| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | July 7, 1992 |
| PATENT TITLE |
Synthesis of tagged polymers by post-polymerization (trans) amidation reaction |
| PATENT ABSTRACT | Polymers tagged with pendant fluorescent groups are prepared by the (trans)amidation derivatization of pre-existing polymers having carbonyl-type pendant groups. Polymers having pendant amide groups wherein the amide nitrogen is substituted with fluorescent moieties, prepared by (trans)amidation drrivatization, are provided. Polymers having pendant (sulfonated) napthalene moieties substituted to amide nitrogen are provided |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | August 20, 1990 |
| PATENT CLAIMS |
We claim: 1. A method of preparing a polymer pendant fluorescent groups comprising: admixing an aqueous solution of water-in-oil latex of a polymer having ##STR6## wherein R is --OR' or --NH.sub.2, R' is hydrogen or a substituent other than hydrogen, R.sub.1, R.sub.2 and R.sub.3 are independently hydrogen or a substituent other than hydrogen, and R.sub.1 is C.sub.n H.sub.2n wherein n is zero or an integer of from 1 to about 10, and salts thereof and mixtures and combinations thereof, with an amine-containing organic fluorescent composition of the Formula III ##STR7## wherein one of R.sub.5 and R.sub.6 may be hydrogen, and within at least one of R.sub.5 and R.sub.6 or within R.sub.5 and R.sub.6 taken together is an organic fluorescent group, wherein said organic fluorescent group includes a polynuclear aromatic ring system; and heating said admixture for a sufficient period of time to accomplish a degree of (trans)amidation derivatization of said polymer. 2. The method of claim 1 wherein said admixture is heated in a closed vessel at a temperature of from about 120 to about 200.degree. C. 3. The method of claim 1 wherein said admixture contains at least 10 weight percent of said polymer. 4. The method of claim 1 wherein said amine-containing organic fluorescent composition is admixed with said polymer in an amount of at least 0.01 weight percent based on total weight of said polymer. 5. The method of claim 1 wherein said (trans)amidation derivatization of said polymer is accomplished to the degree of incorporating into said polymer at least 0.01 weight percent of said amine-containing organic fluorescent composition. 6. The method of claim 1 wherein said amine-containing organic fluorescent composition contains a primary amine. 7. The method of claim 1 wherein said admixture has a pH of from about 3 to about 11. 8. The method of claim 1 wherein said admixture is heated for a time period of from about 1 to about 8 hours. 9. The method of claim 1 wherein said admixture is an aqueous solution of said polymer at a concentration of said polymer of from about 30 to about 40 weight percent, and said polymer has a weight average molecular weight of up to 100,000. 10. The method of claim 1 wherein said admixture is a water-in-oil latex of said polymer and said polymer has a weight average molecular weight of at least 200,000. 11. The method of claim 7 wherein said polymer contains at least 10 mole percent of said mer units of said Formula II. 12. The method of claim 7 wherein said polymer contains at least 50 mole percent of said mer units of said Formula II. 13. The method of claim 7 wherein said mer units are units of acrylic acid, acrylamide, salts thereof, or combinations thereof. 14. The method of claim 1 wherein said aromatic ring system contains sulfonic acid, carboxylic acid, salts thereof, or a plurality thereof substituents(s). 15. The method of claim 1 wherein said aromatic ring system is a naphthalene ring system having from 0 to 3 sulfonic acid substituents or salts thereof. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
TECHNICAL FIELD OF THE INVENTION The present invention is in the technical field of polymer synthesis by the method of post-polymerization derivatization and polymers prepared thereby. More particularly, the present invention is in the technical field of synthesizing polymers having pendant fluorescent groups and polymers prepared thereby. BACKGROUND OF THE INVENTION In many fields that employ polymers it may be desirable to tag or mark such polymers to facilitate monitoring thereof. By the term "monitoring" is meant herein any type of tracing or tracking to determine the location or route of the polymers, and any type of determination of the concentration or amount of the polymer at any given site, including singular or intermittent or continuous monitoring. For instance, it may be desirable to monitor water treatment polymers in water systems, particularly industrial water systems, or to monitor polymers that may be present in waste fluids before disposal, particularly industrial waste fluids, or to monitor the polymer used for down-hole oil well applications, particularly the route taken after introduction down-hole, or to monitor polymers that may be present in fluids