EPA Case # 5034 Docket Number: EPA-HQ-OPP-2011-0067 

THPS (CAS: 55566-30-8) 

RHODIA - SGROOME 1/5 

6 MAY 2011 

Tetrakis (hydroxymethyl) phosphonium sulfate 

THPS 

Chemistry Document 

Steve Groome 

Global Regulatory Affairs & Product Stewardship Manager 

May 2011 

Contents

1. Introduction 

2. Chemistry 

2.1. What is THPS? 

2.2. What are the degradation products of THPS? 

2.3. How much formaldehyde can be produced? 

3. Should THPS be regarded as a formaldehyde releaser? 

4. Conclusions 
EPA Case # 5034 Docket Number: EPA-HQ-OPP-2011-0067 

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6 MAY 2011 

1. Introduction

Albright & Wilson (a company later acquired by Rhodia) first registered THPS in the United States 

in 1995. 

The Agency published a docket in relation to THPS according to the requirements of the review 

program detailed in Section 3(g) of FIFRA. 

Summary document (Docket Number: EPA-HQ-OPP-2011-0067) explains that "The Agency 

believes that THPS may be a formaldehyde releaser because it contains a hydroxymethyl group, 

which is essential to the production of formaldehyde". 

Rhodia and the Agency held an informal conference call on 27 January 2011 to discuss this 

matter. Rhodia agreed to compile an explanatory ‘chemistry document’ for the Agency. That is the 

purpose of this document. 

2. Chemistry

2.1 What is THPS? 

The active substance, Tolcide® PS75, is a 75% solution of THPS in water. 

2.2 What are the degradation products of THPS? 

The manufacturing concentrate and all end use products are stable. The formaldehyde 

concentration in the commercial product is low and is closely controlled. The product of course 

conforms to OSHA requirements (29CFR1910.1048 for hazard communication). 

THPS is always used in water. The degradation in water has been followed analytically on many 

occasions and the following reaction scheme shows the main steps: 

TetrakisHydroxymethylPhosphonium Sulfate (THPS)

HOCH2

C

P

+ CH2OH SO4

2-

CH2OH

CH2OH

2 
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 THPS THPO Formaldehyde Sulfuric acid 

When THPS concentrate is diluted in water to ppm levels, as it is used as a biocide, it will begin to 

hydrolyse in the presence of even the smallest concentrations of dissolved oxygen. The initial 

degradation follows the reaction shown above. 

THPS degrades to form THPO plus small amounts of formaldehyde and sulfuric acid, all of which 

are very soluble in water. 

THPO is tris (hydroxymethyl) phosphine oxide, which is a less reactive compound than THPS. The 

phosphorus in THPO is in a more stable oxidation state. However THPO is not a biocide and it 

was found not to have adverse effects on aquatic organisms tested (invertebrates, benthic 

organisms, fish and algae from freshwater and seawater). 

In addition, in biodegradation studies, or following prolonged hydrolysis, further breakdown can 

occur, resulting in complete mineralization to carbon dioxide, water and inorganic phosphate. 

Rhodia is in possession of studies using radiolabelled THPS which show ready biodegradation to 

give 14CO2. Therefore, as THPO is the major first degradate of THPS, it may also be concluded 

that THPO is not persistent in the environment. 

2.3 How much formaldehyde can be produced? 

When THPS is used as a biocide, analysis confirms THPO as the main degradant. When THPS 

reacts to form THPO the stoichiometry is as follows: 

1 mole THPS → 2 moles THPO + 2 moles formaldehyde + 1 moles sulfuric acid 

406 g THPS → 60 g formaldehyde 

If the dose level of THPS is 10 ppm, it will produce only 1.5 ppm of formaldehyde in solution. As 

we will see in section 3 of this report, this level is insufficient to be efficacious in most 

circumstances. The formaldehyde is produced in water and formaldehyde is readily 

biodegradable. Due to the use pattern of THPS, inhalation of treated water is not foreseeable. 

Dermal exposure to a solution of formaldehyde (at the ppm level) should not give rise to concern. 

For an industrial biocide, short duration and infrequent dermal exposure should be the worst case 

encountered. 

