HK1009148B - Fluorescent petroleum markers - Google Patents

Description

Fluorescent petroleum marking agent

Background

The present invention relates to colorless or near colorless compounds for identifying or tracking petroleum fuels. The invention also relates to agents for coloring and fluorescing base-extractable markers.

A so-called marker is a substance that can be used to trace petroleum products for subsequent detection. The marker is first dissolved in the liquid to be identified and then detected by simple physical or chemical tests on the labelled liquid. Sometimes the government uses a marking agent to ensure that a particular grade of fuel has been delivered the appropriate tax. Oil companies also identify their products to help identify those products that have been diluted or changed. These companies often spend a great deal of money to ensure that the petroleum products they identify meet certain specifications, for example, regarding volatility and octane number, and to provide petroleum products containing detergents and other ingredients with effective secondary packaging. Consumers rely on product names and quality indicators to ensure that the purchased product is of the desired quality.

The gasoline dealer gains more benefit by selling inferior products at a price that the customer is willing to pay corresponding to a high quality logo or logo product. In addition, by diluting the high-quality product with the low-quality product, a high profit can be easily obtained. It is very difficult to manage a merchant who replaces one product for another or blends a premium product with a poor product, since the blended product will qualitatively reveal the presence of ingredients in the premium product. The critical ingredients in a good quality product are usually present in such low amounts that quantitative analysis to detect dilution with a poor quality product is difficult, time consuming and expensive.

Marker systems for fuels and other petroleum products have been proposed to date, but they all suffer from various drawbacks that hinder their effectiveness. For example, many systems lose their color over time, making them difficult to detect after storage. In addition, the agents used to color the marking agents are often difficult to control or have disposal problems. Still further, certain marking agents are dispensed into water. Thus rendering the marker ineffective when stored in an aqueous container.

The present invention provides marking agents that are not visible in liquid petroleum products, but which give rise to fluorescence and/or colour when extracted from petroleum products with suitable colour developers. In addition, the reagent for exhibiting fluorescence is also easy to control and handle by itself.

Summary of The Invention

The present invention includes marker compositions, and compositions comprising a liquid petroleum product and a detectable amount of a marker which is a derivative of 2(3H) furanone wherein the carbon atom number 5 is part of a xanthene system:wherein R1 is an alkyl group having 1 to 8 carbon atoms, or an aryl group. R2, R3, R4 and R5 are hydrogen, chlorine, bromine or C1-C12 alkyl. R1 may be the same or different groups, and R2-R5 may be the same or different groups. The alkyl group may be linear or branched. (3H) The carbon atoms 1 and 2 of the furanone ring may be saturated or there may be an olefinic double bond between them. The hydrogen atoms attached to these carbon atoms may also be substituted in whole or in part by alkyl groups.

In addition, carbon atoms No. 1 and No. 2 of the 3(H) furanone ring may form part of a carbocyclic ring system, particularly a benzo ring system. Especially preferred are 3, 3-disubstituted derivatives of 1(3H) isobenzofuranone in which the carbon atom number 3 forms part of a xanthene system.Wherein R1-R5 are the same as above, and R6 is any combination of hydrogen, bromine or chlorine. Also, the alkyl groups are linear or branched. When R1-R5 are alkyl, they are often C1-C4 alkyl groups.

The present invention also includes a method of making a liquid petroleum product comprising adding to the liquid petroleum product a detectable amount of a marker selected from the group consisting of:andwherein R1 and R2-R6 are the same as described above.

The invention also relates to a method for marking a liquid petroleum product, comprising: a) obtaining a sample of the liquid petroleum product comprising a detectable amount of the marker as described above, b) adding a chromogenic agent to the sample so as to exhibit fluorescence.

Detailed description of the invention

When classified by hue index (third edition of 1975), the compositions of the present invention comprise organic esters of fluorescent dyes of the hydroxyphthalide subclass of xanthene dyes. Among these organic esters, the organic ester of fluorescein (C) is more commonly used₂₀H₁₂O₅). Particularly preferred is 3 ', 6 ' -dihydroxyspiro [ isobenzofuran-1 (3H), 9 ' - (9H) xanthene]Esters of-3-ketones, commonly known as fluorescein, have the following structural formula:wherein R1 is an alkyl or aryl group of 1 to 18 carbon atoms. Also preferred are esters of fluorescein in which the aromatic ring hydrogen atoms 1 ', 2', 4 ', 5', 7 'and 8' and 4, 5, 6, 7 are substituted with non-ionizing substituents such as alkyl, hydrogen, chlorine or bromine. In particular, when R2, R3, R4 and R5 are hydrogen, chlorine or bromine or C1-C12 alkyl and R6 is hydrogen, chlorine or bromine, the present invention includes the compounds described above. R1-R6 may be the same or different groups, and the alkyl group may be straight or branched. For many applications, it is preferred that R2-R6 are hydrogen and R1 is C1-C4 alkyl.