used to wash a manufactured product, for instance a polymer-coated product, to determine the amount of polymer washed or leached therefrom. By fluids or liquids as used herein generally is meant aqueous, non-aqueous, and mixed aqueous/non-aqueous fluid systems. As seen from the above list of possible applications of polymer monitoring, the purpose of such monitoring may be to trace or track or determine the level of the polymer itself, or to trade or track or determine the level of some substance in association with the polymer, or to determine some property of the polymer or substance in association with the polymer, for instance its leachability. Conventional techniques for monitoring polymers are generally time-consuming and labor intensive, and often require the use of bulky and/or costly equipment. Most conventional polymer analysis techniques require the preparation of calibration curves for each type of polymer employed, which is time-consuming and laborious, particularly when a large variety of polymer chemistries are being employed, and the originally prepared calibration curves lose their accuracy if the polymer structures change, for instance an acrylic acid ester mer unit being hydrolyzed to an acrylic acid mer unit. Direct methods wherein the level of functional groups present in a polymer is determined analytically are generally not practical for industrial use, particularly when it is desired to monitor a polymer on a frequent or continuous basis, or when rapid monitoring results are needed. Indirect methods of polymer monitoring may provide more rapid results using simpler techniques, but in many instances faster and/or more accurate determinations are desirable. Polymers tagged with pendant fluorescent groups are generally easily monitored, even when present at low concentrations. Highly fluorescent molecules, that is molecules which have a fluorescent quantum efficiency, or fluorescent quantum yield, within the range of from about 0.1 to about 1.0, and a light absorbance molar extinction coefficient of at least 1,000, are typically detectable at concentration ranges of parts per million ("ppm") to parts per billion ("ppb") or even less. The incorporation of such a highly fluorescent species into a polymer in the amount of one weight percent (based on polymer actives) will permit the detection of such polymer at polymer concentration levels down to 0.1 ppm or less, provided the fluorescent quantum yield and the light absorbance molar extinction coefficient of the fluorescent tagging agent are not significantly adversely affected by its attachment to the polymer. It would be desirable to provide a method of tagging polymers with pendant fluorescent groups by derivatization of a pre-existing polymer and hence avoid problems such as potential side-reactions or other complications that may arise in attempts to incorporate fluorescent monomer units into the polymer during synthesis of such polymer. It would be desirable to provide a method of tagging pre-existing polymers with pendant fluorescent groups so as to permit polymers that are commercially available without such tagging to be converted to tagged polymers. It would be desirable to provide such a method that minimizes the reaction steps required and minimizes the time required for the reaction. It would be desirable to provide such a method that proceeds under reaction conditions that do not have a deleterious effect on the polymer structure. It would be desirable to provide such a method that can utilize as the agent providing the pendant fluorescent groups, inexpensive commercially available compositions. It would be desirable to provide such a method that can employ a wide variety of fluorescent derivatizing agents and hence provide tagged polymers having a wide variety of fluorescent spectra, particularly when it is desired to monitor polymers individually in situations where a plurality of polymers are present. For instance, such polymers may be tagged with distinguishable fluorescent derivatizing agents and hence each polymer may be individually monitored, or the derivatizing agent(s) may be selected so as to monitor the polymer(s) even in the presence of other fluorescent material(s). It would be desirable to provide such a method wherein a reasonably high concentration of polymer starting material may be utilized. It would be desirable to provide such a method that is not dependent on the presence of a single type of pendant group being present on the starting material polymer. It would be desirable to provide such a method that can proceed simultaneously with other post-polymerization derivatizing reactions. It would be desirable to provide polymers tagged with fluorescent groups produced by such a method. It would be desirable to provide such polymers having diverse fluorescent tags. It would be desirable to provide such polymers derivatized at reasonable cost. It would be desirable to provide such polymers having