Sistemas de representación geográfica. Mapas

Cations

Sodium AAS, FES, ICP, ISE, calculation

Potassium AAS, FES, ICP, calculation Calcium and

Magnesium

Titration, AAS, ICP, ISE, gravimetric

Barium and Strontium AAS, ICP, turbidimetric, gravimetric

Iron Titration, colorimetric, ICP

Hardness Titration, AAS, ICP, ISE, calulation

Anions

Chloride Titration, ISE, IC, gravimetric Salinity, Chlorinity

and Chlorisity

Titration and calculation

Carbonate and Bicarbonate

Titration, calculation

Sulfate Gravimetric,

turbidimetric, IC “Organic Acids” Titration, IC

Alkalinity Titration, calculation Dissolved Gases

Oxygen Titration, colorimetric, amperometric with membrane electrode Carbon Dioxide Titration, ISE, calculation Hydrogen Sulfide Colorimetric, ISE, titration Abbreviations:

AAS — Atomic absorption spectrophotometry FES — Flame emission photometry

IC — Ion chromatography

ICP — Inductively coupled plasma spectroscopy ISE — Ion selective electrode

Table 4 Analytical Methods for Neutral Components and Properties COMPONENT ANALYTICAL METHODS Neutral Components

Silica Gravimetric, colorimetric, ICP

Bacterial Content Culturing techniques (see separate Section on

bacterial effects)

Oil Content Colorimetric, gravimetric (see separate Section on Oil-in-Water analysis) Total Residue Suspended Solids Amount Gravimetric Type Colorimetric Particle Size Analysis

Particle size analyzer

Total Dissolved Solids Gravimetric, calculation Properties pH ISE, colorimetric Temperature Thermometer

Turbidity Turbidimeter, colorimetric

Color Colorimetric

Density (or Specific Gravity) Hydrometer, pycnometer Conductivity (or Resistivity) Conductometric, calculation Abbreviations:

AAS — Atomic absorption spectrophotometry FES — Flame emission photometry

IC — Ion chromatography

ICP — Inductively coupled plasma spectroscopy ISE — Ion selective electrode

Gravimetric methods are usually the most accurate and precise analytical methods. They have been used to determine atomic weights to seven significant figures. However, the procedures are frequently long and compli- cated, and they are usually not suitable for field applica- tions. A stoichiometric compound of sufficient insolubil- ity is not always available for each desired component in the sample.

Volumetric or Titrimetric Analysis — In titrimetric analy- sis, a standard solution of accurately known concentra- tion that reacts stoichiometrically with the sought constituent is added stepwise until all the sought com- pound has reacted. This point of complete reaction is called the endpoint. The endpoint is detected by an “indi- cator” added to, or placed in, the solution. The indicator may be a chemical compound that changes color at the endpoint. Colored indicators are available for acid/base, oxidation/reduction, and many precipitation or complex- ation titrations. The indicator may also be electrodes that detect the electrochemical potential (activity) of a particu- lar ion in solution (e.g., a pH electrode for acid/base titrations) or determine the electrical conductivity of the ions in solution. From the concentration and volume of standard solution added to reach the endpoint, we can calculate the amount of sought component in the sample. Thus, for the precipitation titration of chloride ion with a standard solution of silver ion,

Ag++Cl−→AgCl

the endpoint occurs when an amount of silver ion equivalent to the chloride ion has been added. The endpoint can be detected with colored indicators, by electrochemical methods, or by turbidity changes. The amount of chloride ion in the original sample is given by

W V C E= ∗ ∗ where

W = weight of chloride ion present, mg; V = volume of silver standard added, mL;

C = concentration of silver standard, meq/mL; and

Volumetric analysis is convenient, inexpensive, and widely used for a large number of chemical components. It is

frequently done in the laboratory but many kits are available for field analysis.

For accurate and precise volumetric analysis, the end point must be readily and reproducibly detected and correspond exactly to the equivalence point.

E = milliequivalent weight of chloride ions, mg/ meq.

Titrimetric procedures are available for a wide variety of constituents. They are usually quite precise if specified procedures are closely followed. In some cases, interfer- ences in solution will reduce accuracy by giving an end- point indication at a volume other than the equivalence point. Instructions for the titrimetric procedures usually indicate possible interferences and means to eliminate or compensate for them. Small, portable test kits using syringe-type burettes are available for many titrimetric analyses. These are ideal for field analyses.

Colorimetric or Spectrophotometric Analysis — These methods measure the amount of light of a particular wavelength (color) either emitted or absorbed by the sought component. Electromagnetic radiation in the infrared, visible, or ultraviolet regions is most commonly used for oilfield water analyses. Light may interact directly with the sought component or with an added material that will react specifically and stoichiometrically with the sought component.