The marking agents of the present invention also include chemical agents of the formula:wherein R1-R5 are as described above.

For artificial and natural waterways, fluorescein itself has been used as a marking or tracer substance in the form of water-soluble salts, thereby enabling tracing of, for example, rivers and sewers. It can also be used as a diagnostic marker in the human vascular system. It is generally believed that a dye that is colored pale yellow will be most valuable for convenient detection, and will exhibit strong fluorescence even when very diluted. The fluorescence is in the selfBut is observable under light or suitable artificial light sources, especially long-wave ultraviolet light or "black line" lamps. Spectrofluorometers can accurately quantify fluorescein concentrations to parts per billion (10)^-9G/ml). In addition, fluorescein is also known to have low toxicity and be readily biodegradable.

However, fluorescein itself is not suitable for use as a marker for petroleum fuels because it is readily distributed between water and petroleum. As often occurs in fuel storage tanks, when a fuel containing fluorescein comes into contact with water, the compound will distribute between the two phases and render it useless as a quantitative petroleum marker.

By converting fluorescein to an organic diester using an esterification agent in accordance with the present invention, the tendency to bleed (partition) can be minimized or eliminated altogether. The diesters may be derived from organic acids having 1 to 8 carbon atoms, anhydrides or halides thereof. Another advantage of esterification is that the yellowish color of fluorescein itself is reduced to the point where commercial quality is negligible and can be removed entirely in the purified material. This provides a marking substance that is invisible to the human eye for marking in the fuel. Thus, esterification can prevent the marking agent from obscuring the colorant added as a result of regulatory requirements or other reasons.

The ester markers of the present invention can be added to any liquid petroleum product such as fuels, lubricating oils and greases. Examples of the liquid petroleum products of the present invention are gasoline, diesel oil, fuel oil, kerosene and kerosene. When developed, the ester marker is detectable to the naked eye over a wide range of concentrations, but preferably at a concentration of at least about 0.5ppm or 5ppm, more preferably from about 0.5 to about 100 ppm.

Since the markers are essentially colorless in petroleum products, their presence can be detected by their reaction with a color developer. The developer used in the present invention preferably comprises a strong base such as an alkali metal hydroxide, or more preferably a quaternary ammonium hydroxide. The pH of the developer is about 10 to about 14, preferably about 11 to about 13. It is believed that the base hydrolyzes the ester and promotes the formation of highly fluorescent dianions, which may also have different colors. This fluorescence is readily detectable by the naked eye. Provided that only a quantitative indication of the marker is required, the fluorescent "displayed" fuel can be returned to its starting state. In this way, the developer and marker are burned or exhausted with the product so that no potentially harmful waste material accumulates to be disposed of, for example by roadside testing.

Where the fluorescence of the chromogenic markers of the present invention is obscured by other colorants in the petroleum product, the fluorescent dianion may become visible by extraction of the fluorescent dianion into the extraction medium. While this can be accomplished by adding only water as the extraction medium to the sample, it is preferred to use a mixture of water and a phase separation enhancer such as a fatty alcohol, ethylene glycol, or a glycol ether. The use of a phase separation enhancer tends to facilitate the separation of the aqueous and organic phases. In addition, other substances, such as pH buffering salts, may be present in the extraction phase to stabilize the fluorescent anion. Preferred extraction medium mixtures also contain quaternary ammonium hydroxide compounds to provide a simple means of exhibiting fluorescence through the formation of dianions and allowing immediate extraction of the colored dianions into a human suitable medium. Of course, other strong bases, especially alkali metal hydroxides, may also be used.

The extract phase can be visually detected based on the bright yellow to green fluorescence characteristics of the fluorescein-derived dianion. At very low concentrations (about (1-500). times.10)^-9) The fluorescence can be made more easily observable by the naked eye by co-illuminating the extracted dye with long-wave ultraviolet light. In addition, the extracted marker can be detected and quantitatively analyzed by visible light absorption spectroscopy or by spectrofluorimetry. Another advantage of this extraction technique is that it provides the opportunity to concentrate the marker from the petroleum fuel, thereby increasing the sensitivity of the test method.