fluorescent spectra distinguishable from the spectra of the fluorescent derivatizing agents employed. These and other objects are provided by the present invention which is described in more detail below. DISCLOSURE OF THE INVENTION The present invention provides a method of preparing polymers having pendant fluorescent groups by (trans)amidation derivatization of pre-existing polymers by reaction between certain pendant groups on such polymers and certain organic fluorescent derivatizing agents. More specifically, the pendant polymer groups derivatized by the (trans)amidation reaction are carbonyl-type groups of the Formula I: ##STR1## wherein R is --OR' or --NH.sub.2, wherein R' is hydrogen or a substituent other than hydrogen, and salts thereof, and mixtures and combinations thereof. By salts is meant herein alkali metal salts, alkaline earth metal salts, amine salts, alkanol amine salts, and the like. By "substituent" is meant herein a single or multivalent group other than hydrogen covalently bonded to a carbon of an organic molecule. The pendant polymer group derivatized by the (trans)amidation reaction may be contained in a polymer unit or mer unit (a portion of the polymer containing two adjacent backbone carbons) having the structure of Formula II: ##STR2## wherein R.sub.4 is C.sub.n H.sub.2n wherein n is zero or an integer from 1 to about 10, R.sub.1, R.sub.2 and R.sub.3 are independently hydrogen or a substituent other than hydrogen, and R is as defined above for Formula I, and salts thereof, and mixtures or combinations thereof. The organic fluorescent derivatizing agent is an organic fluorescent molecule that contains an amine group, preferably a primary amine group, Such an agent may be represented by the Formula III: ##STR3## wherein one of R.sub.5 and R.sub.6 may be, and preferably is, hydrogen, and within at least one of R.sub.5 and R.sub.6 or within R.sub.5 and R.sub.6 together, is an organic fluorescent group. The (trans)amidation derivatization reaction proceeds as follows: ##STR4## wherein the structure designated Formula IV above is a (trans)amidation derivatized pendant group on the polymer. As seen from Formula IV, the derivatized pendant polymer group is a pendant N-substituted amide group having a fluorescent group within at least one of R.sub.5 and R.sub.6 or within R.sub.5 and R.sub.6 taken together. This reaction is an amidation reaction when R is --OR' and a transamidation reaction when R is --NH.sub.2, and hence the term "(trans)amidation" is used herein to specify either or both forms of the reaction. PREFERRED EMBODIMENTS OF THE INVENTION The (trans)amidation reaction is conducted in an aqueous reaction mixture generally, employing a starting material polymer that is water soluble at the concentration used, or is introduced as held within a latex, and in addition, employing an organic fluorescent derivatizing agent that is water soluble. The reaction mixture preferably is fluid. The product polymer resulting from the (trans)amidation derivatization reaction may possibly have its solubility characteristics so altered by the incorporation of the fluorescent group that it precipitates, or partially precipitates, from the reaction mixture. The (trans)amidation reaction is conducted at elevated temperatures, and generally under pressures that exceed atmospheric pressure. In general it is desirable not to degrade the polymer to any significant extent during the derivatization reaction and generally a reaction in a closed vessel at a temperature of from about 120 to about 200.degree. C., for a time period of from about 1 to about 8 hours, may be suitable, the pressure being dependent in part on the temperature. In preferred embodiment the reaction is conducted at from about 130 to about 155.degree. C. In preferred embodiment the reaction is conducted over a time period of from about 1 to about 6 hours. Nonetheless, the reaction may proceed somewhat at temperatures as low as 75 or 80.degree. C. The derivatization reaction may be conducted with air present in the reaction vessel at times, but it may be desirable to exclude oxygen, for instances by purging the reaction vessel with nitrogen gas prior to the reaction. When it is desirable to reduce the pressure that would otherwise be attained by reaction at a given temperature, the reaction vessel may have its internal pressure reduced prior to closing and raising the temperature for the transamidation reaction. The derivatization reaction may be conducted under varying pH conditions, for instance, at a pH of from about 3 to about 10 or possibly 11, but it is preferable, particularly to avoid hydrolysis or polymer degradation, to conduct the reaction at a moderate pH. Moreover, one of the advantages of the present invention is that the derivatization reaction proceeds at a moderate pH, which preferably is a pH of from about 5 to about 7. Another of the advantages of the present invention is that, for a suitable polymer