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The in situ temperature is usually measured directly or on samples taken with a water sampler (at depth) (e.g. Niskin bottles, Van Dorn Sampler) or a bucket (at surface).
Usually a mercury thermometer is used. Take care to wait for stabilization before reading.
In situ temperature at great depths are measured with a reversible thermometer, mounted on a sampler (e.g. Niskin bottle).
is defined as 0.3285234 times the weight of the silver precipitated as silver halides (halides are inorganic bounds of a metal element with fluorine (F), chlorine (Cl), bromine (Br) or jodine (I)) from 1 kg of sea water, all weighings being in vacuum.
is the quantity determined by volumetric methods and is defined as the weight of the silver precipitated as silver halides from 1000 ml of sea water at a stated temperature.
The content of dissolved salts in sea water is usually expressed as salinity (pro-mill or PSU, Practical Salinity Units). In practice, the salinity is defined in terms of chlorosity by the Knudsen equation:
S ‰ = 0.030 + 1.8050 Cl (g Cl/l) x 1/P
This equation is merely a definition.
P: density of sea water at that chlorisity
- A few drops of sea water are sufficient for salinity readings with a refractometer.
- A volume of 10 ml of sea water is required for the Mohr-Knudsen titration for salinity determination. Standard medicine bottles are ideal for collecting, handling and storage of samples for salinity determinations. After rinsing the bottles thoroughly with the sample, fill them up to the shoulder. They are stoppered using a waxed cork or a cork covered with Parafilm (plastic sheet) under these circumstances no changes in salinity are expected over a period of many years. Once the bottles opened, analysis should be performed within the hour.
The precipitable halogens in a 10 ml volume of sea water are quantified by titration with a silver nitrate solution using a chromate end point (the Mohr titration). The silver nitrate solution is standardized against 10 ml of sea water of known chlorosity.
AgNO3 + NaCl ==> AgCl (white precipitate) + NaNO3
K2CrO4 + 2AgNO3 ==> Ag2CrO4 (red colored precipitate) + 2KNO3
The appearance of the red color indicates the titration end point.
This method gives the chlorosity, which can be converted to chlorinity and salinity (see further).
- A 10 ml pipette
- A (preferably dark) 24 ml burette
- Use of an preferably automatic dispenser for the chromate indicator is advantageous.
- A 100 ml tall form beaker (with magnetic stirrer if available), placed against a white background.
To keep the glassware spotlessly clean: soak periodically for a few minutes in a cold 5 % solution of sodium hydroxyde (NaOH) in methyl alcohol (CH3OH), rinse with diluted nitric acid (HNO3); rinse with distilled water.
dissolve 16,48 g anhydrous sodium chloride (NaCl p.a.) in 1 l aq. dest
(this is equivalent to 10 g Cl/l)
dissolve 47,9 g silver nitrate (AgNO3 p.a.), in 1 l aq. dest
Store this solution in a dark bottle, shake well before use.
dissolve 20 g potassium chromate (K2CrO4 p.a.) in 250 ml aq. dest.
- 10 ml of standard sea water is brought into the 100 ml beaker
- dilute with a few ml aq. dest
- Add 5 drops of indicator solution
- Add AgNO3 from the burette drop per drop until you see a change in color: first orange, then red
Note the volume of AgNO3 added = A
10 ml of sample is titrated as in a)
note the volume of AgNO3 added = a
1. Chlorosity = Cl/l = 10 x a/A g Cl/l
2. Salinity S ‰ = 0.030 + (1.8050 x Cl/l x 1/P)
where: Cl/l is the chlorosity, and P is the density of sea water at that chlorosity
Conversion table: see in annex.
P.S.: It is assumed that the temperature of samples and reagents is the same and approximately 20° C at the time of the titration. If this is not the case, you need to involve a correction factor for the temperature (see Strickland and Parsons).