The marker compounds of the present invention may be synthesized by any conventional method of esterifying phenolic hydroxyl groups. These include direct esterification with acids, reaction with acid halides, especially with acid chlorides, and most preferably with acid anhydrides. Generally, the preferred technique is to react the hydroxyxanthene with an acylating agent as a suitable separate reactant, under aqueous or anhydrous conditions. The esters obtained from lower aliphatic carboxylic acids are relatively high melting solids and can be isolated as such. The esters of higher carboxylic acids tend to be low melting solids, or viscous liquids, which can be isolated as solutions in a suitable solvent.

As previously mentioned, preferred markers obtained by esterification have the following structural formula:r1 is a C1-C8 alkyl or aryl group. Preferably, R1 is a straight or branched chain C1-C4 alkyl or aryl group. In many petroleum applications, R2-R6 are preferably all hydrogen. The presence of halogen atoms in carbocyclic ring systems can provide different shades of visible light and fluorescence after ester hydrolysis. For example, bromine atoms tend to impart a redder hue to the product than hydrogen atoms.

The esters of the invention may be produced and used in dry (usually powder, crystalline or flake) or liquid form. For processing reasons, the liquid form is generally preferred. The esters of the invention can be produced directly in liquid form without the addition of solvents and used directly. However, it is generally preferred that the marking agent be mixed with the solvent for the marking agent, and that the solvent itself be readily soluble in the petroleum product to be marked. Thus, prior to mixing with many petroleum products, the marking agent may be dissolved by conventional techniques in a solvent that is fully compatible with the petroleum product to be marked. Suitable solvents for liquid petroleum products include, for example, aromatic hydrocarbons (especially alkylbenzenes, such as xylene, and naphthalene), aromatic alcohols, especially benzyl alcohol, and aprotic solvents such as formamide, N-dimethylformamide, or 1-methylpyrrolidone. These solvents can be used alone or preferably in the form of a mixture. Aprotic solvents are particularly useful as co-solvents in combination with aromatic hydrocarbon or aromatic alcohol solvents. For example, a composition consisting of about 0.5 to about 10 weight percent of a tagging agent, about 70 to about 80 weight percent of an aromatic hydrocarbon solvent, and about 10 to about 30 weight percent of an aprotic solvent is particularly useful as a composition that is readily soluble in many liquid petroleum products and is stable in such products; that is to say that it remains soluble in the petroleum product after a long time on the market.

Thus, the esters of the invention form stable liquid compositions that are readily soluble in petroleum products, particularly when combined with appropriate solvents. This effectiveness of marker compounds as stable, free-flowing liquids will be much more attractive to the petroleum industry than dry or solid products, since liquids are easily controlled. However, a dry or solid form of the marker can also be used.

The invention will now be illustrated by the following examples, which are not meant to limit the scope of the invention.

Example 1

33.3 grams of fluorescein was added to a 500ml reaction flask already containing 200 grams of glacial acetic acid and 25 grams of acetic anhydride. Then 0.3 g of concentrated sulfuric acid was added and the reaction flask was stoppered. The contents of the flask were then heated from the outside until they began to boil. Boiling was continued at reflux until a sample of the flask contents, as measured by thin layer chromatography, indicated that all of the original fluorescein was converted to its diacetate ester.

The contents of the flask were then cooled below boiling point and slowly added to 600ml of cold water with good stirring. The mixture was stirred to hydrolyze unreacted acetic anhydride, then the product was recovered by filtration in a Buchner funnel, washed with distilled water to remove acetic acid, and then dried at 105 ℃ to constant weight. The product was obtained in almost quantitative yield as milky white crystals. The melting point of this compound was 199-.

Example 2

The above procedure was repeated with 50 g of 2, 7-di-n-hexyl fluorescein replacing 33.2 g of fluorescein. The final product was obtained as a yellow cream solid: 2, 7-n-hexyl-3, 6-diacetoxyfluorescein.

Example 3

The procedure of example 1 was repeated using 65 g of 2, 4, 5, 7-tetrabromo fluorescein in place of 33.2 g of fluorescein. Finally, light yellow powder of 2, 4, 5, 7-tetrabromo-3, 6-diacetoxyfluorescein is obtained.