In the simplest analyses, the material absorbs or emits light of a characteristic wavelength under ambient condi- tions (colorimetry or spectrophotometry). In more com- plex methods, light of a specific wavelength is absorbed or emitted when the sought material is excited in some manner. Atomic absorption spectrophotometry (AAS) is an example of light absorption by a thermally excited mate- rial. Light of specific wavelengths characteristic of a component is emitted when the component is excited electrically (emission spectroscopy), thermally (flame emis- sion spectroscopy, flame photometry, and inductively coupled plasma spectroscopy, ICP), electronically (X-ray spectros- copy), or photochemically (fluorescence spectroscopy).

In turbidimetric analysis, a related optical method, a finely divided precipitate of the sought component is reproduc- ibly formed by the addition of a precipitating agent, usually in the presence of a dispersant. The turbidity of the resulting solution, caused by the absorption and scattering of incident light by the suspended particles, is measured and related to concentration through

The simpler colorimetric and turbidimetric

methods can be used in the field. The more complex procedures requiring extensive equipment are laboratory methods.

similar turbidity measurements on standards. This method is frequently used to determine sulfate as precipitated barium sulfate.

Colorimetric methods range from very simple (requiring little or no additional equipment) to very complex (using complicated and expensive equipment, frequently with computer control and data acquisition and analysis). Simple kits suitable for field sampling and analyses for many components are available from:

Hach Chemical P.O. Box 389

Loveland, CO 80539

In the Hach methods, the color developed by reaction of the desired component with a color-forming reagent is compared with liquid or solid color standards in the kit. Sensitivity and selectivity in field applications can be improved with a portable, battery-operated spectropho- tometer with selective color-forming reagents.

The more complex emission and absorption methods require increasingly complex equipment in a laboratory environment. These methods, however, are usually rapid, more versatile, and less subject to interferences. Some are capable of multiple- element analyses on small sample volumes. Most cations (metallic ions) can be determined by published standard methods using AAS or ICP.

All methods determine concentrations by comparing the amount of light absorbed or emitted with that from standard solutions of known concentrations. The pub- lished standards usually give specific procedures, inter- ferences, and indications of precision and accuracy. Electrochemical Methods — In these methods, some electrochemical property of the sought component is determined and related to concentration or activity. With potentiometric methods, an electrical potential or voltage is measured with an electrode to give an ion activity that is related to ion concentration in a known manner. Ex- amples are the pH electrode and ion selective electrodes for Na+_{, Ca}2+_{, Ag}+_{, S}2-_{, Cl}-_{, CO}

2, etc. These electrodes are

quite specific for each component, relatively free from interferences, and usable over several orders of

Many electrochemical analysis procedures have been adopted for field usage.

concentrations. Rugged electrodes and portable, battery- powered voltmeters are available for field measurements. Concentration can be determined by single-point mea- surements and comparison with standards, or by mul- tiple additions or subtractions of standards. The poten- tiometric electrodes can also be used as indicator

electrodes in volumetric titrations.

In amperometric or voltammetric analyses, the current caused by the oxidation or reduction of an electroactive species is measured and related to concentration. Selec- tivity for individual components is achieved by measur- ing the current at a specific applied voltage at which the desired species is electroactive. Concentrations are usually determined by comparison with currents ob- tained with standards under similar conditions. Ex- amples of these types of measurements are electrodes for dissolved oxygen determination and various forms of voltammetry and polarography. Amperometric methods can also be used as indicator electrodes in titration or electrochemical procedures. Instruments are available for field measurement of dissolved oxygen and various forms of chlorine. Most other determinations are done in the laboratory. A variant method is electrogravimetric (plating) analysis in which the amount of electro- deposited material is determined by weighing.

Coulometric methods measure the number of coulombs (ampere- seconds) required to completely oxidize or reduce an electroactive species. Specificity is achieved by controlling the potential at which the electrochemical reaction occurs. The amount of material reacted is deter- mined from first principles — no chemical standardiza- tion is required. Coulometric methods are very accurate and sensitive because of the availability of precise, accu- rate methods for measuring small numbers of coulombs. These methods are usually not suitable for field analyses. Conductometric analyses measure the current carrying capacity (conductance) of all ions present. The method is not specific for any ion but measures all ions present. It is frequently used to indicate the total dissolved ionic concentration. As such, it can be used as a quality-con- The dispersed and

dissolved water in crude oil can be quickly and accurately determined by an automatic coulometric titrator (Karl Fisher titration).

trol check. Battery-powered instruments are available for field measurements. The method is calibrated by mea- suring the conductance of standard solutions.

Chromatographic Methods — New chromatographic techniques are being developed for the separation, identification, and quantification of anions and cations. In ion chromatography, ions are separated on chromato- graphic columns with special absorbents and eluants. Special in-line colorimetric or electrochemical detectors detect and quantify the separated ions. Identification of the type of separated ion is based on the retention time of the ion on the column. The amount of each separated ionic species is determined from the detector response. Each method is calibrated for retention time and detector response by running standards. Many ion chroma- tographic methods are still in the development stage. They have not been published as “standards methods.”