The refraction - index of a liquid increases with its salinity. This instrument allows determination of the salinity (S ‰) on a 0.02 ml sample. Precision 1 ‰. Adjustment for temperature is provided.
Artificial seawater can be obtained by dissolving sea-salt in deionized water.
Strong air-bubbling can be used to facilitate the dissolving of the salt.
This also causes a number of elements in the salt to react with each other and to form precipitates. These can be removed by filtration.
Dissolving 42 g salt/l gives a salinity of 35 ‰. Check for each brand.
To perform experiments at different salinities:
- Salinities lower than the available seawater:
dilute filtered seawater with aqua dest or prepare with sea-salt and deionized water.
- Salinities higher than the available seawater:
evaporate natural seawater: time consuming, unpractical.
prepare from sea-salt and deionized water.
pH can be measured on small (sub)samples of + 20 ml.
Put one or a few drops of the fluid to be analyzed on the indicator-paper.
After 1 min. the color is compared with the reference - scale. Precision is dependent on the interval-magnitude of this scale.
By means of a glass-electrode in combination with a reference electrode, pH values can be read.
The reference electrode is usually built- in in the glass-electrode.
- At each series of measurements first measure the temperature of the water and adjust the pH meter to this temperature.
- Calibration and measurement
* Ideally the temperature of the calibrating solution should be the same as that of the water to be analyzed.
* Adjust temperature setting of pH meter to the temperature of the fluid to be measured.
* Bring electrode in calibrating solution with pH = 7 and turn button till readout indicates 7.
* Rinse electrode with aqua dest.
* Rinse electrode with aqua dest.
* Dip calibrated and temperature adjusted electrode in water to be analyzed. Stir gently till readout stabilizes.
* Important !
After each series of measurements, rinse the electrode with aqua dest and keep it in 3M KCl solution. This has to be done even if the electrode is not used for even a few minutes !
Remember the glass membrane is easily damaged.
The aim of this section is give a brief overview of the principles of the most widely used methods for the determination of seawater alkalinity.
The alkalinity may be defined as the excess of anions of weak acids in seawater. In other words, alkalinity is a quantitative measure of the ability to react with H+ ions: the higher the alkalinity of a solution, the greater its capacity to react with H+.
In most natural waters, carbonate species are the predominant form of alkalinity, but other weak acids and a wide variety of dissolved or suspended materials may also contribute to alkalinity. Therefore, Total Alkalinity (TALK) is defined as:
TALK = [HCO3-] + 2[CO32-] + [B(OH)4-] + [OH-] - [H+] + [other strong bases]
The alkalinity can be titrated and therfore total alkalinity or titration alkalinity corresponds to the amount of strong acid in milliequivalents (or millimoles) required to neutralize 1 kg of seawater.
The samples are taken from an ordinary hydrocast and transferred to neutral glass bottles with rubber stoppers. The bottles should preferably be aged with hydrochloric acid for several months. Input or release of carbon dioxide has no effect on the total alkalinity, but has an effect on the total carbonate concentration. Biological activity may change the total alkalinity and therefore it is preferable that the analyses be carried out on board ship, if conditions permit. In the case of samples brought back to the laboratory, it is advisable to add the exact amount of acid and sample into the bottles immediately on board.
The original method by Gripenberg has been modified by different workers. However, the principle of the method involves acidification of the water sample with strong hydrochloric acid to a pH of about 3.5. Carbon dioxide is driven off by boiling. The solution is then back titrated with sodium hydroxide to pH 6 using bromothymol blue as an indicator. Carbon dioxide free air is bubbled through the sample during the titration. Only negligible amounts of boric acid are back titrated and thus
TALK = [H+ ]added - [OH- ]added
This method is essentially a single point potentiomentric titration of seawater. The sample is acidified to about pH 3.5. The pH is measured with a high precision instrument and the alkalinity can be calculated from difference between the amount of acid added and the excess acid present. The latter is obtained from already determined emperical activity coefficient values.