Example 4

The procedure of example 1 was repeated substituting 79.0 g of 4, 5, 6, 7-tetrachloro-2, 4, 5, 7-tetrabromo fluorescein for 33.2 g of fluorescein. The diacetate of the starting material was finally obtained as a pale yellow powder.

Example 5

The procedure of example 1 was repeated except that 40 g of butyric anhydride was used instead of 25g of acetic anhydride. The esterification step appeared to be somewhat slow, but the di-n-butoxy ester of fluorescein was finally obtained in almost quantitative yield.

Example 6

33.2 grams of fluorescein contained in a 500ml reaction vial was dissolved in 600ml of cold water by the addition of 16 grams of 50% sodium hydroxide solution. 12 grams of anhydrous sodium carbonate was added to the contents of the flask, then 160ml of xylene solvent was added. 40 g of butyric anhydride are then added dropwise over 60 minutes, during which the biphasic system is stirred at 20-25 ℃. When fluorescein is esterified, the lower aqueous phase is relieved of color and fluorescence and the product dissolves in xylene to form a pale yellow non-fluorescent solution. When all butyric anhydride has been added, the reaction mixture is heated externally to 50-55 ℃ until the esterification has been completed as indicated by thin layer chromatography. The two phases were allowed to separate and the lower aqueous phase containing only traces of unreacted fluorescein was removed. 50 g of 1-methylpyrrolidone (pyrollidone) are added to the remaining upper xylene phase. The contents of the flask were then placed under vacuum and azeotropically distilled for all traces of water and sufficient xylene until the total weight of the reactants was 165 grams. The solution of almost colorless dibutyl ester of fluorescein was filtered and left to store. The solution will have good resistance to crystallization even when stored at 0 ° F for 3 months.

Example 7

The procedure of example 6 was repeated except that the n-butyric anhydride was replaced with an equal amount of isobutyric anhydride. Similar products are obtained, except that the product has even better resistance to crystallization when stored at low temperatures for long periods.

Example 8

The procedure of example 6 was repeated except that 47 g of pivalic anhydride was used instead of 40 g of butyric anhydride. The final di (1, 1, 1-trimethylacetyl) ester of fluorescein is an off-white solid with essentially the same identifying properties as the di-n-butyl ester of example 6.

Example 9

The procedure of example 6 was repeated except for replacing 33.2 g of fluorescein with 50.8 g of 4, 5, 6, 7-tetrachlorofluorescein. The final product was a pale yellow solution which was less stable than the product of example 6 when stored for long periods of time frozen.

Example 10

20 g of the fluorescein diacetate prepared in example 1 were stirred into 50 g of Exxon Aromatic  200 solvent and 30 g of 1-methylpyrrolidone were added. The mixture was heated to 80 ° F until all the ester dissolved, the hot solution was filtered and bottled. The solution showed only a slight tendency to crystallize when stored for extended periods at 0 ° F.

Example 11

50 g of fluorescein diacetate prepared from example 4 were dissolved in 50 g of 1-methylpyrrolidone by gentle heating. The filtered solution has excellent storage stability at 0 ° F.

Example 12

33.2 g of fluorescein were added to 150ml of pyridine to which 36 g of 2-ethylhexanoyl chloride had been added. The mixture was heated to reflux (125 ℃) and boiled overnight. The next morning the reaction mixture was sampled and analyzed by thin layer chromatography, which indicated that the diester-forming reaction was complete. The reaction mixture was then poured into 1 liter of cold water, and the pH was then adjusted to 3 with hydrochloric acid. The isolated product, i.e. the brown oil, was extracted with toluene. The toluene solution was then vacuum stripped to remove all volatile material, leaving 65 grams of a brown oil that readily dissolved in xylene to form a light brown solution.

Example 13

11 g of a 2(3H) furanone derivative called fluorescein succinate, obtained by condensation of one mole of succinic anhydride and two moles of resorcinol under dehydrating conditions, was mixed with 75 g of pyridine. To the resulting mixture was added 25g of lauroyl chloride and the mixture was refluxed (125 ℃) and boiled overnight until a sample of the reaction mixture analyzed by thin layer chromatography indicated that the esterification of fluorescein succinate was complete. The reaction mixture was cooled to 90 ℃ and poured into 1 l of cold water. The mixture was then acidified to pH3 with hydrochloric acid. The product was extracted as a brown oil with 150ml of toluene. The resulting solution was dried, entrained water (entrained water) was removed by azeotropic distillation, and then, residual toluene was removed by vacuum distillation. The final product is a black oil that is readily soluble in xylene to form a light brown solution.

