Potassium Hydrogen Phthalate Application and Performance

Potassium hydrogen phthalate has the molecular formula KHC₈H₄O₄ and a CAS number of    877-24-7. It is a white crystalline powder; its aqueous solution is weakly acidic. In its structure, a benzene ring is linked to a carboxyl group and a potassium carboxylate group. With high purity, stable properties, and a constant molar mass, potassium hydrogen phthalate serves as a standard reagent for acid-base titration to calibrate solutions of strong bases such as sodium hydroxide. It is also used as a raw material for preparing pH buffer solutions and as a reference standard for chromatographic analysis.

Synergistic Cationic Shielding and Anionic Chemistry of Potassium Hydrogen Phthalate

Scientists demonstrate a multifunctional electrolyte additive of potassium hydrogen phthalate (KHP) with a synergistic effect of the cations and anions on the reversible and stable Zn deposition for aqueous Zn metal batteries. Compared with other alkali metal ions, K+ ions achieve a larger ionic radius, smaller stroke radius, and low reduction potential. Therefore, K+ ions dissociated from potassium hydrogen phthalate additives act as electrostatic shielding cations that can effectively balance the electric field distribution, smooth the deposition morphology, and inhibit the formation of dendrites. On the other hand, the anions entering the first shell of Zn hydrate form the contact ionic pairs (CIP), reducing the activity of parasitic H2O and accelerating the desolvation kinetics of Zn hydrates. The comprehensive analysis of the desolvation process elucidates that the KHP additives are reduced to form anion‐derived SEIs, which effectively inhibit corrosion and HER during the long‐term cycling, compensating the weak point of electrostatic shielding cation. Moreover, the potassium hydrogen phthalate additive, used as the main component of the buffer solution, can effectively stabilize the interfacial pH fluctuations caused by the reduction process at the initial stage of the charge storage reaction. Benefiting from these advantages, the electrolyte with the synergistic effect of the cations and anions shows a greatly improved performance in both half and full cells. These findings provide new insights into the development and application of aqueous electrolyte chemistry.[1]

Based on the theoretical calculation and experiment, the effective regulation ability of K ion as an electrostatic shielding cation on the morphology of aqueous Zn ion deposition was first verified systematically. However, maintaining in an aqueous environment, the flat Zn surface is still challenged by slow kinetics, corrosion, hydrogen evolution, and other side reactions, which are obvious under long‐term cycling. A low‐cost, multi‐functional potassium hydrogen phthalate additive was developed to realize a dendrite‐free anode with a long lifespan in aqueous Zn batteries. In particular, the electrostatic shielding K cations and the HP anion‐derived SEIs were expected to resolve the Zn dendrite growth, HER, and corrosion. The electrostatic shielding effect of K ions contributed to modifying the deposition morphology for the formation of dendrite‐free Zn. Moreover, the anions of potassium hydrogen phthalate could enter the first solvation shell of hydrated Zn2+ to form contact ionic pairs, which suppress the parasitic reaction with H2O and accelerate the desolvation kinetics of hydrated Zn2+. Furthermore, as the main component of the buffer solution, KHP can alleviate the change in the pH of the solution caused by competitive HER, thus inhibiting the generation of insoluble by‐products. Therefore, symmetric cells with potassium hydrogen phthalate additives showed a long cycling lifespan of over 4500 h under 1.0 mA cm−2 and even under a high depth of discharging state (44%), they can still maintain stability for over 350 h. Therefore, this work provides new insight into the rational design of emerging multifunctional additives based on the synergistic cations electric shielding effect and anions chemistry for long‐term stable Zn metal batteries.

Purity of potassium hydrogen phthalate

Potassium hydrogen phthalate (KHP) is widely used both as acidimetric standard for acid and base titration analysis and as pH standard. Therefore, well characterised certified reference materials are necessary. Within a framework of BAM and Sigma–Aldrich Production GmbH for certification of primary substances for titration and standard anion solutions a batch of 994 bottles of potassium hydrogen phthalate was certified for its acidimetric purity expressed as a mass fraction. It is intended to be used as an acidimetric standard only. KHP has been determined by precision coulometric acidimetric titration for about 50 years. Since then the procedure was extensively automated. The purity of KHP was subject of CCQM-P36 and CCQM-K34/K34.1 where seven metrology institutes compared their measurement capability to determine the amount content of acid in solid weak acids. All institutes used constant current coulometry. Although there has been a lot of work carried out on potassium hydrogen phthalate -determination using precision coulometry, it is still an extremely extensive method not suitable for high sample throughput. A fundamental apparatus for coulometry is commercially not available. Therefore, individual experimental designs are applied, which are only possible to build up if special know how and knowledge of coulometry procedures and electrochemistry techniques are available. To overcome these problems a special experimental design should be developed to carry out certification analysis of a large batch of a KHP-standard.[2]

The different results for both methods used for the determination of the mass fraction of KHP are summarised. Acidimetric purity of potassium hydrogen phthalate is statistically not different from 100% purity. Very precise volumetric titration with regards to bias and repeatability using optimised procedures shows within uncertainties the same results as measurements with precision coulometry. Whereby the volumetric method is calibrated with a certified reference material and the coulometric determination rely on traceable SI quantities. The purity of potassium hydrogen phthalate is the same with slightly different uncertainties while the throughput of the volumetric titration is significantly higher than that of coulometric titration. This material is well characterised and can be used as an acidimetric standard with low uncertainty. Thus, it forms the basis for traceability of a high number of acid–base titrations.

References

[1]Fu H, Huang S, Wang T, Lu J, Xiong P, Yao K, Byun JS, Li W, Kim Y, Park HS. Synergistic Cationic Shielding and Anionic Chemistry of Potassium Hydrogen Phthalate for Ultrastable Zn─I2 Full Batteries. Adv Mater. 2025 Jan;37(3):e2411686. doi: 10.1002/adma.202411686. Epub 2024 Oct 22. PMID: 39439146; PMCID: PMC11756037.

[2]Recknagel, S., Breitenbach, M., Pautz, J., & Lück, D. (2007). Purity of potassium hydrogen phthalate, determination with precision coulometric and volumetric titration–A comparison. Analytica Chimica Acta, 599(2), 256–263. https://doi.org/10.1016/j.aca.2007.07.062