This method has been widely used and adapted to ship board conditions. From the potentiometric titration of seawater with hydrochloric acid, TALK, CO2 and pH can be evaluated. The calculations are rather complicated and usually a small computer is employed.
For your exercises, a complete automated titrator (DL53 TITRATOR) will be demonstrated to you and basic information about how the equipment works will be explained. You will also have an opportunity to measure your own samples with the titrator.
Since you are not expected to master the operations of the equipment from a single demonstration, an experienced staff will guide you while measuring your samples.
Concentration of oxygen can be measured on small (sub)samples (50 ml) taken in situ (cf. 1.I.I.). When sampling for oxygen concentration measurements, care has to be taken to avoid air-bubbles. To keep turbulence minimal, most sampling devices (e.g. Niskin bottles) are equipped with special taps.
When bringing samples into Winkler bottles, use a plastic tube. To open bottles without getting air-bubbles inside: remove cork very carefully, drop cork on top for closing.
° Mn(OH)2 is formed in the water to be analyzed:
2 MnSO4 + 4NaOH ==> 2Na2SO4 + 2Mn(OH)2
° Mn(OH)2 uses the free O2 in the water to form MnO(OH)2
2 Mn(OH)2 + O2 ==> 2MnO(OH)2
° In the presence of KI, addition of a strong acid (H2SO2) liberates I2
2 MnO(OH)2 + 4H2SO4 + 4KI ==> 2MnSO4 + 2K2SO4 + 6H2O + 2I2
The amount of liberated I2 is equivalent with the dissolved O2 in the water :
2 molecules I2 are formed for each molecule of O2 present
I2 colors the solution brown
I2 is titrated with a standardized solution of Na2S2O3
I2 + 2Na2S2O3 ==> 2NaI + Na2S4O6
End of titration is made visible by adding a little bit of starch which colors the I2 blue.
- glass stoppered BOD bottles
- glass 50 ml pipet
- glass erlenmeyer 100 ml
- glass buret (precision 0.1 or 0.05 ml)
- spatula
1) MnSO4 solution
48 g MnSO4.4H2O in 100 ml dest
2) alkaline iodide:
30 g KI in 50 ml aqua dest + 50 g NaOH in 50 ml aqua dest
3) Na2S2O3
Dilute from stock solution to a 0.01 N solution
4) starch
5) concentrated H2SO4
* Fill 50 or 100 ml Winkler bottles with the water to be analyzed (very gently, avoiding air bubbles)
* Add 0.4 ml MnSO4/100 ml sample
0.2 ml MnSO2/ 50 ml sample
0.4 ml alkaline KI/100 ml sample
0.2 ml alkaline KI/ 50 ml sample
* Close bottle, shake well
* Let the precipitate settle (+ 1/2 hour)
* Add 0.2 ml H2SO4, shake till all the precipitate is dissolved.
Bottles can be conserved for 1-2 days this way (preferably in fridge).
* Shake bottle, bring 50 ml into an erlenmeyer with a pipette
* Add Na2S2O3 drop by drop till color is light-yellow
* Add a little bit of starch with a spatula ==> dark blue
* Carefully add drops of Na2S2O3 till water becomes colorless
Result: the concentration of O2 can be calculated as follows :
(O2) mgl-1 = [Normality Na2S2O3 x 0.25 x 32 x 1000 x volume Na2S2O3 added (ml)] / volume titrated
= [ 0.01 x .25 x 32 x 1000 x vol. Na2S2O3 added (ml) ] / 50
= Vol. Na2S2O3 added (ml) x 1.6
When measuring fluxes in a carbon cycle analysis, it is usually necessary to express the final result in C in stead of O2.
