Determinations in the laboratory
Fats and oils
The determination of substances soluble in cold in ethyl ether includes the sum of fats, oils, hydrocarbons and other substances soluble in cold in ethyl ether of a sample acidified to pH 4.2 and that are not volatile at 70 °C (degrees Celsius).
The presence of some oils is due to the decomposition of aquatic life forms.
Most fats and oils are insoluble in water but can be saponified or emulsified by the action of detergents, alkalis, and other chemicals.
Emulsified fats and oils are extracted from water acidulated to pH 4.2 by contact with organic solvents that also dissolve other organic substances. There is no selective solvent for fats and oils.
Some low-boiling fractions such as kerosene and naphtha evaporate during the analysis and others evaporate at the ether evaporation temperature.
Oils and fats tend to remain in emulsion, but acidifying the sample to pH 4.2 and/or adding sodium chloride helps break this emulsion.
Other substances soluble in ethyl ether that are not fats and oils, in a sample acidified to pH 4.2 and that are not volatile at 70 °C, interfere.
The maximum sensitivity achieved with the technique is 2 mg/L (milligrams per liter).
The sample must be representative, so it is collected from an area that has agitation, where the effluent is not sealed. The effluent container should not be filled because the floating oil can be lost when closing it. To preserve the sample, acidify it to pH 4.2.
Oxygen consumed
The oxygen in permanganate that water consumes when treated with this reagent under certain conditions is called consumed oxygen. Such conditions are concentration of the reagents, heating time, heating temperature and the treatment must be rigorously adjusted to them. The determination approximately measures the organic matter present in the sample, and if there are permanganate-reducing minerals, the correction must be made. It provides an index of the degree of contamination, and is extremely useful when BOD is not practiced, or even as complementary data to the latter.
To determine the oxygen consumed you must:.
- Pour 100 mL (milliliters) of the sample or a suitable dilution into a suitable Erlenmeyer flask (maximum dilution is 1/500), 10 mL of sulfuric acid (1+3) and 10 mL of 0.0125 N potassium permanganate.
- Immerse the Erlenmeyer flask in boiling water. The water must exceed the surface of the interior liquid for proper heating and care must also be taken to ensure that the liquid inside does not spill.
Permanganate oxidizes organic matter.
- After 30 minutes, a faint pink color should remain. If it is very colored, carry out a dilution and if it is not colored, carry out a concentration.
After 30 minutes, there is excess permanganate and the violet color should remain.
- Remove the Erlenmeyer flask from the bath, adding 10 mL of oxalic acid, reaching discoloration (excess oxalic acid remaining). The excess consumed corresponds to the original amount of organic matter. Titrate by return until obtaining a faint pink but resistant color for 3 minutes. This assessment must be carried out during 60-80 °C. The volume used in the return assessment must be less than 5 mL. If it is higher, a higher dilution should be used, with water without permanganate.
The hot titration is carried out at 60-70 °C, adding oxalic acid until discoloration. The excess oxalic acid consumes the organic matter that was at the beginning, that is, the 0.0125N potassium permanganate. Then, potassium permanganate 0.0125N is added by return. The end point is a weak but resistant pink coloration for 3 minutes and is indicated by potassium permanganate.
- Carry out the blank test. In practice, a value less than 0.1 mg/L is unimportant and can be ignored in the correction.
The moment at which the cold titration is performed is when a positive hot titration was performed (the correct dilution was found). It is used to determine if permanganate-reducing minerals are present. Permanganate is used as a titrant and indicator of the end point of the reaction, which is identified by a slight pink color that persists for 3 minutes.
Cold titration is performed to determine if permanganate-reducing minerals are present. The hot and acidic titration is carried out to determine the milligrams of oxygen that the sample completely consumes. The greater the oxygen consumption, the more contaminated it is. Potassium permanganate is reduced from Mn(+7) to Mn(+2) without oxidizing the organic matter, by adding oxalic acid of the same concentration, the excess of which is titrated with 0.0125N potassium permanganate. The excess oxalic acid is equal to the original amount of organic matter.
Cold titration is carried out as follows:.