In the most severe hypothetical case that THPS was to be completely mineralized to carbon 

dioxide, water and phosphate, the degradation would be expected to occur via the production of 

formaldehyde, which itself is rapidly degradable – but only in water and over an extended period of 

time (the normal duration of a ready biodegradability is 28 days). In this worst case, 1 mole of 

THPS would produce 8 moles of formaldehyde. This means 406 g of THPS would produce 240 g 

formaldehyde, and 10 ppm THPS would produce 5.9 ppm formaldehyde in water. It should be 

emphasised that the relative rates of formaldehyde production and removal make it extremely 

unlikely that this concentration of formaldehyde would ever be present in any practical situation. 

 HOH2C P+ CH2OH SO4

2- 

 2 HOCH2 P CH2OH + 2HCHO + H2SO4

2-

2 

CH2OH

CH2OH O

CH2OH

O2
EPA Case # 5034 Docket Number: EPA-HQ-OPP-2011-0067 

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Rhodia has data on the rates of hydrolysis in aerobic and anaerobic conditions; the rates of 

removal of formaldehyde from aqueous solutions (by hydrolysis, biodegradation, etc) are well 

known. 

Formaldehyde is very soluble in water. The corresponding concentration in the headspace can be 

calculated to be very low compared to the concentration in solution. 

3. Should THPS be regarded as a formaldehyde releaser?

Formaldehyde release agents (FRAs) release formaldehyde and their efficacy is comparable to 

that of formaldehyde. Their purpose is to produce formaldehyde in-situ to avoid handling of 

formaldehyde, or to provide slow delivery of formaldehyde. This definition is widely held within the 

industry, and within companies servicing water treatment applications. FRAs are a category of insitu biocide. 

The THPS molecule is the biocidally active agent. The effectiveness of THPS is not dependent 

upon release of formaldehyde. In fact, THPS is more effective than formaldehyde; it is faster 

acting than formaldehyde, and it is effective against ‘formaldehyde-resistant’ bacteria. 

THPS has different modes of action to formaldehyde. Formaldehyde is deactivated by hydrogen 

sulphide, whereas THPS is not. In oilfield applications, THPS is effective in sour systems whereas 

formaldehyde is not. 

The biocidal mode of action of formaldehyde is well documented. It works by protein denaturing 

(cross-linking) of proteins in cell structures (Reference: Hugo (1971), Inhibition and Destruction of 

the Microbial Cell). In contrast a 1986 Albright & Wilson study identified multiple modes of action 

for THPS against sulfate-reducing bacteria: protein denaturing, enzyme destruction and inhibition, 

interference with the ADP-ATP energy cycle, and inhibition of sulfate reduction. 

Although THPS degradation results in the production in solution of small amounts of 

formaldehyde, its biocidal mechanism is not dependent upon the released formaldehyde. In fact, 

the THPS molecule is the active agent and the presence of the THPS molecule results in quicker 

and more complete protein denaturing, and in formation of stronger, irreversible protein crosslinks. 

The cross-linking mechanisms are different for THPS and formaldehyde: 

 THPS mechanism: P-CH2OH + H2N-R  ...P-CH2NH-R 

 Formaldehyde mechanism: HCHO + H2N-R  ...CH2-NH-R 

The presence of the P-C-N bond is critical: such bonds are very stable and result in irreversible 

cross-linking which results in greater biocidal efficacy. 

Because THPS is not a FRA, its use patents are very different to that of FRAs and reflect different 

use patterns. 

For all these reasons, THPS should not be considered a formaldehyde releasing agent. 
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4. Conclusions

In use, the degradation of THPS does result in the production of small quantities of formaldehyde. 

However, these levels do not present a risk to human health or to the environment. The 

concentrations that are produced are unlikely to be effective as a biocide. 

The THPS molecule is biocidally active. The mode of action of THPS is not dependent upon 

released formaldehyde. THPS is more effective and faster acting than formaldehyde. THPS is 

effective against ‘formaldehyde resistant’ bacteria. 

There are many reasons why THPS itself must be considered as the sole active biocide. 

THPS is not a formaldehyde releasing agent, as defined and accepted by the biocides industry.