Example 14

The procedure of example 13 was repeated except for using 35 g of stearoyl chloride instead of 25g of lauroyl chloride. The final product was a light brown waxy solid that was readily soluble in xylene.

Example 15

500mg of the solution obtained in example 7 are dissolved in toluene and brought to 100ml in a graduated flask. 1.0ml of this solution was pipetted into 100ml of premium gasoline (retail) that had been colored Red with 3ppm of Unisol Liquid Red B and contained in a separatory funnel. The gasoline sample contained 10ppm equivalent of fluorescein diacetate as a marker. To the identified gasoline in a separatory funnel was added 5ml of an aqueous solution containing 15% sodium chloride and potassium hydroxide sufficient to raise its pH to 12.0. The two phases were shaken together for 2-3 minutes and then allowed to separate. The result was that the upper gasoline phase retained its light red color, but the lower aqueous phase was dark green fluorescent. The phases are separated and the amount of the strong fluorescent dye is measured by spectrophotometry or spectrofluorimetry. The separated solution needs to be diluted five or more times with more extractant in order to have its absorption/emission characteristics fall within the optimal sensitivity range of the measuring instrument.

Example 16

5ml of the pigmented gasoline prepared in example 10 was blended with 95ml of the off-label gasoline. The mixture was then subjected to the same extraction procedure as in example 15 with brine. Even at this much lower marker concentration, the aqueous extract still has significant fluorescence and the amount of dye can be measured instrumentally by comparison to a calibration standard if desired.

Example 17

5ml of the developer composition are added to 50ml of a gasoline sample identified with 10ppm of the dibutyrate ester of fluorescein, prepared according to example 6; the developer composition is a 10% solution of tetrabutylammonium hydroxide dissolved in diethylene glycol. When the mixture appeared to be clearly visible dark fluorescent green on the red background color of gasoline, the mixture was shaken for 1-2 minutes. If only qualitative measurement of the marking agent in the gasoline is needed, the gasoline with the color and the mark can be returned to the fuel source; thus avoiding the potential problem of separately disposing of hazardous waste. If quantitative measurement of the marker is desired, the measurement can be done by direct spectrophotometry or spectrofluorimetry, depending on the background interference of other components in the fuel. In addition, 5ml of a 10% aqueous solution of sodium chloride in distilled water can be added to the colored, labeled fuel. When the mixture is shaken together for a short period of time, the fluorescent marker will be extracted into the lower aqueous phase, which can be separated and quantitatively analyzed as in example 15.

Example 18

A gasoline solution containing 15ppm of 2, 4, 5, 7-tetrabromo-3, 6-diacetoxyfluorescein as synthesized in example 3 was prepared. The mixture was then subjected to the same color development and extraction procedures as described in example 15. At this point, the separated aqueous phase appeared bright red with orange fluorescence. The eosin dye produced can also be quantitatively analyzed by spectrophotometry or spectrofluorimetry.

Example 19

The procedure of example 13 was repeated using diacetyl ester of 4, 5, 6, 7-tetrachloro-2, 45, 7-tetrabromo fluorescein. The hydrolyzed extraction marker contains the divalent anion of a dye historically referred to as Phloxine B. The marker exhibited a bright cherry red color with dark green fluorescence.

Example 20

5ml of an 8% solution of tetramethylammonium hydroxide in ethylene glycol mono-n-propyl ether was added to 100ml of a gasoline solution containing 15ppm of dibutyl ester of 4, 5, 6, 7-tetrachlorofluorescein as prepared in example 7. The mixture was shaken to show dark green fluorescence. The addition of 5ml of 10% aqueous sodium chloride solution will cause the hydrolyzed marker to be extracted into the lower aqueous phase where it forms an orange-brown solution with dark green fluorescence, which is quite different and easily distinguishable from the fluorescence of the non-chlorinated dye exemplified in example 17.

Example 21

100ml of a substantially colorless toluene solution prepared as in example 14 and containing 30ppm of fluorescein succinate distearate were shaken for one minute with 20ml of a mixture composed of 2 parts of tetramethylammonium hydroxide, 48 parts of ethylene glycol mono-n-propyl ether and 50 parts of water. The mixture is then allowed to separate. The lower aqueous phase had a very light orange-yellow color and a very strong dark green fluorescence.

Applicants' invention has now been described with reference to preferred embodiments. Many modifications may be made to the present invention without departing from its scope.