To convert mg O2.l-1 to mg C.l-1 =
[(MgO2)/l] x [(MW C)/(MW O2)] = [(Mg O2)/l] x (12/32) = [(Mg O2)/l] x 0.375
To convert to ml l -1 : 1 mole of a gas has a volume of 22,4 l at 1 atm and a temperature of 4° C
==> (Mg O2/l) x (1/32) x (22.4l) = [(Mg O2)/l] x 0.7 = (lO2)/l
Standardization on Na2S2O3 (0.01N-solution)
==> mg O2 x 0.7 = ml O2
l l
KIO3 0.1N
KI 10 %
H2SO4 conc.
- Dry KIO3 at 105° C for 1 hour
- Prepare a stock solution of KIO3 0.1N by weighing 0,3567 gr KIO3 and dissolve it in 100 ml
- Dilute this solution 10 times to get a 0.01N-solution
- With a analytical pypette bring 10 ml KIO3 0.01N into an erlenmeyer add 10 ml of a 10 % KI-solution and a few drops of conc H2SO4 add about 50 ml distillated water and with Na2S2O3 0.01N like discribed earlier.
The normality of Na2S2O3 0.01N is :
(normality of KIO3)/(normality of Na2S2O3) = (ml KIO3 0.01 used)/(ml Na2S2O3 0.01N used)
(0.01 N)/X = (10 ml)/(ml Na2S2O3)
X = [(0.01N) x (ml Na2S2O3)]/(10 ml)
Sea water samples are taken with Niskin bottles and sub-samples are brought into clean polyethylene vials, which are stored in the refrigerator. Ammonium should be analyzed immediately after sampling. The samples for nitrate and silicate analyses can be kept in the fridge for several hours before concentration changes occur.
For nitrites and phosphates, the analysis should be carried out within two hours after sampling.
In case immediate analysis is impossible, samples should be filtered on 0.45 µm porosity membrane filter or GF/C filters and deep frozen. Nitrate, silicates and phosphates can be correctly measured on such samples, whereas ammonium and nitrites can no longer be quantified on these preserved samples.
The methods for phosphate, nitrate and silicium are described.
The method relies on the formation of a phosphorus-molybdate complex and its subsequent reduction to highly colored compounds that can be quantified with a spectrophotometer at a specific wavelength.
The sea water sample is allowed to react with a composite reagent containing molybdic acid, ascorbic acid, and trivalent antimony.
The resulting complex heteropoly acid is reduced to give a blue solution (phosphomolybdate complex), of which the absorption is measured at 885 nm.
- Polyethylene bottles of 130 ml volume, with a mark at the 100 ml level
- spectrophotometer
- thermostated water bath (only in case the room temperature is below 15° C or above 30° C).
Disolve 15 g of ammonium paramolybdate ((NH4)6Mo7O24.4H2O p.a) in 500 ml aq. dest. Store in a plastic bottle out of direct sunlight. This solution is stable indefinitely, even at room temperature.
Add 140 ml of concentrated sulphuric acid (H2SO4 p.a.) little by little to 900 ml aq. dest. Allow the solution to cool and store in a glass bottle at room temperature.
Dissolve 27 g of ascorbic acid (C6H8O6 p.a.) in 500 ml aq. dest. in deep freeze !
Store the solution in a plastic bottle in the deep-freezer. Thaw for use and refreeze at once. The solution is then stable for many months, but should not be kept at room temperature for more than a week.
Dissolve 0.34 g of potassium antimonyl-tartrate (C8H4K2O12Sb2.3H2O) in 250 ml aq. dest, warm if necessary. Store in a glass or plastic bottle. The solution is stable for many months, even at room temperature.
mix together: 100 ml ammonium molybdate solution
250 ml sulphuric acid solution
100 ml ascorbic acid solution
50 ml potassium antimonyl-tartrate solution
Prepare this reagent in a quantity sufficient for the samples for use at hand.
The quantity mentioned above is for about 50 samples.
Do not store this solution for more than 6 hours.