- Pour 100 mL of raw sample (or a dilution of it using water over permanganate) into an Erlenmeyer flask.
- Add 10 mL of 1+3 sulfuric acid.
- Titrate with 0.0125 N potassium permanganate until a weak but resistant pink color.
The amount of oxygen is calculated by the following expression:.
(N-Nf-Nb)*100*(f/(V.r)).
N: amount of potassium permanganate used in mL of the return titration.
Nf: amount of potassium permanganate in mL used in the cold titration.
Nb: amount of potassium permanganate used in the target.
r: the dilution factor.
f: permanganate correction factor.
For example, the volumes of permanganate consumed are:.
Hot=4.2 mL.
Cold=0.5 mL.
Blank=0.2 mL.
Sample volume=100 mL.
Dilution=1/10.
Oxygen consumed=(4.2-0.5-0.2)*100*(1/(100*1/10))=35 mg/L.
Another example, the cold titration of 100 mL of sample, diluted 250 times, consumed 1.6 mg/L of 0.0125N potassium permanganate and the hot titration 4.2 mL. The reagent blank consumed 0.2 mL of the same titrant.
Oxygen consumed=(4.2-1.6-0.2)*100*(1/(100*1/250))=600 mg/L.
Before starting the test, verify the chloride value in the sample. If the value is greater than 500 mg/L, a dilution must be made that allows working with a lower concentration or alkaline digestion must be carried out.
The procedure is:
- Place 100 mL of sample in an Erlenmeyer flask (undiluted or the dilution used in the treatment).
- Add 0.5 mL of sodium hydroxide and 10 mL of potassium permanganate.
- Heat for 30 minutes.
- Add 5 mL of oxalic acid solution.
- Assess by carrying out the blank test in parallel.
To avoid heating with a water bath, electric irons or lighters can be used, but this causes large variations in the results. Only by rigorously adjusting to the method can comparable results be obtained.
To avoid excessive permanganate consumption, dilutions are set for effluent classes: for sewage liquids, dilutions vary between 1 and 10%; for sewage effluents from purification plants, dilutions vary between 25 and 50%; for polluted rivers, dilutions vary between 25 and 50%.
When we calculate the oxygen consumed, we must take into account that we are working with lighters, so tests cannot be carried out with flammable and explosive materials such as ether. In addition, you work with sulfuric acid and oxalic acid, so gloves, overalls and closed-toed shoes must be used.
Ammoniacal nitrogen
In samples with high ammonia content, distillation is omitted and the sample is subjected to direct nesslerization. Pretreatment with zinc or aluminum sulfate in an alkaline medium allows calcium, magnesium, iron and sulfur ions to precipitate, which can cause turbidity in the presence of Nessler's reagent. The addition of EDTA prevents the precipitation of residual Ca and Mg, in the presence of Nessler's reagent.
A number of aliphatic or aromatic amines, alcohols, aldehydes, ketones have been found to cause turbidity or a parasitic green-yellow coloration in the presence of Nessler's reagent. There is no recommended procedure to eliminate interferences by resorting to distillation when they cannot be avoided.
Under optimal conditions, the reagent allows the detection of one milligram of ammoniacal N in 50 mL of solution. Data playback below mg may be erratic.
At the beginning, the calibration curve must be assembled, to do this:
- Measure the aliquots of the ammonia standard solution represented in the table in 50 mL volumetric flasks.
- Dilute to 50 mL with ammonia-free water.
- Add 2 drops of Seignette's salt solution and 1 mL of Nessler's reagent.
- Let it rest away from light for 10 minutes.
- Read the percentage of transmittance in the spectrophotometer at 420 nm (nanometers) adjusting 100% T to the reagent blank.
To determine fats and oils:.
- The raw sample must be homogenized before measuring the sample aliquot.
- Measure 50 mL of the raw sample with a 50 mL measuring cylinder.
- Acidify with pH 4.2.
Acidification of the sample helps to break the emulsion of fats and oils with water or saponified.
- Add 2 drops of heliantine.
