samples that are low in BOD (<5 mg L-1).

Keep samples at or below 4°C during compositing. Limit compositing period to 24 hours after sample collection. To assist in the preservation, refrigerate at 1-4°C and store in the dark but do not freeze. Plastic containers should be made from high-density polyethylene (HDPE), polypropylene, polycarbonate, or a fluoropolymer (e.g. Teflon).

• Metals — total and dissolved metals and metalloids

Many metals are toxic to aquatic animals and can bioaccumulate in aquatic food chains. Metals commonly determined include aluminum (Al), silver (Ag), arsenic (As), boron (B), barium (Ba), beryllium (Be), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), potassium (K), lithium (Li), magnesium (Mg), manganese (Mn), molybdenum (Mo), sodium(Na), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), tin (Sn), titanium (Ti), uranium (U), vanadium (V) and zinc (Zn).

Total metals can be analyzed by digesting the sample using a concentrated nitric/hydrochloric acid added to an unfiltered water sample prior to analysis (APHA, 1998)).

Dissolved metals are determined by analyzing those metals in a filtered sample that passes through a 0.45 µm membrane filter (APHA, 1998).

Before analysis of a field-filtered, field-acidified sample, extra dilute acid is added to the filtered sample, to ensure dissolution of any precipitates formed after filtration. The sample must not be filtered when determining total metals (which include those metals bound to the particulate matter in the sample); otherwise, the same collection procedure is followed.

For heavy metals;

a) Filter samples as soon as possible after collection preferably on site. Wash filter paper first with sample before filtration. Do not re-use filter paper.

b) Do not use plastic samples made from low-density polyethylene (LDPE). Appropriate sample container plastics are high-density polyethylene (HDPE), polypropylene, polycarbonate or fluoropolymer (e.g. Nalgene, Teflon (polytetrafluoroethylene, PTFE)).

c) 125 ml Nalgene bottles are now used for heavy metals as HDPE bottles have been found to leach trace quantities of Zn over time.

d) For soluble metals analysis, if the sample has high particulate matter content, it may be necessary to pre-filter using a glass fibre filter paper (GFC 1.2 µm).

e) Acidification is not needed for the analysis of Li, K, and Na in samples because these metals in solution are stable for 1 month without acidification.

• Total water hardness (as CaCO3)

Water hardness is a measure of the capacity of water to precipitate soap. This is mainly due to the presence of calcium and magnesium ions in the water.

Total hardness is now defined as the sum of calcium and magnesium concentrations in water, expressed as calcium carbonate equivalents in milligrams per liter according to the following formula (APHA, 1998):
Hardness equivalent CaCO3/L = 2.497 [Ca, mg/L] + 4.118 Mg, mg/L].

Take 125 ml of unfiltered sample in a plastic container plastic with a Teflon-lined cap. Use new pre-cleaned acid rinsed bottles. Decant from collection vessel, ensuring sample bottle is pre-rinsed thrice with sample water (3 × 20 mL) before final collection. Fill to the shoulder of the bottle. Samples can be stored for 7 days. Units of measurement are mg/L (mg CaCO3/L).

• Total acidity and total alkalinity (as CaCO3)

The total alkalinity of water is a measure of its acid-neutralizing capacity to a designated pH. It is the sum of all titratable bases-carbonates, bicarbonates, hydroxides, borates, phosphates, silicates, and other bases if present. For analysis of total alkalinity (APHA, 1998) requires titration with a standard hydrochloric acid solution to an end-point pH of 3.7 (i.e. the methyl orange endpoint).

Total acidity is a quantitative measure of the capacity of water to react with a strong base. To determine total acidity APHA, 1998 requires titration with a standard sodium hydroxide solution to an end-point of pH 8.3 (i.e. the phenolphthalein end-point).

Take 125 ml of unfiltered sample in new pre-cleaned bottles or plastic containers. Collect directly into a sample bottle or transfer into a sample bottle from the collection vessel. Pre-rinse sample bottle thrice with sample water (3×20mL) before final collection.

Fill the container completely to the top to exclude air and cap tightly. Refrigerate at 1-4°C but do not freeze and analyze within 1 day.

Both total acidity and alkalinity are expressed as mg/L (mg CaCO3/L). Both total alkalinity and total acidity require separate bottles for analysis. Analysis method acidity and alkalinity method 2310 B and 2320 B. (APHA, 1998).

• True color

Colouring water samples result from the presence of natural metallic ions (iron and manganese), humus and peat materials, plankton, weeds, and industrial wastes. True color refers to the color of water upon removal of suspended solids or when the sample has been filtered.

Take a 500 ml filtered water sample in pre-cleaned plastic or glass bottle with Teflon lined cap. Wash bottles in phosphate-free detergent and rinse thrice with tap water and thrice with de-ionized water.

Or, collect the sample in a clean sample container before filtration. Filter sample through 0.45µm pore diameter cellulose acetate(membrane) filter C. Place filtered water sample in a sample bottle, after rinsing three times three times with filtered sample water(3 × 20 mL) before final collection.

Refrigerate at 1-4°C, store, and transport in the dark without freezing. Fill to just below the shoulder of the bottle but do not fill. Analyze within 2 days. Units of measurement are color units or platinum-cobalt units (PCU, Pt/Co, or Pt-Co units).

Gilvin — color

Gilvin describes the natural dissolved organic carbon compounds that give water a brown coloration (absorb light in the 400-440 nm wavelength and reduce the blue region of the spectrum). Gilvin, or soluble humic color, is the major component absorbing light in inland waters.

There are many compounds and compound classes that makeup gilvin. The amount of light transmitted through the water is an important indicator of the potential for primary production within a wetland system. Measurement of color provides information on the light climate of a wetland and its capacity for primary productivity.

Take 500 ml filtered sample in pre-cleaned plastic or glass bottle directly or transfer from collection vessel. Pre-rinse bottle thrice with sample water (3 × 20mL) before final collection. Filter sample through 0.45µm pore diameter cellulose acetate (membrane) filter B.

Fill the container to the top to exclude air, cap tightly refrigerates at 1-4°C, store, and transport in the dark but do not freeze. Analyze within 2 days.

The unit of measurement is Gilvin440/m. Note that freezing may produce irreversible changes in gilvin color.