- dissolve 0.816 g of anhydrous potassium dihydrogen phosphate (KH2PO4 p.a.) (MW = 136) in 1000 ml aq. dest. Store in a dark bottle with 1 ml of chloroform (CHCl3), the solution is stable for many months. This solution thus contains 6 µmol KH2PO4/ml.
or 6.0 µg- at P (microgram - atoms of phosphate phosphorus)/ml
- dilute 10 ml of this solution to 1000 ml with aq. dest Store in a dark bottle with 1 ml of chloroform added, the solution is stable for about a month.
1 ml = 6.0 x 10-2 µg-at P
- prepare five standards, using between 0,25 and 10 ml (0,25 - 0,5 - 1 - 2 - 4 - 8 - 10) of the diluted phosphate solution. Make up to 100 ml with aq.dest.
- transfer the solution to dry bottles and fill two more bottles with 100 ml of aq. dest to serve as blanks.
- add 10 ml of the mixed reagent solution and mix immediately.
- after at least 5 minutes and within the first 2-3 hours, measure the extinction of the solution in a spectrophotometer against aq. dest at a wavelength of 885 mm.
- using the standards' concentrations (µg at P/l) and the corrected extinction values obtained from the spectrophotometer (corrected extinction = extinction standard - extinction reagent blanks), make a calibration plot.
- calculate the calibration curve Extinction = a . concentration (µg at P/l) + b
- allow the samples to come to room temperature
- to 100 ml of sample, add 10.0 ml of mixed reagent solution and mix at once
- see a)
- calculate the phosphate concentration in microgram-atoms of phosphate phosphorus (µg - at P/l) from the expression:
µg - at P/l = (corrected extinction - b)/a
(a and b are obtained from the calibration curve)
P.S.: To exclude effects of turbidity, it is best to filter the samples on 0.45 µm porosity
1. Membrane filters or GF/C filters before analysis
2. If the obtained samples' E falls beyond the range of the standard Extinction, include additional standards in the calibration curve.
The nitrate in sea water is reduced almost quantitatively to nitrite when a sample is run through a column containing cadmium (Cd) filings loosely coated with metallic copper (Cu).
NO3 - + 2H+ + 2e- ==> NO2 - + H2O
Cd° ==> Cd2+ + 2e-
The nitrite thus produced is quantified by diazoting with sulfanilamide and coupling with N- (1-naphtyl) ethylenediamine to form a highly colored azo dye
NH2SO2C6H4NH2.HCl + HNO2 ==> NH2SO2C6H4N ==> NCl + 2H2O
NH2SO2C6H4N == NCl + C10H7NHCH2CH2NH2.Cl ==> NH2SO2C6H4N = NC10H6NHCH2CH2NH2.2HCl + HCl
The extinction of the colored complex is measured spectrophotometrically at 543 nm.
- Glass-tube to prepare the Cd-Cu column
- 125 ml erlenmeyer flasks
- A spectrophotometer
- 50 ml graduated cylinders
- Concentrated ammonium chloride solution
dissolve 125 g of ammonium chloride (NH4Cl p.a.) in 500 ml aq. dest, store in a glass or plastic bottle.
- Diluted ammonium chloride solution
dilute 50 ml of the concentrated solution to 2000 ml with aq. dest, store in a glass or plastic bottle.
- Cadmium-copper filings
Stir about 100 g of cadmium filings (enough for 2 columns) with 500 ml of a 2 % W/V solution of coppersulphatepentahydrate (CuSO4.5H2O), until the blue color has left the solution and semi-colloidal copper particles begin to enter the supernatant liquid.
Roll very fine copper turnings between fingers and thumb to make a small plug and push this in the bottom of a reductor column, (glass or quartz wool can be used if you can not obtain very fine copper "wool" turnings).
Fill the column with diluted ammonium chloride solution, or the supernatant liquor from the preparation of Cd-Cu turnings above, and pour in sufficient Cd-Cu mixture to produce a column of about 30 cm of length.
Add the filings a little at a time, tapping the column hard after each addition to make sure that the filings are well settled.
Wash the column thoroughly with diluted ammonium chloride solution.