Heliantin is used to verify the action of acids. Although it can be corroborated with a peachimeter, the change from yellow-orange to red is used. It is also used to differentiate the phases, since after adding the ether, the ampoule is shaken and allowed to rest to observe the 2 phases: ethereal phase and aqueous phase.
- Transfer to the decanting ampoule and add 50 mL of ether there. Shake and let stand until phase separation occurs. The aqueous phase is extracted in a beaker and the ethereal phase in a crystallizer. The crystallizer must be previously weighed. Then the contents of the crystallizer evaporate.
It should not be shaken violently because the emulsion will be produced again. It is worked under a hood with a correct closure because ether is flammable and toxic, so the test is not carried out along with others such as the oxygen consumed where lighters are used. The extractor is working correctly. Use gloves, face mask, glasses, overalls, long pants and closed shoes. Use stoves with temperature control security systems. The work table is clean. The ether is transferred to the ampoule using a test tube, which is easy to manipulate; the valve must be perfectly closed. Have perfectly located where the chemical protection elements (antiseptics, alcohol, iodine), fire extinguishers, and first aid kit are located.
For colorimetric determinations, in the case of turbid and colored samples, pretreatment must be applied to the sample. The procedure is:
- Place 300 mL of sample in a 1000 mL Erlenmeyer flask.
- Add 3 drops of 10% w/v aluminum sulfate and 3 mL of 50% w/w sodium hydroxide solution.
Once the reagents are added, a stopper is added to the sample or covered with aluminum foil. Aluminum sulfate is a coagulant which allows the flocculation and subsequent sedimentation of the flocs. Coagulation is carried out with the formation of aluminum hydroxide. With the appropriate pH, it can be left in the refrigerator until sedimentation occurs. If sedimentation is not verified, 2 mL of the supernatant liquid is taken and the sample is diluted or pretreated again. If flocculation is not verified, the sample is centrifuged.
- Cover and shake gently in a circular motion.
The pH must be controlled successive times; if the flocs have already formed, the next step is not necessary.
- Adjust the pH to 6.5-7.5 with the addition of drops of diluted acid and base.
- Let it decant (30 min-24h) and store in the refrigerator. Use the supernatant to perform colorimetric determinations.
Phosphates
The determination is carried out using the Murphy-Riley Method. The phosphate ion reacts with the ammonium molybdate complex, which when reduced with ascorbic acid gives a complex of defined composition that is molybdenum blue.
The pH does not intervene, copper does not interfere up to 10 mg/L, arsenic does not interfere up to 0.05 mg/L, the presence of oxidants and reducers does not seriously disturb the accuracy of the method.
The procedure is as follows:
- Fill both tubes with 5 mL of pretreated sample.
- Place a tube in position A of the comparator.
- Measurement of PO4(-1). Add 6 drops of the reagent for determination of PO4(-1) and close the tube and mix.
- PO4(-2) measurement. Add 6 drops of the reagent for determination of PO4(-2) and close the tube and mix.
- Place the comparator in position B and wait 10 minutes.
- Open the tube and move along the color scale until it matches that of the pretreated sample. Read the concentration.
- Clean the tubes very well and close.
The result corresponds to the phosphate content expressed as phosphorus PO4-P and is equivalent to the sum of the concentrations of PO4(-1) and PO4(-2).
The phosphorus content in the form of phosphates is determined using a colorimetric kit.
Traditional Murphy-Riley method.
The method is suitable for determining the concentration of various forms of phosphorus in sewage liquids, drinking water and water courses such as organic phosphorus, polyphosphates and orthophosphates.
Advantages.
To determine orthophosphates:.
- Filter 50 mL of the pretreated sample.
- Place 8 mL of the mixed reagent and fill with the filtrate.
- Wait 10 minutes for color development.
- Observe the absorbance at 890 nm (nanometers).
- Determine the phosphorus content in the calibration curve using the standard phosphorus solution.
- The concentration of orthophosphates will be:.
C=n*50/V.
n: concentration of standard whose color is the same as that of the sample (mg/L).
V: volume used for determination (ml).
To determine total inorganic phosphorus:.
- Place 20 of the pretreated sample in a 125 mL Erlenmeyer flask.
- Add 1 mL of 5N sulfuric acid.