starting material, the derivatization reaction may be conducted employing a high concentration of starting material polymer, which is particularly advantageous for the commercial use of the derivatization reaction. Commercially, a starting material polymer concentration of at least about 10 weight percent is desirable, but if a low water solubility polymer is used, a lower concentration may be necessary to maintain a fluid reaction mixture. For water soluble polymers with weight average molecular weights of up to 50,000 to 200,000, or up to 50,000 to 100,000, starting material polymer concentrations of from about 10 to about 45 weight percent are desirable, and from about 30 to about 45 weight percent are very preferred. As noted above, the starting material polymer may be a polymer containing polymer units of the Formula II above. Such polymer units may be derived from acrylic acid, wherein the n of R.sub.4 is zero, each of R.sub.1, R.sub.2 and R.sub.3 are hydrogen, and R is --OH. Such polymer units may be derived from acrylamide, similar to acrylic acid except R is --NH.sub.2. Such units may be methacrylic acid or methacrylamide wherein R.sub.2 is methyl, or for instance itaconic acid wherein R.sub.2 is --CO.sub.2 H, R.sub.4 is --CH.sub.2 --, and R is --OH. Such polymer units may contain carboxylic acid esters, such as methylacrylate wherein R is --OR' and R' is methyl. It is believed that existing N-substituted amide pendant groups do not to any significant extent participate in the (trans)amidation reaction. The presence of nonparticipating pendant groups on the starting material polymer generally will not interfere with the (trans)amidation reaction, and hence the present invention may be used to tag a wide variety of pre-existing polymers. Only a very low level of fluorescent groups need to be incorporated into a polymer to provide fluorescent detectibility to the polymer. When highly fluorescent groups are employed, the fluorescent derivatizing agent may be required in amounts as low as 0.01 weight percent based on total weight of the polymer, and generally an amount of about 1 weight percent based on total weight of polymer should be sufficient. While the maximum amount of fluorescent groups that may be incorporated into the polymer is limited only by the reactive sites available for a given polymer, there is generally no practical purpose for excessively tagging a polymer. Since it is believed that the derivatization reaction does not, under the preferred and advantageous reaction conditions of the invention, proceed to the exhaustion of all fluorescent derivatizing agent and the exhaustion of all reactive sites on the polymer, it is preferred to employ a polymer having more reactive sites (pendant groups of the Formula I) than required for exhaustion of the fluorescent derivatizing agent used. Hence while a 1 to 1 mole ratio of polymer reactive sites to fluorescent derivatizing agent is possible, and in fact less polymer reactive sites could reasonably be employed, it is desirable to have an excess of polymer reactive sites, and a soluble polymer of any significant molecule weight will have such an excess generally. In preferred embodiment, the starting material polymers of the present invention are water soluble polymers having molecular weights of from about 2,000 to about 100,000, and more preferably to about 50,000. In further preferred embodiment, the starting material polymers of the present invention are water soluble polymers having at least 10 mole percent, and more preferably at least 30 or 50 mole percent, of mer units of the Formula II. In ever more preferred embodiment, such polymers have at least 70 mole percent of mer units of the Formula II. Nonetheless the (trans)amidation reaction of the present invention is believed to proceed with as little as about 1 weight percent of such mer units, particularly if excess derivatizing agent is used. In another preferred embodiment, the starting material polymers of the present invention are employed in the form of lattices or emulsions, rather than aqueous solutions. Some polymers are manufactured in latex form, and for such polymers it is highly advantageous to utilize them in that form. High molecular weight polymers are often manufactured as water-in-oil lattices, and this manufacturing technique is advantageous for polymers of such high molecular weight that solution polymerization is not practical. Lower molecular weight polymers could also be made in water-in-oil form, but generally there is no practical reason for employing this technique, which is more expensive generally than solution polymerization. Thus while there is no theoretical minimum molecular weight for a polymer employed in the present invention in latex form, in preferred embodiment such polymer has a molecular weight of at least 200,000, and more preferably at least 500,000, and even more preferably at least 1,000,000. The fluorescent group(s) of the organic