The flow rate must be such that 100 ml of solution takes 8 to 12 minutes to flow completely through the column.
If flow time is less than 8 minutes: pack more Cu or glass wool at the outlet of the column
If flow time is more than 12 minutes: loosen the packing at the base of the column
Finally add a small plug of copper or glass wool to the top of the column to prevent cadmium filings being washed into the top chamber when solutions are added to the column.
When not in use, columns must be left with the metal filings completely covered with diluted ammonium chloride solution.
To regenerate a column that has lost efficiency of reduction:
- empty the column into a beaker
- wash the filings with 100 ml of 5 % (weight per. volume) hydrochloric acid (HCl)
- decant the acid
- repeat the procedure once more
- wash the metal with 100 ml portions of aq. dest until the pH of the wash exceeds the value 5
- decant the liquid to leave the metal as dry as possible
- retreat the metal with copper sulphate solution as described above.
- Sulphanilamide solution
dissolve 5 g of sulphanilamide (C6H6N2O2S) in a mixture of 50 ml of concentrated hydrochloric acid (HCl) and about 300 ml of aq. dest. Dilute to 500 ml with aq. dest. This solution is stable for months.
- N- (1-naphtyl)-ethylenediamine dihydrochloride solution:
dissolve 0.50 g of the dihydrochloride in 500 ml aq. dest.
Store the solution in a dark bottle, it should be renewed monthly.
This should be carried out using synthetic sea water or natural sea water with a nitrate concentration less than 1 µg-at N/l.
The concentration-extinction relationship is strictly linear and the factor need therefore be obtained at only one level of nitrate concentration.
- Synthetic sea water
dissolve 310 g of sodium chloride (NaCl p.a.), 100 g of magnesium sulfate (MgSO4.7H2O p.a.) and 0.50 g of sodium bicarbonate (NaHCO3.H2O p.a.) in 10 l aq. dest.
- Standard nitrate solution
- dissolve 1.02 g of potassium nitrate (KNO3 p.a.) in 1000 ml aq. dest. The solution is stable indefinitely in the absence of evaporation.
1 ml = 10.0 µg- at N
Dilute 4.00 ml of this solution to 2000 ml with synthetic sea water. This solution should be stored in a dark bottle and prepared freshly immediately before use.
concentration = 20 µg- at N/l
* As a reagent blank: pour 110 ml aq. dest in a 125 ml erlenmeyer
* Pour about 110 ml of the diluted nitrate standard solution into a clean dry 125 ml erlenmeyer flask, do this in triplo
* Add 1 ml of naphtylethylenediamine solution, mix immediately
* After 10 minutes to 2 hours, measure the extinction of the solution against aq. dest at a wavelenght of 543 nm.
* Correct the mean of the three extinctions by the blank extinction
* Calculate the factor F from the expression: F = 20.0 / E
Where E is the mean extinction of the three values corrected for a blank.
- bring 50.0 ml of sea water (filtered on a 0.45 µm porosity membrane filter or GF/C filter if turbidity is too high) in a 125 ml erlenmeyer flask.
1. Add 2.0 ml of concentrated ammonium chloride to the sample in the Erlenmeyer flask. Mix the solution and pour about 5 ml onto the top of the column and allow it to pass through.
2. Add the remainder of the sample to the column and place the drained Erlenmeyer flask under the collection tube. When 40 ml has passed through the column, drain the collection tube into the flask, rinse the flask with this effluent, drain it and replace beneath the collection tube.
3. Collect a further 50 ml in the collection tube and rapidly empty it into the Erlenmeyer flask. The column will not be quite empty and should then be allowed to drain until flow ceases.
- Add 1.0 ml of sulphanilamide solution, mix and allow the reagents to react for 2-8 minutes.
- Add 1 ml of naphtylethylenediamine solution, mix immediately.
- After 10 minutes to 2 hours, measure the extinction of the solution against aq. dest at a wavelength of 543 nm.
- Correct the measured extinction for the reagent blank.