Claims (30)

1. A composition comprising a petroleum product and a detectable amount of a marker selected from the group consisting of:andwherein R1 is C1-C18 alkyl, or aryl, R2, R3, R4, and R5 are hydrogen, chlorine, bromine, or C1-C12 alkyl, and R6 is hydrogen, chlorine, or bromine; wherein the marking agent exhibits fluorescence or color when contacted with a color-developing agent that converts the marking agent to a divalent anion.

2. The composition of claim 1 wherein said marker is present in said petroleum product in an amount of at least 0.5 ppm.

3. The composition of claim 1, wherein the amount of the marker is at least 5 ppm.

4. The composition of claim 1, wherein the marker is present in an amount of 0.5ppm to 100 ppm.

5. The composition of claim 1, wherein R1 is selected from the group consisting of C1-C4.

6. The composition of claim 5, wherein R2-R6 are hydrogen.

7. A method of producing a petroleum product comprising adding a detectable amount of a marker to the petroleum product, said marker being selected from the group consisting of:andwherein R1 is C1-C18 alkyl or aryl, R2, R3, R4 and R5 are hydrogen, chlorine, bromine or C1-C12 alkyl, and R6 is hydrogen, bromine or chlorine.

8. The method of claim 7, wherein the amount of marker is at least 0.5 ppm.

9. The method of claim 7, wherein the amount of marker is at least 5 ppm.

10. The method of claim 7, wherein the marker is used in an amount of 0.5ppm to 100 ppm.

11. The method of claim 7, wherein R1 is selected from the group consisting of C1-C4.

12. The method of claim 11, wherein R2-R5 are hydrogen.

13. The method of claim 11, wherein the marker is in a liquid state when added to the petroleum product.

14. A method of identifying a petroleum product containing a marking agent, comprising: a) obtaining a sample of a petroleum product comprising a detectable amount of a marker selected from the group consisting of:andwherein R1 is C1-C18 alkyl or aryl, R2, R3, R4 and R5 are hydrogen, chlorine, bromine or C1-C12 alkyl, R6 is hydrogen, chlorine or bromine; and

b) a color-developing agent comprising a strong base is added to the sample, which color-developing agent exhibits color and fluorescence when contacted with the marker.

15. The method of claim 14, wherein the developer has a pH of 10 to 14.

16. The method of claim 14, wherein the base is selected from the group consisting of alkali metal hydroxides.

17. The method of claim 14, wherein the base is a quaternary ammonium hydroxide.

18. The method of claim 17, wherein the base has a pH of 11 to 13.

19. The method of claim 14 wherein an extraction medium is added to said sample.

20. The method of claim 19 wherein the extraction medium and the liquid petroleum product are combined in a 1: 17 volume ratio.

21. The method of claim 19, wherein the extraction medium is a mixture comprising water and a phase separation enhancer selected from the group consisting of aliphatic alcohols, aromatic alcohols, glycols, or glycol ethers.

22. The method of claim 21 wherein said mixture further comprises a quaternary ammonium hydroxide compound.

23. A method of identifying a petroleum product comprising: a) obtaining a sample of a petroleum product comprising a detectable amount of a marker selected from the group consisting of:andb) adding a color developing agent comprising a strong base to the marking agent; c) extracting the marking agent into an extraction medium.

24. A liquid marking agent for liquid petroleum products comprising: a) a marker selected from the group consisting of:andwherein R1 is C1-C18 alkyl, or aryl, R2, R3, R4, and R5 are hydrogen, chlorine, bromine, or C1-C12 alkyl, and R6 is hydrogen, chlorine, or bromine; and

b) a solvent in a weight ratio to the marking agent of at least 1: 2.

25. The liquid marker of claim 24, wherein the solvent is selected from the group consisting of aromatic hydrocarbons, aromatic alcohols, and aprotic solvents, alone or in combination.

26. The liquid marking agent of claim 24, wherein the weight ratio of the solvent to the marking agent is at least 1: 1.

27. The liquid marking agent of claim 24, comprising 0.5 to 10 weight percent of the marking agent; 70-80 wt% of aromatic hydrocarbon or aromatic alcohol solvent and 10-30 wt% of aprotic solvent.

28. The liquid marker of claim 27, wherein the aprotic solvent is selected from the group consisting of 1-methylpyrrolidone, N-dimethylformamide, and formamide.

29. The liquid marker of claim 24, wherein R1 is selected from the group consisting of C1-C4.

30. The liquid marking agent of claim 29, wherein R2-R6 are hydrogen.