derivatizing agent in preferred embodiment contains an aromatic ring system, including polynuclear aromatic ring systems, which may also contain various functional groups. The molecule containing such aromatic ring system must also contain an amine radical, preferably a primary amine radical, which amine radical may or may not be directly substituted to the aromatic ring system. Other various functional groups that may be present in such molecule are carboxylic acid, sulfonic acid, hydroxyl, nitrile, keto, amine, substituted amine, and the like and such functional groups may or may not be directly substituted to the aromatic ring system. An extremely important advantage of the present invention is the number and variety of such organic fluorescent amine-containing compositions that are commercially available as water soluble compositions, and others that may become water soluble, or more water soluble, upon formation of the salts thereof. The fluorescent derivatizing agents useful for the present invention are available with fluorescent spectral characteristics that can distinguish one agent from the others. Thus a plurality of polymers may be each tagged with a different fluorescent group so as to be able to distinguish between such polymers by virtue of their resultant unique spectral characteristics. In a preferred embodiment, with reference to Formula III above, the organic fluorescent derivatizing agent may be defined as one in which one of R.sub.5 and R.sub.6 is hydrogen and the other of R.sub.5 and R.sub.6 is an (alkylene)aromatic ring system in which the aromatic ring system may contain other substituents, particularly sulfonicacid, carboxylic acid and salts thereof and the aromatic ring system may be a polynuclear aromatic ring system. By (alkylene)aromatic ring system is meant that the aromatic ring system may be either bonded to its amine nitrogen through a carbon within the ring system or bonded to such nitrogen through an alkylene group or a combination of an alkylene group and a functional group, for instance when the (alkylene)aromatic ring system is an N-alkylene substituted aromatic amide. The amine group of the organic fluorescent derivatizing agent may be covalently bonded directly to the agent's aromatic ring system, or it may be bonded to the ring system through an alkyl group. In other words, the group bonded to the ring system may be an amine, an amine substituted alkyl group, or an amine substituted alkyl group containing further substitution. In one preferred embodiment of the invention, the amine-containing fluorescent derivatizing agent is one containing an alkyl amine, and is so preferred because the (trans)amidation reaction proceeds efficiently with such an agent. In a different preferred embodiment of the invention, the amine-containing fluorescent derivatizing agent is one containing a primary amine covalently bonded to the ring system, and is so preferred because such agents are generally readily available at very low cost. A very useful group of organic fluorescent derivatizing agents contain a naphthalene ring system, which ring system is substituted with at least one primary amine and at least one sulfonic acid group. There are many compositions within this group that are commercially available at reasonable cost, and there are many compositions within this group, distinguished from one another by the number of amine and sulfonic acid substituents and, for a given number of each substituent, by the position of each substituent on the naphthalene ring system. Below in Table I is a list of commercially available compositions within this group, identified as to number of substituents and ring positions, which list is not meant to be exhaustive of the compositions within this group. TABLE I __________________________________________________________________________ Composition Substituent at Specified Ring Position Designation 1 2 3 4 5 6 7 8 __________________________________________________________________________ a --NH.sub.2 --SO.sub.3 H -- -- -- -- -- -- b --NH.sub.2 -- -- --SO.sub.3 H -- -- -- -- c --NH.sub.2 -- -- -- --SO.sub.3 H -- -- -- d --NH.sub.2 -- -- -- -- --SO.sub.3 H -- -- e --NH.sub.2 -- -- -- -- -- --SO.sub.3 H -- f --NH.sub.2 -- -- -- -- -- -- --SO.sub.3 H g --SO.sub.3 H --NH.sub.2 -- -- -- -- -- -- h -- --NH.sub.2 -- -- --SO.sub.3 H -- -- -- i -- --NH.sub.2 -- -- -- --SO.sub.3 H -- -- j -- --NH.sub.2 -- -- -- -- --SO.sub.3 H -- k -- --NH.sub.2 -- -- -- -- -- --SO.sub.3 H l --SO.sub.3 H --NH.sub.2 -- -- --SO.sub.3 H -- -- -- m -- --NH.sub.2 -- --SO.sub.3 H -- -- -- --SO.sub.3 H n -- --NH.sub.2 -- -- --SO.sub.3 H -- --SO.sub.3 H -- o -- --NH.sub.2 -- -- -- --SO.sub.3 H -- --SO.sub.3 H p -- --NH.sub.2 --SO.sub.3 H -- -- --SO.sub.3 H -- -- q --NH.sub.2 -- -- --SO.sub.3 H -- -- -- --SO.sub.3 H r --NH.sub.2 -- --SO.sub.3 H -- -- -- -- --SO.sub.3 H s --NH.sub.2 -- -- --SO.sub.3 H -- -- --SO.sub.3 H -- t --NH.sub.2 -- --SO.sub.3 H -- -- -- --SO.sub.3 