- Calculate the nitrate-nitrogren concentration in microgram-atoms of nitrogen per liter (µg-at N/l) as:
µg- at N/l = corrected extinction x F
(corrected extinction = measured extinction-extinction of reagent blank)
The sea water sample is stored in polyethylene bottles in the dark for maximum a day. If longer storage is planned deep freezing of the samples is better.
The sample is allowed to react with molybdate under acid conditions (1 < pH < 2) to form the silicomolybdate, phosphomolybdate and arsenomolybdate complexes.
A reducing solution, containing metol and oxalic acid, is then added to give a blue reduction compound and to simultaneously decompose the arsenomolybdate and phosphomolybdate complexes. In this way, interference of arsenate and phosphate is eliminated. The extinction of the resulting solution is measured at 810 nm.
- 50 ml graduated glass measuring cylinders
Cleaning: fill them with chromic-sulphuric acid cleaning mixture, rinse thoroughly with aq. dest just before use.
- polyethylene bottles
- spectrophotometer
Dissolve 4.0 g of ammonium molybdate ((NH4)6MO7O24.4H2O p.a.) in about 300 ml aq. dest.
Add 12.0 ml of concentrated hydrochloric acid (HCl p.a.), mix and make the volume to 500 ml with aq. dest.
Store this solution in a polyethylene bottle, in which it is stable for many months if not exposed to direct sunlight.
Dissolve 6 g of anhydrous sodium sulphite (Na2SO3 p.a.) in 500 ml aq. dest
Add 10 g of metol (p-methylaminophenol sulphate) ((HOC6H4NHCH3)2.H2SO4)
When the metol has dissolved, filter the solution through a Whatman N° 1 filter paper and store it in a glass bottle which is tightly stoppered. This solution should be prepared fresh monthly.
Prepare a saturated oxalic acid by shaking 50 g oxalic acid dihydrate ((COOH)2.2H2O p.a.) with 500 ml aq. dest.
Decant the solution from the crystals for use
This solution can be kept indefinitely in a glass bottle
Add 250 ml of concentrated sulphuric acid (H2SO4 p.a.) little by little into 250 ml aq. dest. Allow to cool to room temperature and make the volume to 500 ml with a little extra aq. Dest.
Mix 10 ml of metol-sulphite solution with 60 ml of oxalic acid solution
Add slowly, with mixing, 60 ml of the 50 % V/V sulphuric acid solution.
Make the mixture to a volume of 300 ml with aq. dest.
This solution should be prepared for immediate use.
- Standard silicate solution : 1g/1000ml
Weigh 0.960 g of fine powdered ammonium silicofluoride ((NH4)2SiF6), crush any lumps if present.
Dissolve the salt by stirring it with 50-100 ml aq. dest in a plastic beaker using a nickel spatula.
Transfer the solution to a 1000 ml measuring flask, rinse the beaker thoroughly and make the volume to the mark with aq. dest.
Mix and transfer the solution to a polyethylene bottle for storage as soon as possible (while the solution rapidly picks up silica from glass)
The solution is stable indefinitely
1 ml = 5 µg-at Si (microgram-atoms of silicate silicon)
Dilute 10 ml of this solution to 500 ml with synthetic sea water, this diluted solution should be used within a few hours.
1 ml = 0.1 µg- at Si
- Synthetic sea water
Dissolve 25 g sodium chloride (NaCl) and 8 g of magnesium sulphate heptahydrate (MgSO4.7H2O) in 1000 ml aq. dest. This water is equivalent to sea water with salinity 28 ‰. This solution must be stored in polyethylene bottles.
* Add 10 ml of molybdate solution to 7 dry 50-ml measuring cylinders.
As reagent blanks, add 25.0 ml of synthetic sea water to two of the measuring cylinder, restopper and mix, allow to stand for 10 minutes.
For the standards, add 5.0 - 25.0 ml of the diluted silicon standard to five of the 50 ml measuring cylinders. Make up to 25.0 ml with aq. dest.