H -- u --NH.sub.2 -- --SO.sub.3 H -- -- -- SO.sub.3 H -- -- v --NH.sub.2 -- --SO.sub.3 H -- -- --SO.sub.3 H -- --SO.sub.3 H w --NH.sub.2 -- -- --SO.sub.3 H -- --SO.sub.3 H -- --SO.sub.3 H x -- --NH.sub.2 -- --SO.sub.3 H -- --SO.sub.3 H -- --So.sub.3 H y --SO.sub.3 H --NH.sub.2 -- -- --SO.sub.3 H -- --SO.sub.3 H -- aa -- --NH.sub.2 --SO.sub.3 H -- -- --SO.sub.3 H -- --SO.sub.3 H __________________________________________________________________________ The amine-containing naphthalene compositions set forth in Table I above include amino naphthalene monosulfonic acids (Compositions a through k), amino naphthalene disulfonic acids (Compositions l through u), and amino naphthalene trisulfonic acids (Compositions v through aa), and various of these compositions are supplied as, or are available as, their sodium and/or potassium salt(s). There are numerous commercially available products that contain a fluorescent group and either a primary or secondary amine, or a plurality of same. For instance, Aldrich Chemical Company, Inc., lists in excess of 50 of such products in addition to naphthalene derivatives. Such compositions are generally dyes or dye intermediates, and include some compositions that do not have a polynuclear aromatic ring system, such as benzene derivatives and compositions containing a plurality of benzene rings. Again the amine group required for the present invention may be a primary amine covalently bonded directly to the aromatic ring or it may be a secondary amine bonded to the aromatic ring, or it may be bonded to another group in turn bonded to the aromatic ring. There is no need generally for the (trans)amidation reaction to proceed to extent where all of the organic fluorescent derivatizing agent is incorporated into the polymer. Residual fluorescent derivatizing agent is generally innocuous, and when desired may be separated from the product polymer, such as by selective precipitation of the polymer from the liquid system employed in the (trans)amidation reaction. In preferred embodiment, the product polymer has different spectral properties than the organic fluorescent derivatizing agent, and hence the presence of product polymer successfully tagged by the (trans)amidation reaction can be determined merely by the presence of its particular major fluorescent peak(s). The following Examples 1 through 37 demonstrate the advantages of the present invention, including without limitation the successful fluorescent tagging of pre-existing polymers without any deleterious effect on the polymer of any significance, using a wide variety of amine-containing fluorescent derivatizing agents, The derivatizing agents used were all commercially available, and many of such agents are commercially available at low cost. In some instances, noted in more detail below, the fluorescent spectras of the derivatized polymers differ from the fluorescent spectras of the derivatizing agents used, and hence one may confirm that fluorescent groups have been incorporated into the polymer by fluorescent spectral analysis without isolating the product polymer from any residual derivatizing agent that may be present. The synthesis method is exemplied using reaction mixtures containing high concentrations of polymer starting material, moderate pH conditions, a single reaction step, and generally low levels of derivatizing agent. The synthesis method proceeds successfully in short time periods. The polymers employed vary as to the amount of pendant groups of Formula I, and a number of the polymers employed have diverse pendant groups outside of Formula I. Exemplified are derivatizations using a single type of pendant group within Formula I, that is, an acrylic acid homopolymer, and a number of polymers containing both (meth)acrylic acid and acrylamide units. The polymers employed as starting material polymers in Examples 1 to 37 each have more than 70 mole percent of mer units within Formula II and weight average molecular weights of less than 100,000. The following abbreviations are used for the polymer units of these and other Examples: ______________________________________ AA Acrylic acid AcAm Acrylamide SMA Sulfomethylacrylamide VA Vinyl acetate MAA Methacrylic acid ACN Acrylonitrile ______________________________________ It has been found that the (trans)amidation derivatization of the present invention may proceed together with other post-polymerization derivatization reactions that are feasible under the reaction conditions. For instance, in Examples 9, 10, 13, 20, and 22 to 32, a sulfomethylation derivatization reaction is conducted simultaneously using a formaldehyde-bisulfite (HOCH.sub.2 SO.sub.3 Na) derivatizing agent. In a number of the synthesis Examples that follow, sodium hydroxide is added to the reaction mixture for the purpose of neutralizing the fluorescent derivatizing agent by forming the salt form, which is more water soluble. |
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