* Rapidly add the reducing reagent (15 ml) so as to make the volume exactly to 50 ml and mix immediately.
* Allow the solution to stand for 2-3 hours to complete the reduction of the silicomolybdate complex.
* Measure the extinctions of the blanks and the standards against aq. dest at a wavelength of 810 nm
* Make a calibration plot: (corrected extiction in function of the Standard concentration (µg - at Si/l).
* Calculate the calibration curve. Extinction = a . concentration (µg at Si/l) +b
* Add 10 ml of molybdate solution to a dry 50 ml measuring cylinder.
* Add 25.0 ml of the sea water (can be filtered on a 0.45 µm porosity membrane filter or a GF/C filter in case turbidity is too high), stopper the cylinder, mix and allow the solution to stand for 10 minutes.
* Following: see a)
* Correct the measured extinction by subtracting the mean of the two reagent blanks.
* Calculate the silicate concentration in microgram-atoms of silicate silicon per liter from the expression
µg-at Si/l = (corrected extinction - b)/a
(a and b obtained from the calibration curve)
In alikaline medium the dissolved NH3 reacts with hypochlorite (HClO) and form a monochlooramine. This will form a blue indofenol in oxydising medium, and in the presence of a fenol. At 20°C and with nitroprussaat (Na2(Fe(CN)5NO).2H2O) as a caterlyst, this reaction will take 12 hours.
Precipitation of Ca and Mg in basic medium is avoided by complexation with sodiumcitraatdihydraat.
Dissolve subsequently 17.5 g of phenol (p.a.) and 0.2 g of sodiumnitroprusside (Na2(Fe(CN)5NO).2H2O) in MilliQ water and dilute to 500 ml with MilliQ water. Transfer the solution to a dispenser and label it as ‘Reagent 1’ (R1). Store in a refrigerator until use.
Dissolve subsequently 140 g of tri-sodiumcitrate-dihydrat (p.a.) and 11 g NaOH (p.a.) in MilliQ water. When complete dissolution is reached, add 20 ml of sodium hypochorite (Javel 10° BE). Dilute the solution to 500 ml with MilliQ water, transfer to a dispenser and store in a refrigerator until use. Note: do not store this reagent for more than 14 days!!!
Add 3 ml of reagent 1 (R1) to 100 ml of your standard solutions. Shake well and then add another 3 ml of reagent 2 (R2) and shake thoroughly. Store in a dark place for a minimum of 12h but not more than 48h (preferably overnight)..
Measure the absorbance of the your standard solutions at 630 nm wavelength. Use highly purified water as a blank. For better results prepare your standard solutions in dublicate, so that you finally work with a mean value.
Measure 100 ml of your seawater sample and treat it exactly the same as you did for the standard solutions. Several replicates are generally advised.
- 100 ml polyethylene bottles
- spectrophotometer
If natural samples are to be measured, cuvettes with an optical path of 10 cm should be used as ammonium concentrations may be very low in natural seawater.
The light environment in the sea is very important for many biological aspects.
The quantity and quality of light in the sea fluctuate depending on tide, weather conditions, angular distribution.
- expressed as an illumination (only visible part of spectrum (300 nm 760 nm)), as a luminous flux per unit area: LUX
- expressed as an energy unit (whole spectrum including UV and infra red (IR)): per unit area: cal/cm2
- expressed as a quantum: micro-Einstein per second per m2
µE s-1 m-2 recently most used
The lab measurements (see primary production) will be carried out using a LUX-meter !
Depending on selective absorption and scattering due to seawater itself and the suspended matter every H2O column has its specific transparency.
Light penetration can be expressed by a vertical extinction coefficient k
Id = Io e-kd
With Io: incoming light intensity
Id: incoming light intensity at depth d
k varies with wave length
In lab these different light intensities at different depth are simulated using bags or bottles with different transparency filters (see primary production).